36. Input/output <stdio.h>

36.1. Introduction

The header <stdio.h> declares three types, several macros, and many functions for performing input and output.

The types declared are size_t (described in Common definitions <stddef.h>);

FILE

which is an object type capable of recording all the information needed to control a stream, including its file position indicator, a pointer to its associated buffer (if any), an error indicator that records whether a read/write error has occurred, and an end-of-file indicator that records whether the end of the file has been reached; and

fpos_t

which is an object type other than an array type capable of recording all the information needed to specify uniquely every position within a file.

The macros are NULL (described in Common definitions <stddef.h>);

_IOFBF
_IOLBF
_IONBF

which expand to integer constant expressions with distinct values, suitable for use as the third argument to the setvbuf function;

BUFSIZ

which expands to an integer constant expression that is the size of the buffer used by the setbuf function;

EOF

which expands to an integer constant expression, with type int and a negative value, that is returned by several functions to indicate end-of-file, that is, no more input from a stream;

FOPEN_MAX

which expands to an integer constant expression that is the minimum number of files that the implementation guarantees can be open simultaneously;

FILENAME_MAX

which expands to an integer constant expression that is the size needed for an array of char large enough to hold the longest file name string that the implementation

..index:: L_tmpnam macro, SEEK_CUR macro, SEEK_END macro, SEEK_SET macro, TMP_MAX macro

guarantees can be opened; [1]

L_tmpnam

which expands to an integer constant expression that is the size needed for an array of char large enough to hold a temporary file name string generated by the tmpnam function;

SEEK_CUR
SEEK_END
SEEK_SET

which expand to integer constant expressions with distinct values, suitable for use as the third argument to the fseek function;

TMP_MAX

which expands to an integer constant expression that is the maximum number of unique

file names that can be generated by the tmpnam function;

stderr
stdin
stdout

which are expressions of type “pointer to FILE” that point to the FILE objects associated, respectively, with the standard error, input, and output streams.

The header <wchar.h> declares a number of functions useful for wide character input and output. The wide character input/output functions described in that subclause provide operations analogous to most of those described here, except that the fundamental units internal to the program are wide characters. The external representation (in the file) is a sequence of “generalized” multibyte characters, as described further in Files.

The input/output functions are given the following collective terms:

  • The wide character input functions — those functions described in Extended multibyte and wide character utilities <wchar.h> that perform input into wide characters and wide strings: fgetwc, fgetws, getwc, getwchar, fwscanf, wscanf, vfwscanf and vwscanf.
  • The wide character output functions — those functions described in Extended multibyte and wide character utilities <wchar.h> that perform output from wide characters and wide strings: fputwc, fputws, putwc, putwchar, fwprintf, wprintf, vfwprintf and vwprintf.
  • The wide character input/output functions the union of the ungetwc function, the wide character input functions, and the wide character output functions.
  • The byte input/output functions — those functions described in this subclause that perform input/output: fgetc, fgets, fprintf, fputc, fputs, fread, fscanf, fwrite, getc, getchar, gets, perror, printf, putc, putchar, puts, scanf, ungetc, vfprintf, vfscanf, vprintf and vscanf.

Forward references: files (Files), the fseek function (The fseek function), streams (Streams), the tmpnam function (The tmpnam function), <wchar.h> (Extended multibyte and wide character utilities <wchar.h>).

[1]If the implementation imposes no practical limit on the length of file name strings, the value of FILENAME_MAX should instead be the recommended size of an array intended to hold a file name string. Of course, file name string contents are subject to other system-specific constraints; therefore all possible strings of length FILENAME_MAX cannot be expected to be opened successfully.

36.2. Streams

Input and output, whether to or from physical devices such as terminals and tape drives, or whether to or from files supported on structured storage devices, are mapped into logical data streams, whose properties are more uniform than their various inputs and outputs. Two forms of mapping are supported, for text streams and for binary streams. [2]

A text stream is an ordered sequence of characters composed into lines, each line consisting of zero or more characters plus a terminating new-line character. Whether the last line requires a terminating new-line character is implementation-defined. Characters may have to be added, altered, or deleted on input and output to conform to differing conventions for representing text in the host environment. Thus, there need not be a oneto- one correspondence between the characters in a stream and those in the external representation. Data read in from a text stream will necessarily compare equal to the data that were earlier written out to that stream only if: the data consist only of printing characters and the control characters horizontal tab and new-line; no new-line character is immediately preceded by space characters; and the last character is a new-line character. Whether space characters that are written out immediately before a new-line character appear when read in is implementation-defined.

A binary stream is an ordered sequence of characters that can transparently record internal data. Data read in from a binary stream shall compare equal to the data that were earlier written out to that stream, under the same implementation. Such a stream may, however, hav e an implementation-defined number of null characters appended to the end of the stream.

Each stream has an orientation. After a stream is associated with an external file, but before any operations are performed on it, the stream is without orientation. Once a wide character input/output function has been applied to a stream without orientation, the stream becomes a wide-oriented stream. Similarly, once a byte input/output function has been applied to a stream without orientation, the stream becomes a byte-oriented stream. Only a call to the freopen function or the fwide function can otherwise alter the orientation of a stream. (A successful call to freopen removes any orientation.) [3]

Byte input/output functions shall not be applied to a wide-oriented stream and wide character input/output functions shall not be applied to a byte-oriented stream. The remaining stream operations do not affect, and are not affected by, a stream’s orientation, except for the following additional restrictions:

  • Binary wide-oriented streams have the file-positioning restrictions ascribed to both text and binary streams.
  • For wide-oriented streams, after a successful call to a file-positioning function that leaves the file position indicator prior to the end-of-file, a wide character output function can overwrite a partial multibyte character; any file contents beyond the byte(s) written are henceforth indeterminate.

Each wide-oriented stream has an associated mbstate_t object that stores the current parse state of the stream. A successful call to fgetpos stores a representation of the value of this mbstate_t object as part of the value of the fpos_t object. A later successful call to fsetpos using the same stored fpos_t value restores the value of the associated mbstate_t object as well as the position within the controlled stream.

Environmental limits

An implementation shall support text files with lines containing at least 254 characters, including the terminating new-line character. The value of the macro BUFSIZ shall be at least 256.

Forward references: the freopen function (The freopen function), the fwide function (The fwide function), mbstate_t (Introduction), the fgetpos function (The fgetpos function), the fsetpos function (The fsetpos function).

[2]An implementation need not distinguish between text streams and binary streams. In such an implementation, there need be no new-line characters in a text stream nor any limit to the length of a
[3]The three predefined streams stdin, stdout and stderr are unoriented at program startup.

36.3. Files

A stream is associated with an external file (which may be a physical device) by opening a file, which may involve creating a new file. Creating an existing file causes its former contents to be discarded, if necessary. If a file can support positioning requests (such as a disk file, as opposed to a terminal), then a file position indicator associated with the stream is positioned at the start (character number zero) of the file, unless the file is opened with append mode in which case it is implementation-defined whether the file position indicator is initially positioned at the beginning or the end of the file. The file position indicator is maintained by subsequent reads, writes, and positioning requests, to facilitate an orderly progression through the file.

Binary files are not truncated, except as defined in The fopen function. Whether a write on a text stream causes the associated file to be truncated beyond that point is implementation-defined.

When a stream is unbuffered, characters are intended to appear from the source or at the destination as soon as possible. Otherwise characters may be accumulated and transmitted to or from the host environment as a block. When a stream is fully buffered, characters are intended to be transmitted to or from the host environment as a block when a buffer is filled. When a stream is line buffered, characters are intended to be transmitted to or from the host environment as a block when a new-line character is encountered. Furthermore, characters are intended to be transmitted as a block to the host environment when a buffer is filled, when input is requested on an unbuffered stream, or when input is requested on a line buffered stream that requires the transmission of characters from the host environment. Support for these characteristics is implementation-defined, and may be affected via the setbuf and setvbuf functions.

A file may be disassociated from a controlling stream by closing the file. Output streams are flushed (any unwritten buffer contents are transmitted to the host environment) before the stream is disassociated from the file. The value of a pointer to a FILE object is indeterminate after the associated file is closed (including the standard text streams). Whether a file of zero length (on which no characters have been written by an output stream) actually exists is implementation-defined.

The file may be subsequently reopened, by the same or another program execution, and its contents reclaimed or modified (if it can be repositioned at its start). If the main function returns to its original caller, or if the exit function is called, all open files are closed (hence all output streams are flushed) before program termination. Other paths to program termination, such as calling the abort function, need not close all files properly.

The address of the FILE object used to control a stream may be significant; a copy of a FILE object need not serve in place of the original.

At program startup, three text streams are predefined and need not be opened explicitly — standard input (for reading conventional input), standard output (for writing conventional output) and standard error (for writing diagnostic output). As initially opened, the standard error stream is not fully buffered; the standard input and standard output streams are fully buffered if and only if the stream can be determined not to refer to an interactive device.

Functions that open additional (nontemporary) files require a file name, which is a string. The rules for composing valid file names are implementation-defined. Whether the same file can be simultaneously open multiple times is also implementation-defined.

Although both text and binary wide-oriented streams are conceptually sequences of wide characters, the external file associated with a wide-oriented stream is a sequence of multibyte characters, generalized as follows:

  • Multibyte encodings within files may contain embedded null bytes (unlike multibyte encodings valid for use internal to the program).
  • A file need not begin nor end in the initial shift state. [4]

Moreover, the encodings used for multibyte characters may differ among files. Both the nature and choice of such encodings are implementation-defined.

The wide character input functions read multibyte characters from the stream and convert them to wide characters as if they were read by successive calls to the fgetwc function. Each conversion occurs as if by a call to the mbrtowc function, with the conversion state described by the stream’s own mbstate_t object. The byte input functions read characters from the stream as if by successive calls to the fgetc function.

The wide character output functions convert wide characters to multibyte characters and write them to the stream as if they were written by successive calls to the fputwc function. Each conversion occurs as if by a call to the wcrtomb function, with the conversion state described by the stream’s own mbstate_t object. The byte output functions write characters to the stream as if by successive calls to the fputc function.

In some cases, some of the byte input/output functions also perform conversions between multibyte characters and wide characters. These conversions also occur as if by calls to the mbrtowc and wcrtomb functions.

An encoding error occurs if the character sequence presented to the underlying mbrtowc function does not form a valid (generalized) multibyte character, or if the code value passed to the underlying wcrtomb does not correspond to a valid (generalized) multibyte character. The wide character input/output functions and the byte input/output functions store the value of the macro EILSEQ in errno if and only if an encoding error occurs.

Environmental limits

The value of FOPEN_MAX shall be at least eight, including the three standard text streams.

Forward references: the exit function (The exit function), the fgetc function (The fgetc function), the fopen function (The fopen function), the fputc function (The fputc function), the setbuf function (The setbuf function), the setvbuf function (The setvbuf function), the fgetwc function (The fgetwc function), the fputwc function (The fputwc function), conversion state (Extended multibyte/wide character conversion utilities), the mbrtowc function (The mbrtowc function), the wcrtomb function (The wcrtomb function).

[4]Setting the file position indicator to end-of-file, as with fseek(file, 0, SEEK_END), has undefined behavior for a binary stream (because of possible trailing null characters) or for any stream with state-dependent encoding that does not assuredly end in the initial shift state.

36.4. Operations on files

36.4.1. The remove function

Synopsis

#include <stdio.h>
int remove(const char *filename);

Description

The remove function causes the file whose name is the string pointed to by filename to be no longer accessible by that name. A subsequent attempt to open that file using that name will fail, unless it is created anew. If the file is open, the behavior of the remove function is implementation-defined.

Returns

The remove function returns zero if the operation succeeds, nonzero if it fails.

36.4.2. The rename function

Synopsis

#include <stdio.h>
int rename(const char *old, const char *new);

Description

The rename function causes the file whose name is the string pointed to by old to be henceforth known by the name given by the string pointed to by new. The file named old is no longer accessible by that name. If a file named by the string pointed to by new exists prior to the call to the rename function, the behavior is implementation-defined.

Returns

The rename function returns zero if the operation succeeds, nonzero if it fails, [5] in which case if the file existed previously it is still known by its original name.

[5]Among the reasons the implementation may cause the rename function to fail are that the file is open or that it is necessary to copy its contents to effectuate its renaming.

36.4.3. The tmpfile function

Synopsis

#include <stdio.h>
FILE *tmpfile(void);

Description

The tmpfile function creates a temporary binary file that is different from any other existing file and that will automatically be removed when it is closed or at program termination. If the program terminates abnormally, whether an open temporary file is removed is implementation-defined. The file is opened for update with “wb+” mode.

Recommended practice

It should be possible to open at least TMP_MAX temporary files during the lifetime of the program (this limit may be shared with tmpnam) and there should be no limit on the number simultaneously open other than this limit and any limit on the number of open files (FOPEN_MAX).

Returns

The tmpfile function returns a pointer to the stream of the file that it created. If the file cannot be created, the tmpfile function returns a null pointer.

Forward references: the fopen function (The fopen function).

36.4.4. The tmpnam function

Synopsis

#include <stdio.h>
char *tmpnam(char *s);

Description

The tmpnam function generates a string that is a valid file name and that is not the same as the name of an existing file. [6] The function is potentially capable of generating TMP_MAX different strings, but any or all of them may already be in use by existing files and thus not be suitable return values.

The tmpnam function generates a different string each time it is called.

The implementation shall behave as if no library function calls the tmpnam function.

Returns

If no suitable string can be generated, the tmpnam function returns a null pointer. Otherwise, if the argument is a null pointer, the tmpnam function leaves its result in an internal static object and returns a pointer to that object (subsequent calls to the tmpnam function may modify the same object). If the argument is not a null pointer, it is assumed to point to an array of at least L_tmpnam chars; the tmpnam function writes its result in that array and returns the argument as its value.

Environmental limits

The value of the macro TMP_MAX shall be at least 25.

[6]Files created using strings generated by the tmpnam function are temporary only in the sense that their names should not collide with those generated by conventional naming rules for the implementation. It is still necessary to use the remove function to remove such files when their use is ended, and before program termination.

36.5. File access functions

36.5.1. The fclose function

Synopsis

#include <stdio.h>
int fclose(FILE *stream);

Description

A successful call to the fclose function causes the stream pointed to by stream to be flushed and the associated file to be closed. Any unwritten buffered data for the stream are delivered to the host environment to be written to the file; any unread buffered data are discarded. Whether or not the call succeeds, the stream is disassociated from the file and any buffer set by the setbuf or setvbuf function is disassociated from the stream (and deallocated if it was automatically allocated).

Returns

The fclose function returns zero if the stream was successfully closed, or EOF if any errors were detected.

36.5.2. The fflush function

Synopsis

#include <stdio.h>
int fflush(FILE *stream);

Description

If stream points to an output stream or an update stream in which the most recent operation was not input, the fflush function causes any unwritten data for that stream to be delivered to the host environment to be written to the file; otherwise, the behavior is undefined.

If stream is a null pointer, the fflush function performs this flushing action on all streams for which the behavior is defined above.

Returns

The fflush function sets the error indicator for the stream and returns EOF if a write error occurs, otherwise it returns zero.

Forward references: the fopen function (The fopen function).

36.5.3. The fopen function

Synopsis

#include <stdio.h>
FILE *fopen(const char * restrict filename, const char * restrict mode);

Description

The fopen function opens the file whose name is the string pointed to by filename, and associates a stream with it.

The argument mode points to a string. If the string is one of the following, the file is open in the indicated mode. Otherwise, the behavior is undefined. [7]

r open text file for reading
w truncate to zero length or create text file for writing
a append; open or create text file for writing at end-of-file
rb open binary file for reading
wb truncate to zero length or create binary file for writing
ab append; open or create binary file for writing at end-of-file
r+ open text file for update (reading and writing)
w+ truncate to zero length or create text file for update
a+ append; open or create text file for update, writing at end-of-file
r+b or rb+ open binary file for update (reading and writing)
w+b or wb+ truncate to zero length or create binary file for update
a+b or ab+ append; open or create binary file for update, writing at end-of-file

Opening a file with read mode ('r' as the first character in the mode argument) fails if the file does not exist or cannot be read.

Opening a file with append mode ('a' as the first character in the mode argument) causes all subsequent writes to the file to be forced to the then current end-of-file, regardless of intervening calls to the fseek function. In some implementations, opening a binary file with append mode ('b' as the second or third character in the above list of mode argument values) may initially position the file position indicator for the stream beyond the last data written, because of null character padding.

When a file is opened with update mode ('+' as the second or third character in the above list of mode argument values), both input and output may be performed on the associated stream. However, output shall not be directly followed by input without an intervening call to the fflush function or to a file positioning function (fseek, ``fsetpos or rewind), and input shall not be directly followed by output without an intervening call to a file positioning function, unless the input operation encounters end- of-file. Opening (or creating) a text file with update mode may instead open (or create) a binary stream in some implementations.

When opened, a stream is fully buffered if and only if it can be determined not to refer to an interactive device. The error and end-of-file indicators for the stream are cleared.

Returns

The fopen function returns a pointer to the object controlling the stream. If the open operation fails, fopen returns a null pointer.

Forward references: file positioning functions (File positioning functions).

[7]If the string begins with one of the above sequences, the implementation might choose to ignore the remaining characters, or it might use them to select different kinds of a file (some of which might not conform to the properties in Streams).

36.5.4. The freopen function

Synopsis

#include <stdio.h>
FILE *freopen(const char * restrict filename, const char * restrict mode, FILE * restrict stream);

Description

The freopen function opens the file whose name is the string pointed to by filename and associates the stream pointed to by stream with it. The mode argument is used just as in the fopen function. [8]

If filename is a null pointer, the freopen function attempts to change the mode of the stream to that specified by mode, as if the name of the file currently associated with the stream had been used. It is implementation-defined which changes of mode are permitted (if any), and under what circumstances.

The freopen function first attempts to close any file that is associated with the specified stream. Failure to close the file is ignored. The error and end-of-file indicators for the stream are cleared.

Returns

The freopen function returns a null pointer if the open operation fails. Otherwise, freopen returns the value of stream.

[8]The primary use of the freopen function is to change the file associated with a standard text stream (stderr, stdin or stdout), as those identifiers need not be modifiable lvalues to which the value returned by the fopen function may be assigned.

36.5.5. The setbuf function

Synopsis

#include <stdio.h>
void setbuf(FILE * restrict stream, char * restrict buf);

Description

Except that it returns no value, the setbuf function is equivalent to the setvbuf function invoked with the values _IOFBF for mode and BUFSIZ for size, or (if buf is a null pointer), with the value _IONBF for mode.

Returns

The setbuf function returns no value.

Forward references: the setvbuf function (The setvbuf function).

36.5.6. The setvbuf function

Synopsis

#include <stdio.h>
int setvbuf(FILE * restrict stream, char * restrict buf, int mode, size_t size);

Description

The setvbuf function may be used only after the stream pointed to by stream has been associated with an open file and before any other operation (other than an unsuccessful call to setvbuf) is performed on the stream. The argument mode determines how stream will be buffered, as follows: _IOFBF causes input/output to be fully buffered; _IOLBF causes input/output to be line buffered; _IONBF causes input/output to be unbuffered. If buf is not a null pointer, the array it points to may be used instead of a buffer allocated by the setvbuf function [9] and the argument size specifies the size of the array; otherwise, size may determine the size of a buffer allocated by the setvbuf function. The contents of the array at any time are indeterminate.

Returns

The setvbuf function returns zero on success, or nonzero if an invalid value is given for mode or if the request cannot be honored.

[9]The buffer has to have a lifetime at least as great as the open stream, so the stream should be closed before a buffer that has automatic storage duration is deallocated upon block exit.

36.6. Formatted input/output functions

The formatted input/output functions shall behave as if there is a sequence point after the actions associated with each specifier. [10]

[10]The fprintf functions perform writes to memory for the %n specifier.

36.6.1. The fprintf function

Synopsis

#include <stdio.h>
int fprintf(FILE * restrict stream, const char * restrict format, ...);

Description

The fprintf function writes output to the stream pointed to by stream, under control of the string pointed to by format that specifies how subsequent arguments are converted for output. If there are insufficient arguments for the format, the behavior is undefined. If the format is exhausted while arguments remain, the excess arguments are evaluated (as always) but are otherwise ignored. The fprintf function returns when the end of the format string is encountered.

The format shall be a multibyte character sequence, beginning and ending in its initial shift state. The format is composed of zero or more directives: ordinary multibyte characters (not %), which are copied unchanged to the output stream; and conversion pecifications, each of which results in fetching zero or more subsequent arguments, converting them, if applicable, according to the corresponding conversion specifier, and then writing the result to the output stream.

Each conversion specification is introduced by the character %. After the %, the following appear in sequence:

  • Zero or more flags (in any order) that modify the meaning of the conversion specification.
  • An optional minimum field width. If the converted value has fewer characters than the field width, it is padded with spaces (by default) on the left (or right, if the left adjustment flag, described later, has been given) to the field width. The field width takes the form of an asterisk * (described later) or a nonnegative decimal integer. [11]
  • An optional precision that gives the minimum number of digits to appear for the d, i, o, u, x and X conversions, the number of digits to appear after the decimal-point character for a, A, e, E, f and F conversions, the maximum number of significant digits for the g and G conversions, or the maximum number of bytes to be written for s conversions. The precision takes the form of a period (.) followed either by an asterisk * (described later) or by an optional decimal integer; if only the period is specified, the precision is taken as zero. If a precision appears with any other conversion specifier, the behavior is undefined.
  • An optional length modifier that specifies the size of the argument.
  • A conversion specifier character that specifies the type of conversion to be applied.

As noted above, a field width, or precision, or both, may be indicated by an asterisk. In this case, an int argument supplies the field width or precision. The arguments specifying field width, or precision, or both, shall appear (in that order) before the argument (if any) to be converted. A negative field width argument is taken as a - flag followed by a positive field width. A negative precision argument is taken as if the precision were omitted.

The flag characters and their meanings are:

- The result of the conversion is left-justified within the field. (It is right-justified if this flag is not specified.)

+ The result of a signed conversion always begins with a plus or minus sign. (It begins with a sign only when a negative value is converted if this flag is not specified.) [12]

space If the first character of a signed conversion is not a sign, or if a signed conversion results in no characters, a space is prefixed to the result. If the space and + flags both appear, the space flag is ignored.

# The result is converted to an “alternative form”. For o conversion, it increases the precision, if and only if necessary, to force the first digit of the result to be a zero (if the value and precision are both 0, a single 0 is printed). For x (or X) conversion, a nonzero result has 0x (or 0X) prefixed to it. For a, A, e, E, f, F, g and G conversions, the result of converting a floating-point number always contains a decimal-point character, even if no digits follow it. (Normally, a decimal-point character appears in the result of these conversions only if a digit follows it.) For g and G conversions, trailing zeros are not removed from the result. For other conversions, the behavior is undefined.

0 For d, i, o, u, x, X, a, A, e, E, f, F, g and G conversions, leading zeros (following any indication of sign or base) are used to pad to the field width rather than performing space padding, except when converting an infinity or NaN. If the 0 and - flags both appear, the 0 flag is ignored. For d, i, o, u, x and X conversions, if a precision is specified, the 0 flag is ignored. For other conversions, the behavior is undefined.

The length modifiers and their meanings are:

hh Specifies that a following d, i, o, u, x or X conversion specifier applies to a signed char or unsigned char argument (the argument will have been promoted according to the integer promotions, but its value shall be converted to signed char or unsigned char before printing); or that a following n conversion specifier applies to a pointer to a signed char argument.

h Specifies that a following d, i, o, u, x or X conversion specifier applies to a short int or unsigned short int argument (the argument will have been promoted according to the integer promotions, but its value shall be converted to short int or unsigned short int before printing); or that a following n conversion specifier applies to a pointer to a short int argument.

l (ell) Specifies that a following d, i, o, u, x or X conversion specifier applies to a long int or unsigned long int argument; that a following n conversion specifier applies to a pointer to a long int argument; that a following c conversion specifier applies to a wint_t argument; that a following s conversion specifier applies to a pointer to a wchar_t argument; or has no effect on a following a, A, e, E, f, F, g or G conversion specifier.

ll (ell-ell) Specifies that a following d, i, o, u, x or X conversion specifier applies to a long long int or unsigned long long int argument; or that a following n conversion specifier applies to a pointer to a long long int argument.

j Specifies that a following d, i, o, u, x or X conversion specifier applies to an intmax_t or uintmax_t argument; or that a following n conversion specifier applies to a pointer to an intmax_t argument.

z Specifies that a following d, i, o, u, x or X conversion specifier applies to a size_t or the corresponding signed integer type argument; or that a following n conversion specifier applies to a pointer to a signed integer type corresponding to size_t argument.

t Specifies that a following d, i, o, u, x or X conversion specifier applies to a ptrdiff_t or the corresponding unsigned integer type argument; or that a following n conversion specifier applies to a pointer to a ptrdiff_t argument.

L Specifies that a following a, A, e, E, f, F, g or G conversion specifier applies to a long double argument.

If a length modifier appears with any conversion specifier other than as specified above, the behavior is undefined.

The conversion specifiers and their meanings are:

d,i The int argument is converted to signed decimal in the style [-]dddd. The precision specifies the minimum number of digits to appear; if the value being converted can be represented in fewer digits, it is expanded with leading zeros. The default precision is 1. The result of converting a zero value with a precision of zero is no characters.

o,u,x,X The unsigned int argument is converted to unsigned octal (o), unsigned decimal (u), or unsigned hexadecimal notation (x or X) in the style dddd; the letters abcdef are used for x conversion and the letters ABCDEF for X conversion. The precision specifies the minimum number of digits to appear; if the value being converted can be represented in fewer digits, it is expanded with leading zeros. The default precision is 1. The result of converting a zero value with a precision of zero is no characters.

f,F A double argument representing a floating-point number is converted to decimal notation in the style [-]ddd.ddd, where the number of digits after the decimal-point character is equal to the precision specification. If the precision is missing, it is taken as 6; if the precision is zero and the # flag is not specified, no decimal-point character appears. If a decimal-point character appears, at least one digit appears before it. The value is rounded to the appropriate number of digits.

A double argument representing an infinity is converted in one of the styles [-]inf or [-]infinity — which style is implementation-defined. A double argument representing a NaN is converted in one of the styles [-]nan or [-]nan (n-char-sequence) — which style, and the meaning of any n-char-sequence, is implementation-defined. The F conversion specifier produces INF, INFINITY or NAN instead of inf, infinity or nan respectively. [13]

e,E A double argument representing a floating-point number is converted in the style [-]d.ddd e\(\pm dd\) , where there is one digit (which is nonzero if the argument is nonzero) before the decimal-point character and the number of digits after it is equal to the precision; if the precision is missing, it is taken as 6; if the precision is zero and the # flag is not specified, no decimal-point character appears. The value is rounded to the appropriate number of digits. The E conversion specifier produces a number with E instead of e introducing the exponent. The exponent always contains at least two digits, and only as many more digits as necessary to represent the exponent. If the value is zero, the exponent is zero.

A double argument representing an infinity or NaN is converted in the style of an f or F conversion specifier.

g,G A double argument representing a floating-point number is converted in style f or e (or in style F or E in the case of a G conversion specifier), depending on the value converted and the precision. Let \(P\) equal the precision if nonzero, 6 if the precision is omitted, or 1 if the precision is zero. Then, if a conversion with style E would have an exponent of \(X\):

  • if \(P > X \geq -4\), the conversion is with style f (or F) and precision \(P - (X + 1)\).
  • otherwise, the conversion is with style e (or E) and precision \(P - 1\).

Finally, unless the # flag is used, any trailing zeros are removed from the fractional portion of the result and the decimal-point character is removed if there is no fractional portion remaining.

A double argument representing an infinity or NaN is converted in the style of an f or F conversion specifier.

a,A A double argument representing a floating-point number is converted in the style [-]0xh.hhhh \(p \pm d\), where there is one hexadecimal digit (which is nonzero if the argument is a normalized floating-point number and is otherwise unspecified) before the decimal-point character [14] and the number of hexadecimal digits after it is equal to the precision; if the precision is missing and FLT_RADIX is a power of 2, then the precision is sufficient for an exact representation of the value; if the precision is missing and FLT_RADIX is not a power of 2, then the precision is sufficient to distinguish [15] values of type double, except that trailing zeros may be omitted; if the precision is zero and the # flag is not specified, no decimal-point character appears. The letters abcdef are used for a conversion and the letters ABCDEF for A conversion. The A conversion specifier produces a number with X and P instead of x and p. The exponent always contains at least one digit, and only as many more digits as necessary to represent the decimal exponent of 2. If the value is zero, the exponent is zero. A double argument representing an infinity or NaN is converted in the style of an f or F conversion specifier.

c If no l length modifier is present, the int argument is converted to an unsigned char, and the resulting character is written.

If an l length modifier is present, the wint_t argument is converted as if by an ls conversion specification with no precision and an argument that points to the initial element of a two-element array of wchar_t, the first element containing the wint_t argument to the lc conversion specification and the second a null wide character.

s If no l length modifier is present, the argument shall be a pointer to the initial element of an array of character type. [16] Characters from the array are written up to (but not including) the terminating null character. If the precision is specified, no more than that many bytes are written. If the precision is not specified or is greater than the size of the array, the array shall contain a null character.

If an l length modifier is present, the argument shall be a pointer to the initial element of an array of wchar_t type. Wide characters from the array are converted to multibyte characters (each as if by a call to the wcrtomb function, with the conversion state described by an mbstate_t object initialized to zero before the first wide character is converted) up to and including a terminating null wide character. The resulting multibyte characters are written up to (but not including) the terminating null character (byte). If no precision is specified, the array shall contain a null wide character. If a precision is specified, no more than that many bytes are written (including shift sequences, if any), and the array shall contain a null wide character if, to equal the multibyte character sequence length given by the precision, the function would need to access a wide character one past the end of the array. In no case is a partial multibyte character written. [17]

p The argument shall be a pointer to void. The value of the pointer is converted to a sequence of printing characters, in an implementation-defined manner.

n The argument shall be a pointer to signed integer into which is written the number of characters written to the output stream so far by this call to fprintf. No argument is converted, but one is consumed. If the conversion specification includes any flags, a field width, or a precision, the behavior is undefined.

% A % character is written. No argument is converted. The complete conversion specification shall be %%.

If a conversion specification is invalid, the behavior is undefined. If any argument is not the correct type for the corresponding conversion specification, the behavior is undefined.

In no case does a nonexistent or small field width cause truncation of a field; if the result of a conversion is wider than the field width, the field is expanded to contain the conversion result.

For a and A conversions, if FLT_RADIX is a power of 2, the value is correctly rounded to a hexadecimal floating number with the given precision.

Recommended practice

For a and A conversions, if FLT_RADIX is not a power of 2 and the result is not exactly representable in the given precision, the result should be one of the two adjacent numbers in hexadecimal floating style with the given precision, with the extra stipulation that the error should have a correct sign for the current rounding direction.

For e, E, f, F, g and G conversions, if the number of significant decimal digits is at most DECIMAL_DIG, then the result should be correctly rounded. [18] If the number of significant decimal digits is more than DECIMAL_DIG but the source value is exactly representable with DECIMAL_DIG digits, then the result should be an exact representation with trailing zeros. Otherwise, the source value is bounded by two adjacent decimal strings \(L < U\), both having DECIMAL_DIG significant digits; the value of the resultant decimal string \(D\) should satisfy \(L \leq D \leq U\), with the extra stipulation that the error should have a correct sign for the current rounding direction. Returns

The fprintf function returns the number of characters transmitted, or a negative value if an output or encoding error occurred.

Environmental limits

The number of characters that can be produced by any single conversion shall be at least 4095.

EXAMPLE 1 To print a date and time in the form “Sunday, July 3, 10:02” followed by \(\pi\) to five decimal places:

#include <math.h>
#include <stdio.h>
/* ... */
char *weekday, *month; // pointers to strings
int day, hour, min;
fprintf(stdout, "%s, %s %d, %.2d:%.2d\n",
weekday, month, day, hour, min);
fprintf(stdout, "pi = %.5f\n", 4 * atan(1.0));

EXAMPLE 2 In this example, multibyte characters do not have a state-dependent encoding, and the members of the extended character set that consist of more than one byte each consist of exactly two bytes, the first of which is denoted here by a and the second by an uppercase letter.

Forward references: conversion state (Extended multibyte/wide character conversion utilities), the wcrtomb function (The wcrtomb function).

[11]Note that 0 is taken as a flag, not as the beginning of a field width.
[12]The results of all floating conversions of a negative zero, and of negative values that round to zero, include a minus sign.
[13]When applied to infinite and NaN values, the -, + and space flag characters have their usual meaning; the # and 0 flag characters have no effect.
[14]Binary implementations can choose the hexadecimal digit to the left of the decimal-point character so that subsequent digits align to nibble (4-bit) boundaries.
[15]The precision p is sufficient to distinguish values of the source type if \(16^{p-1} > b^n\) where \(b\) is FLT_RADIX and \(n\) is the number of base-b digits in the significand of the source type. A smaller \(p\) might suffice depending on the implementation’s scheme for determining the digit to the left of the decimal-point character.
[16]No special provisions are made for multibyte characters.
[17]Redundant shift sequences may result if multibyte characters have a state-dependent encoding.
[18]For binary-to-decimal conversion, the result format’s values are the numbers representable with the given format specifier. The number of significant digits is determined by the format specifier, and in the case of fixed-point conversion by the source value as well.

36.6.2. The fscanf function

Synopsis

#include <stdio.h>
int fscanf(FILE * restrict stream,
const char * restrict format, ...);

Description

The fscanf function reads input from the stream pointed to by stream, under control of the string pointed to by format that specifies the admissible input sequences and how they are to be converted for assignment, using subsequent arguments as pointers to the objects to receive the converted input. If there are insufficient arguments for the format, the behavior is undefined. If the format is exhausted while arguments remain, the excess arguments are evaluated (as always) but are otherwise ignored.

The format shall be a multibyte character sequence, beginning and ending in its initial shift state. The format is composed of zero or more directives: one or more white-space characters, an ordinary multibyte character (neither % nor a white-space character), or a conversion specification. Each conversion specification is introduced by the character %. After the %, the following appear in sequence:

  • An optional assignment-suppressing character *.
  • An optional decimal integer greater than zero that specifies the maximum field width (in characters).
  • An optional length modifier that specifies the size of the receiving object.
  • A conversion specifier character that specifies the type of conversion to be applied.

The fscanf function executes each directive of the format in turn. If a directive fails, as detailed below, the function returns. Failures are described as input failures (due to the occurrence of an encoding error or the unavailability of input characters), or matching failures (due to inappropriate input).

A directive composed of white-space character(s) is executed by reading input up to the first non-white-space character (which remains unread), or until no more characters can be read.

A directive that is an ordinary multibyte character is executed by reading the next characters of the stream. If any of those characters differ from the ones composing the directive, the directive fails and the differing and subsequent characters remain unread. Similarly, if end-of-file, an encoding error, or a read error prevents a character from being read, the directive fails.

A directive that is a conversion specification defines a set of matching input sequences, as described below for each specifier. A conversion specification is executed in the following steps:

Input white-space characters (as specified by the isspace function) are skipped, unless the specification includes a [, c or n specifier. [19]

An input item is read from the stream, unless the specification includes an n specifier. An input item is defined as the longest sequence of input characters which does not exceed any specified field width and which is, or is a prefix of, a matching input sequence. [20] The first character, if any, after the input item remains unread. If the length of the input item is zero, the execution of the directive fails; this condition is a matching failure unless end-of-file, an encoding error, or a read error prevented input from the stream, in which case it is an input failure.

Except in the case of a % specifier, the input item (or, in the case of a %n directive, the count of input characters) is converted to a type appropriate to the conversion specifier. If the input item is not a matching sequence, the execution of the directive fails: this condition is a matching failure. Unless assignment suppression was indicated by a *, the result of the conversion is placed in the object pointed to by the first argument following the format argument that has not already received a conversion result. If this object does not have an appropriate type, or if the result of the conversion cannot be represented in the object, the behavior is undefined.

The length modifiers and their meanings are:

hh Specifies that a following d, i, o, u, x, X or n conversion specifier applies to an argument with type pointer to signed char or unsigned char.

h Specifies that a following d, i, o, u, x, X or n conversion specifier applies to an argument with type pointer to short int or unsigned short int.

l (ell) Specifies that a following d, i, o, u, x, X or n conversion specifier applies to an argument with type pointer to long int or unsigned long int; that a following a, A, e, E, f, F, g or G conversion specifier applies to an argument with type pointer to double; or that a following c, s or [ conversion specifier applies to an argument with type pointer to wchar_t.

ll (ell-ell) Specifies that a following d, i, o, u, x, X or n conversion specifier applies to an argument with type pointer to long long int or unsigned long long int.

j Specifies that a following d, i, o, u, x, X or n conversion specifier applies to an argument with type pointer to intmax_t or uintmax_t.

z Specifies that a following d, i, o, u, x, X or n conversion specifier applies to an argument with type pointer to size_t or the corresponding signed integer type.

t Specifies that a following d, i, o, u, x, X or n conversion specifier applies to an argument with type pointer to ptrdiff_t or the corresponding unsigned integer type.

L Specifies that a following a, A, e, E, f, F, g or G conversion specifier applies to an argument with type pointer to long double.

If a length modifier appears with any conversion specifier other than as specified above, the behavior is undefined.

The conversion specifiers and their meanings are:

d Matches an optionally signed decimal integer, whose format is the same as expected for the subject sequence of the strtol function with the value 10 for the base argument. The corresponding argument shall be a pointer to signed integer.

i Matches an optionally signed integer, whose format is the same as expected for the subject sequence of the strtol function with the value 0 for the base argument. The corresponding argument shall be a pointer to signed integer.

o Matches an optionally signed octal integer, whose format is the same as expected for the subject sequence of the strtoul function with the value 8 for the base argument. The corresponding argument shall be a pointer to unsigned integer.

u Matches an optionally signed decimal integer, whose format is the same as expected for the subject sequence of the strtoul function with the value 10 for the base argument. The corresponding argument shall be a pointer to unsigned integer.

x Matches an optionally signed hexadecimal integer, whose format is the same as expected for the subject sequence of the strtoul function with the value 16 for the base argument. The corresponding argument shall be a pointer to unsigned integer.

a,e,f,g Matches an optionally signed floating-point number, infinity, or NaN, whose format is the same as expected for the subject sequence of the strtod function. The corresponding argument shall be a pointer to floating.

c Matches a sequence of characters of exactly the number specified by the field width (1 if no field width is present in the directive). [21] If no l length modifier is present, the corresponding argument shall be a pointer to the initial element of a character array large enough to accept the sequence. No null character is added.

If an l length modifier is present, the input shall be a sequence of multibyte characters that begins in the initial shift state. Each multibyte character in the sequence is converted to a wide character as if by a call to the mbrtowc function, with the conversion state described by an mbstate_t object initialized to zero before the first multibyte character is converted. The corresponding argument shall be a pointer to the initial element of an array of wchar_t large enough to accept the resulting sequence of wide characters. No null wide character is added.

s Matches a sequence of non-white-space characters. [22]

If no l length modifier is present, the corresponding argument shall be a pointer to the initial element of a character array large enough to accept the sequence and a terminating null character, which will be added automatically.

If an l length modifier is present, the input shall be a sequence of multibyte characters that begins in the initial shift state. Each multibyte character is converted to a wide character as if by a call to the mbrtowc function, with the conversion state described by an mbstate_t object initialized to zero before the first multibyte character is converted. The corresponding argument shall be a pointer to the initial element of an array of wchar_t large enough to accept the sequence and the terminating null wide character, which will be added automatically.

[ Matches a nonempty sequence of characters from a set of expected characters (the scanset). [22]

If no l length modifier is present, the corresponding argument shall be a pointer to the initial element of a character array large enough to accept the sequence and a terminating null character, which will be added automatically.

If an l length modifier is present, the input shall be a sequence of multibyte characters that begins in the initial shift state. Each multibyte character is converted to a wide character as if by a call to the mbrtowc function, with the conversion state described by an mbstate_t object initialized to zero before the first multibyte character is converted. The corresponding argument shall be a pointer to the initial element of an array of wchar_t large enough to accept the sequence and the terminating null wide character, which will be added automatically.

The conversion specifier includes all subsequent characters in the format string, up to and including the matching right bracket (]). The characters between the brackets (the scanlist) compose the scanset, unless the character after the left bracket is a circumflex (^), in which case the scanset contains all characters that do not appear in the scanlist between the circumflex and the right bracket. If the conversion specifier begins with [] or [^], the right bracket character is in the scanlist and the next following right bracket character is the matching right bracket that ends the specification; otherwise the first following right bracket character is the one that ends the specification. If a - character is in the scanlist and is not the first, nor the second where the first character is a ^, nor the last character, the behavior is implementation-defined.

p Matches an implementation-defined set of sequences, which should be the same as the set of sequences that may be produced by the %p conversion of the fprintf function. The corresponding argument shall be a pointer to a pointer to void. The input item is converted to a pointer value in an implementation-defined manner. If the input item is a value converted earlier during the same program execution, the pointer that results shall compare equal to that value; otherwise the behavior of the %p conversion is undefined.

n No input is consumed. The corresponding argument shall be a pointer to signed integer into which is to be written the number of characters read from the input stream so far by this call to the fscanf function. Execution of a %n directive does not increment the assignment count returned at the completion of execution of the fscanf function. No argument is converted, but one is consumed. If the conversion specification includes an assignment-suppressing character or a field width, the behavior is undefined.

% Matches a single % character; no conversion or assignment occurs. The complete conversion specification shall be %%.

If a conversion specification is invalid, the behavior is undefined.

The conversion specifiers A, E, F, G and X are also valid and behave the same as, respectively, a, e, f, g and x.

Trailing white space (including new-line characters) is left unread unless matched by a directive. The success of literal matches and suppressed assignments is not directly determinable other than via the %n directive.

Returns

The fscanf function returns the value of the macro EOF if an input failure occurs before any conversion. Otherwise, the function returns the number of input items assigned, which can be fewer than provided for, or even zero, in the event of an early matching failure.

EXAMPLE 1 The call:

#include <stdio.h>
/* ... */
int n, i; float x; char name[50];
n = fscanf(stdin, "%d%f%s", &i, &x, name);

with the input line:

25 54.32E-1 thompson

will assign to n the value 3, to i the value 25, to x the value 5.432, and to name the sequence thompson\0.

EXAMPLE 2 The call:

#include <stdio.h>
/* ... */
int i; float x; char name[50];
fscanf(stdin, "%2d%f%*d %[0123456789]", &i, &x, name);

with input:

56789 0123 56a72

will assign to i the value 56 and to x the value 789.0, will skip 0123, and will assign to name the sequence 56\0. The next character read from the input stream will be a.

EXAMPLE 3 To accept repeatedly from stdin a quantity, a unit of measure, and an item name:

#include <stdio.h>
/* ... */
int count; float quant; char units[21], item[21];
do {
  count = fscanf(stdin, "%f%20s of %20s", &quant, units, item);
  fscanf(stdin,"%*[^\n]");
} while (!feof(stdin) && !ferror(stdin));

If the stdin stream contains the following lines:

2 quarts of oil
-12.8degrees Celsius
lots of luck
10.0LBS
of
dirt
100ergs of energy

the execution of the above example will be analogous to the following assignments:

quant = 2; strcpy(units, "quarts"); strcpy(item, "oil");
count = 3;
quant = -12.8; strcpy(units, "degrees");
count = 2; // "C" fails to match "o"
count = 0; // "l" fails to match "%f"
quant = 10.0; strcpy(units, "LBS"); strcpy(item, "dirt");
count = 3;
count = 0; // "100e" fails to match "%f"
count = EOF;

EXAMPLE 4 In:

#include <stdio.h>
/* ... */
int d1, d2, n1, n2, i;
i = sscanf("123", "%d%n%n%d", &d1, &n1, &n2, &d2);

the value 123 is assigned to d1 and the value 3 to n1. Because %n can never get an input failure the value of 3 is also assigned to n2`. The value of d2 is not affected. The value 1 is assigned to i.

Forward references: the strtod, strtof and strtold functions (The strtod, strtof and strtold functions), the strtol, strtoll, strtoul and strtoull functions (The strtol, strtoll, strtoul and strtoull functions), conversion state (Extended multibyte/wide character conversion utilities), the wcrtomb function (The wcrtomb function).

[19]These white-space characters are not counted against a specified field width.
[20]fscanf pushes back at most one input character onto the input stream. Therefore, some sequences that are acceptable to strtod, strtol etc. are unacceptable to fscanf.
[21]No special provisions are made for multibyte characters in the matching rules used by the c, s and [ conversion specifiers — the extent of the input field is determined on a byte-by-byte basis. The resulting field is nevertheless a sequence of multibyte characters that begins in the initial shift state.

36.6.3. The printf function

Synopsis

#include <stdio.h>
int printf(const char * restrict format, ...);

Description

The printf function is equivalent to fprintf with the argument stdout interposed before the arguments to printf.

Returns

The printf function returns the number of characters transmitted, or a negative value if an output or encoding error occurred.

36.6.4. The scanf function

Synopsis

#include <stdio.h>
int scanf(const char * restrict format, ...);

Description

The scanf function is equivalent to fscanf with the argument stdin interposed before the arguments to scanf.

Returns

The scanf function returns the value of the macro EOF if an input failure occurs before any conversion. Otherwise, the scanf function returns the number of input items assigned, which can be fewer than provided for, or even zero, in the event of an early matching failure.

36.6.5. The snprintf function

Synopsis

#include <stdio.h>
int snprintf(char * restrict s, size_t n, const char * restrict format, ...);

Description

The snprintf function is equivalent to fprintf, except that the output is written into an array (specified by argument s) rather than to a stream. If n is zero, nothing is written, and s may be a null pointer. Otherwise, output characters beyond the n-1st are discarded rather than being written to the array, and a null character is written at the end of the characters actually written into the array. If copying takes place between objects that overlap, the behavior is undefined.

36.6.6. The sprintf function

Synopsis

#include <stdio.h>
int sprintf(char * restrict s, const char * restrict format, ...);

Description

The sprintf function is equivalent to fprintf, except that the output is written into an array (specified by the argument s) rather than to a stream. A null character is written at the end of the characters written; it is not counted as part of the returned value. If copying takes place between objects that overlap, the behavior is undefined.

Returns

The sprintf function returns the number of characters written in the array, not counting the terminating null character, or a negative value if an encoding error occurred.

36.6.7. The sscanf function

Synopsis

#include <stdio.h>
int sscanf(const char * restrict s, const char * restrict format, ...);

Description

The sscanf function is equivalent to fscanf, except that input is obtained from a string (specified by the argument s) rather than from a stream. Reaching the end of the string is equivalent to encountering end-of-file for the fscanf function. If copying takes place between objects that overlap, the behavior is undefined.

Returns

The sscanf function returns the value of the macro EOF if an input failure occurs before any conversion. Otherwise, the sscanf function returns the number of input items assigned, which can be fewer than provided for, or even zero, in the event of an early matching failure.

36.6.8. The vfprintf function

Synopsis

#include <stdarg.h>
#include <stdio.h>
int vfprintf(FILE * restrict stream, const char * restrict format, va_list arg);

Description

The vfprintf function is equivalent to fprintf, with the variable argument list replaced by arg, which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls). The vfprintf function does not invoke the va_end macro. [22]

Returns

The vfprintf function returns the number of characters transmitted, or a negative value if an output or encoding error occurred.

EXAMPLE The following shows the use of the vfprintf function in a general error-reporting routine.

#include <stdarg.h>
#include <stdio.h>
void error(char *function_name, char *format, ...)
{
  va_list args;
  va_start(args, format);
  // print out name of function causing error
  fprintf(stderr, "ERROR in %s: ", function_name);
  // print out remainder of message
  vfprintf(stderr, format, args);
  va_end(args);
}

36.6.9. The vfscanf function

Synopsis

#include <stdarg.h>
#include <stdio.h>
int vfscanf(FILE * restrict stream, const char * restrict format, va_list arg);

Description

The vfscanf function is equivalent to fscanf, with the variable argument list replaced by arg, which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls). The vfscanf function does not invoke the va_end macro. [24]

Returns

The vfscanf function returns the value of the macro EOF if an input failure occurs before any conversion. Otherwise, the vfscanf function returns the number of input items assigned, which can be fewer than provided for, or even zero, in the event of an early matching failure.

[24](1, 2, 3, 4, 5, 6) As the functions vfprintf, vfscanf, vprintf, vscanf, vsnprintf, vsprintf and vsscanf invoke the va_arg macro, the value of arg after the return is indeterminate.

36.6.10. The vprintf function

Synopsis

#include <stdarg.h>
#include <stdio.h>
int vprintf(const char * restrict format, va_list arg);

Description

The vprintf function is equivalent to printf, with the variable argument list replaced by arg, which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls). The vprintf function does not invoke the va_end macro. [24]

Returns

The vprintf function returns the number of characters transmitted, or a negative value if an output or encoding error occurred.

36.6.11. The vscanf function

Synopsis

#include <stdarg.h>
#include <stdio.h>
int vscanf(const char * restrict format, va_list arg);

Description

The vscanf function is equivalent to scanf, with the variable argument list replaced by arg, which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls). The vscanf function does not invoke the va_end macro. [24]

Returns

The vscanf function returns the value of the macro EOF if an input failure occurs before any conversion. Otherwise, the vscanf function returns the number of input items assigned, which can be fewer than provided for, or even zero, in the event of an early matching failure.

36.6.12. The vsnprintf function

Synopsis

#include <stdarg.h>
#include <stdio.h>
int vsnprintf(char * restrict s, size_t n, const char * restrict format, va_list arg);

Description

The vsnprintf function is equivalent to snprintf, with the variable argument list replaced by arg, which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls). The vsnprintf function does not invoke the va_end macro. [24] If copying takes place between objects that overlap, the behavior is undefined.

Returns

The vsnprintf function returns the number of characters that would have been written had n been sufficiently large, not counting the terminating null character, or a negative value if an encoding error occurred. Thus, the null-terminated output has been completely written if and only if the returned value is nonnegative and less than n.

36.6.13. The vsprintf function

Synopsis

#include <stdarg.h>
#include <stdio.h>
int vsprintf(char * restrict s, const char * restrict format, va_list arg);

Description

The vsprintf function is equivalent to sprintf, with the variable argument list replaced by arg, which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls). The vsprintf function does not invoke the va_end macro. [24] If copying takes place between objects that overlap, the behavior is undefined.

Returns

The vsprintf function returns the number of characters written in the array, not counting the terminating null character, or a negative value if an encoding error occurred.

36.6.14. The vsscanf function

Synopsis

#include <stdarg.h>
#include <stdio.h>
int vsscanf(const char * restrict s, const char * restrict format, va_list arg);

Description

The vsscanf function is equivalent to sscanf, with the variable argument list replaced by arg, which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls). The vsscanf function does not invoke the va_end macro. [24]

Returns

The vsscanf function returns the value of the macro EOF if an input failure occurs before any conversion. Otherwise, the vsscanf function returns the number of input items assigned, which can be fewer than provided for, or even zero, in the event of an early matching failure.

[22]As the functions vfprintf, vfscanf, vprintf, vscanf, vsnprintf, vsprintf and vsscanf invoke the va_arg macro, the value of arg after the return is indeterminate.

36.7. Character input/output functions

36.7.1. The fgetc function

Synopsis

#include <stdio.h>
int fgetc(FILE *stream);

Description

If the end-of-file indicator for the input stream pointed to by stream is not set and a next character is present, the fgetc function obtains that character as an unsigned char converted to an int and advances the associated file position indicator for the stream (if defined).

Returns

If the end-of-file indicator for the stream is set, or if the stream is at end-of-file, the end-of-file indicator for the stream is set and the fgetc function returns EOF. Otherwise, the fgetc function returns the next character from the input stream pointed to by stream. If a read error occurs, the error indicator for the stream is set and the fgetc function returns EOF. [23]

36.7.2. The fgets function

Synopsis

#include <stdio.h>
char *fgets(char * restrict s, int n, FILE * restrict stream);

Description

The fgets function reads at most one less than the number of characters specified by n from the stream pointed to by stream into the array pointed to by s. No additional characters are read after a new-line character (which is retained) or after end-of-file. A null character is written immediately after the last character read into the array.

Returns

The fgets function returns s if successful. If end-of-file is encountered and no characters have been read into the array, the contents of the array remain unchanged and a null pointer is returned. If a read error occurs during the operation, the array contents are indeterminate and a null pointer is returned.

[23]An end-of-file and a read error can be distinguished by use of the feof and ferror functions.

36.7.3. The fputc function

Synopsis

#include <stdio.h>
int fputc(int c, FILE *stream);

Description

The fputc function writes the character specified by c (converted to an unsigned char ) to the output stream pointed to by stream, at the position indicated by the associated file position indicator for the stream (if defined), and advances the indicator appropriately. If the file cannot support positioning requests, or if the stream was opened with append mode, the character is appended to the output stream.

Returns

The fputc function returns the character written. If a write error occurs, the error indicator for the stream is set and fputc returns EOF.

36.7.4. The fputs function

Synopsis

#include <stdio.h>
int fputs(const char * restrict s, FILE * restrict stream);

Description

The fputs function writes the string pointed to by s to the stream pointed to by stream. The terminating null character is not written.

Returns

The fputs function returns EOF if a write error occurs; otherwise it returns a nonnegative value.

36.7.5. The getc function

Synopsis

#include <stdio.h>
int getc(FILE *stream);

Description

The getc function is equivalent to fgetc, except that if it is implemented as a macro, it may evaluate stream more than once, so the argument should never be an expression with side effects.

Returns

The getc function returns the next character from the input stream pointed to by stream. If the stream is at end-of-file, the end-of-file indicator for the stream is set and getc returns EOF. If a read error occurs, the error indicator for the stream is set and getc returns EOF.

36.7.6. The getchar function

Synopsis

#include <stdio.h>
int getchar(void);

Description

The getchar function is equivalent to getc with the argument stdin.

Returns

The getchar function returns the next character from the input stream pointed to by stdin. If the stream is at end-of-file, the end-of-file indicator for the stream is set and getchar returns EOF. If a read error occurs, the error indicator for the stream is set and getchar returns EOF.

36.7.7. The gets function

Synopsis

#include <stdio.h>
char *gets(char *s);

Description

The gets function reads characters from the input stream pointed to by stdin, into the array pointed to by s, until end-of-file is encountered or a new-line character is read. Any new-line character is discarded, and a null character is written immediately after the last character read into the array.

Returns

The gets function returns s if successful. If end-of-file is encountered and no characters have been read into the array, the contents of the array remain unchanged and a null pointer is returned. If a read error occurs during the operation, the array contents are indeterminate and a null pointer is returned.

36.7.8. The putc function

Synopsis

#include <stdio.h>
int putc(int c, FILE *stream);

Description

The putc function is equivalent to fputc, except that if it is implemented as a macro, it may evaluate stream more than once, so that argument should never be an expression with side effects.

Returns

The putc function returns the character written. If a write error occurs, the error indicator for the stream is set and putc returns EOF.

36.7.9. The putchar function

Synopsis

#include <stdio.h>
int putchar(int c);

Description

The putchar function is equivalent to putc with the second argument stdout.

Returns

The putchar function returns the character written. If a write error occurs, the error indicator for the stream is set and putchar returns EOF.

36.7.10. The puts function

Synopsis

#include <stdio.h>
int puts(const char *s);

Description

The puts function writes the string pointed to by s to the stream pointed to by stdout, and appends a new-line character to the output. The terminating null character is not written.

Returns

The puts function returns EOF if a write error occurs; otherwise it returns a nonnegative value.

36.7.11. The ungetc function

Synopsis

#include <stdio.h>
int ungetc(int c, FILE *stream);

Description

The ungetc function pushes the character specified by c (converted to an unsigned char ) back onto the input stream pointed to by stream. Pushed-back characters will be returned by subsequent reads on that stream in the reverse order of their pushing. A successful intervening call (with the stream pointed to by stream) to a file positioning function (fseek, fsetpos or rewind) discards any pushed-back characters for the stream. The external storage corresponding to the stream is unchanged.

One character of pushback is guaranteed. If the ungetc function is called too many times on the same stream without an intervening read or file positioning operation on that stream, the operation may fail.

If the value of c equals that of the macro EOF, the operation fails and the input stream is unchanged.

A successful call to the ungetc function clears the end-of-file indicator for the stream. The value of the file position indicator for the stream after reading or discarding all pushed-back characters shall be the same as it was before the characters were pushed back. For a text stream, the value of its file position indicator after a successful call to the ungetc function is unspecified until all pushed-back characters are read or discarded. For a binary stream, its file position indicator is decremented by each successful call to the ungetc function; if its value was zero before a call, it is indeterminate after the call.

Returns

The ungetc function returns the character pushed back after conversion, or EOF if the operation fails.

Forward references: file positioning functions (File positioning functions).

36.8. Direct input/output functions

36.8.1. The fread function

Synopsis

#include <stdio.h>
size_t fread(void * restrict ptr, size_t size, size_t nmemb, FILE * restrict stream);

Description

The fread function reads, into the array pointed to by ptr, up to nmemb elements whose size is specified by size, from the stream pointed to by stream. For each object, size calls are made to the fgetc function and the results stored, in the order read, in an array of unsigned char exactly overlaying the object. The file position indicator for the stream (if defined) is advanced by the number of characters successfully read. If an error occurs, the resulting value of the file position indicator for the stream is indeterminate. If a partial element is read, its value is indeterminate.

Returns

The fread function returns the number of elements successfully read, which may be less than nmemb if a read error or end-of-file is encountered. If size or nmemb is zero, fread returns zero and the contents of the array and the state of the stream remain unchanged.

36.8.2. The fwrite function

Synopsis

#include <stdio.h>
size_t fwrite(const void * restrict ptr, size_t size, size_t nmemb, FILE * restrict stream);

Description

The fwrite function writes, from the array pointed to by ptr, up to nmemb elements whose size is specified by size, to the stream pointed to by stream. For each object, size calls are made to the fputc function, taking the values (in order) from an array of unsigned char exactly overlaying the object. The file position indicator for the stream (if defined) is advanced by the number of characters successfully written. If an error occurs, the resulting value of the file position indicator for the stream is indeterminate.

Returns

The fwrite function returns the number of elements successfully written, which will be less than nmemb only if a write error is encountered. If size or nmemb is zero, fwrite returns zero and the state of the stream remains unchanged.

36.9. File positioning functions

36.9.1. The fgetpos function

Synopsis

#include <stdio.h>
int fgetpos(FILE * restrict stream, fpos_t * restrict pos);

Description

The fgetpos function stores the current values of the parse state (if any) and file position indicator for the stream pointed to by stream in the object pointed to by pos. The values stored contain unspecified information usable by the fsetpos function for repositioning the stream to its position at the time of the call to the fgetpos function.

Returns

If successful, the fgetpos function returns zero; on failure, the fgetpos function returns nonzero and stores an implementation-defined positive value in errno.

Forward references: the fsetpos function (The fsetpos function).

36.9.2. The fseek function

Synopsis

#include <stdio.h>
int fseek(FILE *stream, long int offset, int whence);

Description

The fseek function sets the file position indicator for the stream pointed to by stream. If a read or write error occurs, the error indicator for the stream is set and fseek fails.

For a binary stream, the new position, measured in characters from the beginning of the file, is obtained by adding offset to the position specified by whence. The specified position is the beginning of the file if whence is SEEK_SET, the current value of the file position indicator if SEEK_CUR, or end-of-file if SEEK_END. A binary stream need not meaningfully support fseek calls with a whence value of SEEK_END.

For a text stream, either offset shall be zero, or offset shall be a value returned by an earlier successful call to the ftell function on a stream associated with the same file and whence shall be SEEK_SET.

After determining the new position, a successful call to the fseek function undoes any effects of the ungetc function on the stream, clears the end-of-file indicator for the stream, and then establishes the new position. After a successful fseek call, the next operation on an update stream may be either input or output.

Returns

The fseek function returns nonzero only for a request that cannot be satisfied.

Forward references: the ftell function (The ftell function).

36.9.3. The fsetpos function

Synopsis

#include <stdio.h>
int fsetpos(FILE *stream, const fpos_t *pos);

Description

The fsetpos function sets the mbstate_t object (if any) and file position indicator for the stream pointed to by stream according to the value of the object pointed to by pos, which shall be a value obtained from an earlier successful call to the fgetpos function on a stream associated with the same file. If a read or write error occurs, the error indicator for the stream is set and fsetpos fails.

A successful call to the fsetpos function undoes any effects of the ungetc function on the stream, clears the end-of-file indicator for the stream, and then establishes the new parse state and position. After a successful fsetpos call, the next operation on an update stream may be either input or output.

Returns

If successful, the fsetpos function returns zero; on failure, the fsetpos function returns nonzero and stores an implementation-defined positive value in errno.

36.9.4. The ftell function

Synopsis

#include <stdio.h>
long int ftell(FILE *stream);

Description

The ftell function obtains the current value of the file position indicator for the stream pointed to by stream. For a binary stream, the value is the number of characters from the beginning of the file. For a text stream, its file position indicator contains unspecified information, usable by the fseek function for returning the file position indicator for the stream to its position at the time of the ftell call; the difference between two such return values is not necessarily a meaningful measure of the number of characters written or read.

Returns

If successful, the ftell function returns the current value of the file position indicator for the stream. On failure, the ftell function returns -1L and stores an implementation-defined positive value in errno.

36.9.5. The rewind function

Synopsis

#include <stdio.h>
void rewind(FILE *stream);

Description

The rewind function sets the file position indicator for the stream pointed to by stream to the beginning of the file. It is equivalent to

(void)fseek(stream, 0L, SEEK_SET)

except that the error indicator for the stream is also cleared.

Returns

The rewind function returns no value.

36.10. Error-handling functions

36.10.1. The clearerr function

Synopsis

#include <stdio.h>
void clearerr(FILE *stream);

Description

The clearerr function clears the end-of-file and error indicators for the stream pointed to by stream.

Returns

The clearerr function returns no value.

36.10.2. The feof function

Synopsis

#include <stdio.h>
int feof(FILE *stream);

Description

The feof function tests the end-of-file indicator for the stream pointed to by stream.

Returns

The feof function returns nonzero if and only if the end-of-file indicator is set for stream.

36.10.3. The ferror function

Synopsis

#include <stdio.h>
int ferror(FILE *stream);

Description

The ferror function tests the error indicator for the stream pointed to by stream.

Returns

The ferror function returns nonzero if and only if the error indicator is set for stream.

36.10.4. The perror function

Synopsis

#include <stdio.h>
void perror(const char *s);

Description

The perror function maps the error number in the integer expression errno to an error message. It writes a sequence of characters to the standard error stream thus: first (if s is not a null pointer and the character pointed to by s is not the null character), the string pointed to by s followed by a colon (:) and a space; then an appropriate error message string followed by a new-line character. The contents of the error message strings are the same as those returned by the strerror function with argument errno.

Returns

The perror function returns no value.

Forward references: the strerror function (The strerror function).