16. The C Standard Library

This chapter and rest of book eflects chapter 7 of n1124.pdf. This chapter begins with assert.h. We will see three parts of each function/macro as much as possible. From specification, compiler and example point of view in that order.

16.1. Introduction

16.1.1. Definitions of terms

A string is a contiguous sequence of characters terminated by and including the first null character. The term multibyte string is sometimes used instead to emphasize special processing given to multibyte characters contained in the string or to avoid confusion with a wide string. A pointer to a string is a pointer to its initial (lowest addressed) character. The length of a string is the number of bytes preceding the null character and the value of a string is the sequence of the values of the contained characters, in order.

The decimal-point character is the character used by functions that convert floating-point numbers to or from character sequences to denote the beginning of the fractional part of such character sequences. [1] It is represented in the text and examples by a period, but may be changed by the setlocale function.

A null wide character is a wide character with code value zero.

A wide string is a contiguous sequence of wide characters terminated by and including the first null wide character. A pointer to a wide string is a pointer to its initial (lowest addressed) wide character. The length of a wide string is the number of wide characters preceding the null wide character and the value of a wide string is the sequence of code values of the contained wide characters, in order.

A shift sequence is a contiguous sequence of bytes within a multibyte string that (potentially) causes a change in shift state. A shift sequence shall not have a corresponding wide character; it is instead taken to be an adjunct to an adjacent multibyte character. [2]

Forward references: character handling (14.4), the setlocale function (14.11.1.1).

[1]The functions that make use of the decimal-point character are the numeric conversion functions (14.20.1, 14.24.4.1) and the formatted input/output functions (14.19.6, 14.24.2).
[2]For state-dependent encodings, the values for MB_CUR_MAX and MB_LEN_MAX shall thus be large enough to count all the bytes in any complete multibyte character plus at least one adjacent shift sequence of maximum length. Whether these counts provide for more than one shift sequence is the implementation’s choice.

16.1.2. Standard Headers

Each library function is declared, with a type that includes a prototype, in a header, [3] whose contents are made available by the #include preprocessing directive. The header declares a set of related functions, plus any necessary types and additional macros needed to facilitate their use. Declarations of types described in this clause shall not include type qualifiers, unless explicitly stated otherwise.

The standard headers are:

<assert.h>  <inttypes.h> <signal.h>  <stdlib.h>
<complex.h> <iso646.h>   <stdarg.h>  <string.h>
<ctype.h>   <limits.h>   <stdbool.h> <tgmath.h>
<errno.h>   <locale.h>   <stddef.h>  <time.h>
<fenv.h>    <math.h>     <stdint.h>  <wchar.h>
<float.h>   <setjmp.h>   <stdio.h>   <wctype.h>

If a file with the same name as one of the above < and > delimited sequences, not provided as part of the implementation, is placed in any of the standard places that are searched for included source files, the behavior is undefined.

Standard headers may be included in any order; each may be included more thanonce in a giv en scope, with no effect different from being included only once, except that the effect of including <assert.h> depends on the definition of NDEBUG (see 14.2). If used, a header shall be included outside of any external declaration or definition, and it shall first be included before the first reference to any of the functions or objects it declares, or to any of the types or macros it defines. However, if an identifier is declared or defined in more than one header, the second and subsequent associated headers may be included after the initial reference to the identifier. The program shall not have any macros with names lexically identical to keywords currently defined prior to the inclusion.

Any definition of an object-like macro described in this clause shall expand to code that is fully protected by parentheses where necessary, so that it groups in an arbitrary expression as if it were a single identifier.

Any declaration of a library function shall have external linkage.

A summary of the contents of the standard headers is given in annex B.

Forward references: diagnostics (14.2).

[3]A header is not necessarily a source file, nor are the < and > delimited sequences in header names necessarily valid source file names.

16.1.3. Reserved Identifiers

Each header declares or defines all identifiers listed in its associated subclause, and optionally declares or defines identifiers listed in its associated future library directions subclause and identifiers which are always reserved either for any use or for use as file scope identifiers.

  • All identifiers that begin with an underscore and either an uppercase letter or another underscore are always reserved for any use.
  • All identifiers that begin with an underscore are always reserved for use as identifiers with file scope in both the ordinary and tag name spaces.
  • Each macro name in any of the following subclauses (including the future library directions) is reserved for use as specified if any of its associated headers is included; unless explicitly stated otherwise (see 14.1.4).
  • All identifiers with external linkage in any of the following subclauses (including the future library directions) are always reserved for use as identifiers with external linkage. [4]
  • Each identifier with file scope listed in any of the following subclauses (including the future library directions) is reserved for use as a macro name and as an identifier with file scope in the same name space if any of its associated headers is included.

No other identifiers are reserved. If the program declares or defines an identifier in a context in which it is reserved (other than as allowed by 14.1.4), or defines a reserved identifier as a macro name, the behavior is undefined.

If the program removes (with #undef) any macro definition of an identifier in the first group listed above, the behavior is undefined.

[4]The list of reserved identifiers with external linkage includes `errno, ath_errhandling, setjmp, and va_end.

16.1.4. Use of Library Functions

Each of the following statements applies unless explicitly stated otherwise in the detailed descriptions that follow: If an argument to a function has an invalid value (such as a value outside the domain of the function, or a pointer outside the address space of the program, or a null pointer, or a pointer to non-modifiable storage when the corresponding parameter is not const-qualified) or a type (after promotion) not expected by a function with variable number of arguments, the behavior is undefined. If a function argument is described as being an array, the pointer actually passed to the function shall have a value such that all address computations and accesses to objects (that would be valid if the pointer did point to the first element of such an array) are in fact valid. Any function declared in a header may be additionally implemented as a function-like macro defined in the header, so if a library function is declared explicitly when its header is included, one of the techniques shown below can be used to ensure the declaration is not affected by such a macro. Any macro definition of a function can be suppressed locally by enclosing the name of the function in parentheses, because the name is then not followed by the left parenthesis that indicates expansion of a macro function name. For the same syntactic reason, it is permitted to take the address of a library function even if it is also defined as a macro. [5] The use of #undef to remove any macro definition will also ensure that an actual function is referred to. Any invocation of a library function that is implemented as a macro shall expand to code that evaluates each of its arguments exactly once, fully protected by parentheses where necessary, so it is generally safe to use arbitrary expressions as arguments. Likewise, those function-like macros described in the following subclauses may be invoked in an expression anywhere a function with a compatible return type could be called. [6] All object-like macros listed as expanding to integer constant expressions shall additionally be suitable for use in #if preprocessing directives.

Provided that a library function can be declared without reference to any type defined in a header, it is also permissible to declare the function and use it without including its associated header.

There is a sequence point immediately before a library function returns.

The functions in the standard library are not guaranteed to be reentrant and may modify objects with static storage duration. [7]

[5]This means that an implementation shall provide an actual function for each library function, even if it also provides a macro for that function.
[6]Such macros might not contain the sequence points that the corresponding function calls do.
[7]Thus, a signal handler cannot, in general, call standard library functions.