# 29. Signal handling <signal.h>¶

The header <signal.h> declares a type and two functions and defines several macros, for handling various signals (conditions that may be reported during program execution).

The type defined is

sig_atomic_t

which is the (possibly volatile-qualified) integer type of an object that can be accessed as an atomic entity, even in the presence of asynchronous interrupts.

The macros defined are

SIG_DFL
SIG_ERR
SIG_IGN

which expand to constant expressions with distinct values that have type compatible with the second argument to, and the return value of, the signal function, and whose values compare unequal to the address of any declarable function; and the following, which

expand to positive integer constant expressions with type int and distinct values that are the signal numbers, each corresponding to the specified condition:

SIGABRT abnormal termination, such as is initiated by the abort function
SIGFPE an erroneous arithmetic operation, such as zero divide or an operation resulting in overflow
SIGILL detection of an invalid function image, such as an invalid instruction
SIGINT receipt of an interactive attention signal
SIGSEGV an invalid access to storage
SIGTERM a termination request sent to the program

An implementation need not generate any of these signals, except as a result of explicit calls to the raise function. Additional signals and pointers to undeclarable functions, with macro definitions beginning, respectively, with the letters SIG and an uppercase letter or with SIG_ and an uppercase letter, 1 may also be specified by the implementation. The complete set of signals, their semantics, and their default handling is implementation-defined; all signal numbers shall be positive.

1

The names of the signal numbers reflect the following terms (respectively): abort, floating-point exception, illegal instruction, interrupt, segmentation violation, and termination.

## 29.1. Specify signal handling¶

### 29.1.1. The signal function¶

Synopsis

#include <signal.h>
void (*signal(int sig, void (*func)(int)))(int);


Description

The signal function chooses one of three ways in which receipt of the signal number sig is to be subsequently handled. If the value of func is SIG_DFL, default handling for that signal will occur. If the value of func is SIG_IGN, the signal will be ignored. Otherwise, func shall point to a function to be called when that signal occurs. An invocation of such a function because of a signal, or (recursively) of any further functions called by that invocation (other than functions in the standard library), is called a signal handler.

When a signal occurs and func points to a function, it is implementation-defined whether the equivalent of signal(sig, SIG_DFL); is executed or the implementation prevents some implementation-defined set of signals (at least including sig) from occurring until the current signal handling has completed; in the case of SIGILL, the implementation may alternatively define that no action is taken. Then the equivalent of (*func)(sig); is executed. If and when the function returns, if the value of sig is SIGFPE, SIGILL, SIGSEGV or any other implementation-defined value corresponding to a computational exception, the behavior is undefined; otherwise the program will resume execution at the point it was interrupted.

If the signal occurs as the result of calling the abort or raise function, the signal handler shall not call the raise function.

5 If the signal occurs other than as the result of calling the abort or raise function, the behavior is undefined if the signal handler refers to any object with static storage duration other than by assigning a value to an object declared as volatile sig_atomic_t or the signal handler calls any function in the standard library other than the abort function, the _Exit function or the signal function with the first argument equal to the signal number corresponding to the signal that caused the invocation of the handler. Furthermore, if such a call to the signal function results in a SIG_ERR return, the value of errno is indeterminate. 2

At program startup, the equivalent of

signal(sig, SIG_IGN);


may be executed for some signals selected in an implementation-defined manner; the equivalent of

signal(sig, SIG_DFL);


is executed for all other signals defined by the implementation.

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

Returns

If the request can be honored, the signal function returns the value of func for the most recent successful call to signal for the specified signal sig. Otherwise, a value of SIG_ERR is returned and a positive value is stored in errno.

Forward references: the abort function (The abort function), the exit function (The exit function), the _Exit function (The _Exit function).

2

If any signal is generated by an asynchronous signal handler, the behavior is undefined.

## 29.2. Send signal¶

### 29.2.1. The raise function¶

Synopsis

#include <signal.h>
int raise(int sig);


Description

The raise function carries out the actions described in The signal function for the signal sig. If a signal handler is called, the raise function shall not return until after the signal handler does.

Returns

The raise function returns zero if successful, nonzero if unsuccessful.