void (*signal(int sig, void (*func)(int)))(int);
IEEE Std 1003.1-2017
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 shall occur. If the value of func is
SIG_IGN, the signal shall be ignored. Otherwise, the application shall ensure that func points 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 a:
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. (If the value of sig is
SIGILL, the implementation may
alternatively define that no action is taken.) Next the equivalent of:
is executed. If and when the function returns, if the value of sig was
SIGSEGV or any other
implementation-defined value corresponding to a computational exception, the behavior is undefined. Otherwise, the program shall
resume execution at the point it was interrupted. The ISO C standard places a restriction on applications relating to the use
raise() from signal handlers. This
restriction does not apply to POSIX applications, as POSIX.1-2017 requires
be async-signal-safe (see Signal Actions).
the process is multi-threaded, or if the process is
single-threaded and a signal handler is executed other than as the result of:
The process calling
orsigqueue()` to generate a signal that is not blocked
A pending signal being unblocked and being delivered before the call that unblocked it returns
the behavior is undefined if the signal handler refers to any object
errno with static storage duration other than by assigning a value to an object declared as
sig_atomic_t, or if the signal handler calls any function defined in this standard other than one of the
functions listed in Signal Concepts.
At program start-up, the equivalent of:
is executed for some signals, and the equivalent of:
is executed for all other signals (see
signal() function shall not change the setting of
errno if successful.
If the request can be honored,
signal() shall return the value of func for the most recent call to
for the specified signal sig. Otherwise,
SIG_ERR shall be returned and a positive value shall be stored in
signal() function shall fail if:
EINVAL- The sig argument is not a valid signal number or an attempt is made to catch a signal that cannot be caught or ignore a signal that cannot be ignored.
signal() function may fail if:
EINVAL- An attempt was made to set the action to
SIG_DFLfor a signal that cannot be caught or ignored (or both).