SIGNAL(3P)

💖 SIGNAL(3P) 💖

okay bestie, welcome to the POSIX Programmer's Manual page for signal — aka signal management ✨ (yes that's the NAME section, I'm folding it in, we're being cute AND efficient 💅)

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🌸 PROLOG 🌸

so like, real talk before we start: this manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ — so if you're on Linux, go consult the corresponding Linux manual page for the deets on Linux behavior. orrr the interface might not even be implemented on Linux at all 🙈. no cap, check first.

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✨ SYNOPSIS ✨

#include <signal.h>

void (*signal(int sig, void (*func)(int)))(int);

yes the return type is that whole thing. it's giving "pointer to function returning void taking int returning pointer to..." — iconic, chaotic, we love her 👑

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🦋 DESCRIPTION 🦋

The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1‐2017 defers to the ISO C standard.

The signal() function chooses one of three ways in which receipt of the signal number sig is to be subsequently handled:

• if func is SIG_DFL, default handling for that signal shall occur
• if 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". that's the term, bestie, write it down ✍️

When a signal occurs, and func points to a function, it is implementation-defined whether the equivalent of a:

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. (If the value of sig is SIGILL, the implementation may alternatively define that no action is taken.) Next the equivalent of:

(*func)(sig);

is executed. If and when the function returns, if the value of sig was SIGFPE, SIGILL, or 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 of raise() from signal handlers. This restriction does not apply to POSIX applications, as POSIX.1‐2008 requires raise() to be async-signal-safe (see Section 2.4, Signal Actions). phew 🫶

If 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 abort(), raise(), kill(), pthread_kill(), or sigqueue() to generate a signal that is not blocked
• A pending signal being unblocked and being delivered before the call that unblocked it returns

then the behavior is undefined 💀 if the signal handler refers to any object other than errno with static storage duration other than by assigning a value to an object declared as volatile sig_atomic_t, or if the signal handler calls any function defined in this standard other than one of the functions listed in Section 2.4, Signal Concepts.

🚨 bestie this is the scary part. in a signal handler you get errno and volatile sig_atomic_t assignments and the functions in Section 2.4 and that's it. anything else is undefined behavior, fr fr.

At program start-up, the equivalent of:

signal(sig, SIG_IGN);

is executed for some signals, and the equivalent of:

signal(sig, SIG_DFL);

is executed for all other signals (see exec).

The signal() function shall not change the setting of errno if successful. so polite of her 🎀

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💫 RETURN VALUE 💫

If the request can be honored, signal() shall return the value of func for the most recent call to signal() for the specified signal sig. Otherwise, SIG_ERR shall be returned and a positive value shall be stored in errno.

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🌈 ERRORS 🌈

The 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. rude but ok

The signal() function may fail if:

• EINVAL — An attempt was made to set the action to SIG_DFL for a signal that cannot be caught or ignored (or both).

The following sections are informative. 👀

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🍓 EXAMPLES 🍓

None. that's it. nothing. empty. the void. 💀

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🧚 APPLICATION USAGE 🧚

The sigaction() function provides a more comprehensive and reliable mechanism for controlling signals; new applications should use sigaction() rather than signal(). like genuinely, bestie, sigaction() is the main character here and signal() is... her quirky older cousin 💅

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💜 RATIONALE 💜

None.

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⭐ FUTURE DIRECTIONS ⭐

None.

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👑 SEE ALSO 👑

• Section 2.4, Signal Concepts
• exec
• pause()
• raise()
• sigaction()
• sigsuspend()
• waitid()

The Base Definitions volume of POSIX.1‐2017, <signal.h>

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📜 COPYRIGHT 📜

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .

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and that's the whole page bestie!! 🫶 this one was brought to you by the fine people at IEEE / The Open Group, 2017 ✨ mwah, stay safe out there, and remember: don't call printf in a signal handler, I'm begging you 😭💖