signal.h(7POSIX)

📦 version 2017a, in debian bookworm, debian trixie, ubuntu jammy, ubuntu noble, ubuntu resolute, ubuntu stonking

✨ PROLOG ✨

Ok bestie, real talk before we even get into it: this manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux. 💅

🎀 NAME 🎀

So like, <signal.h> — signals. That's the whole name. Short, iconic, does exactly what it says on the tin. 👑

🌸 SYNOPSIS 🌸

#include <signal.h>

That's it. That's the whole vibe. Slap that include in and you're in business. 🦋

💖 DESCRIPTION 💖

Okay so here's the thing, some of the functionality described on this reference page extends the ISO C standard. So if you want those extra symbols to even be visible in this header, your application shall define the appropriate feature test macro (see the System Interfaces volume of POSIX.1-2017, Section 2.2, The Compilation Environment). No macro, no symbols, no cap. 🚫

The Signal-Handling Macros 🫶

The <signal.h> header shall define the following macros, which shall expand to constant expressions with distinct values that have a type compatible with the second argument to, and the return value of, the signal() function, and whose values shall compare unequal to the address of any declarable function. (Yes that's a lot of requirements, they're just making sure you can't confuse these with a real function pointer, ok? 😭)

  • SIG_DFL — Request for default signal handling.
  • SIG_ERR — Return value from signal() in case of error.
  • SIG_HOLD — Request that signal be held.
  • SIG_IGN — Request that signal be ignored.

Types It Pulls In From Elsewhere 🧚

The <signal.h> header shall define the pthread_t, size_t, and uid_t types as described in <sys/types.h>.

The <signal.h> header shall define the timespec structure as described in <time.h>.

The <signal.h> header shall define the pthread_attr_t type as described in <sys/types.h>.

Types It Defines Itself ✨

The <signal.h> header shall define the following data types:

  • sig_atomic_t — Possibly volatile-qualified integer type of an object that can be accessed as an atomic entity, even in the presence of asynchronous interrupts. (yes, even then. it's built different.)
  • sigset_t — Integer or structure type of an object used to represent sets of signals.
  • pid_t — As described in <sys/types.h>.

The sigevent Structure 🌈

The <signal.h> header shall define the sigevent structure, which shall include at least the following members:

int              sigev_notify            Notification type.
int              sigev_signo             Signal number.
union sigval     sigev_value             Signal value.
void           (*sigev_notify_function)(union sigval)
                                         Notification function.
pthread_attr_t *sigev_notify_attributes  Notification attributes.

And the <signal.h> header shall define the following symbolic constants for the values of sigev_notify:

  • SIGEV_NONE — No asynchronous notification is delivered when the event of interest occurs.
  • SIGEV_SIGNAL — A queued signal, with an application-defined value, is generated when the event of interest occurs.
  • SIGEV_THREAD — A notification function is called to perform notification.

The sigval Union 💜

The sigval union shall be defined as:

int    sival_int    Integer signal value.
void  *sival_ptr    Pointer signal value.

Realtime Signals, aka the Good Stuff 🔥

The <signal.h> header shall declare the SIGRTMIN and SIGRTMAX macros, which shall expand to positive integer expressions with type int, but which need not be constant expressions. (Yeah, read that again. They're macros but not necessarily constants. It's giving chaos but it's legal. 🙈)

These macros specify a range of signal numbers that are reserved for application use and for which the realtime signal behavior specified in this volume of POSIX.1-2017 is supported. The signal numbers in this range do not overlap any of the signals specified in the table below.

The range SIGRTMIN through SIGRTMAX inclusive shall include at least {RTSIG_MAX} signal numbers.

It is implementation-defined whether realtime signal behavior is supported for other signals.

The Signal Macros 🎀

The <signal.h> header shall define the following macros that are used to refer to the signals that occur in the system. Signals defined here begin with the letters SIG followed by an uppercase letter. The macros shall expand to positive integer constant expressions with type int and distinct values. The value 0 is reserved for use as the null signal (see kill()). Additional implementation-defined signals may occur in the system.

Now, important lore drop: the ISO C standard only requires the signal names SIGABRT, SIGFPE, SIGILL, SIGINT, SIGSEGV, and SIGTERM to be defined. An implementation need not generate any of these six signals, except as a result of explicit use of interfaces that generate signals, such as raise(), kill(), the General Terminal Interface (see Section 11.1.9, Special Characters), and the kill utility, unless otherwise stated (see, for example, the System Interfaces volume of POSIX.1-2017, Section 2.8.3.3, Memory Protection). 🫠

The following signals shall be supported on all implementations (default actions are explained below the table).

The Signal Table 👑

  • SIGABRT — Default Action: A — Process abort signal.
  • SIGALRM — Default Action: T — Alarm clock.
  • SIGBUS — Default Action: A — Access to an undefined portion of a memory object.
  • SIGCHLD — Default Action: I — Child process terminated, stopped, or continued.
  • SIGCONT — Default Action: C — Continue executing, if stopped.
  • SIGFPE — Default Action: A — Erroneous arithmetic operation.
  • SIGHUP — Default Action: T — Hangup.
  • SIGILL — Default Action: A — Illegal instruction.
  • SIGINT — Default Action: T — Terminal interrupt signal.
  • SIGKILL — Default Action: T — Kill (cannot be caught or ignored).
  • SIGPIPE — Default Action: T — Write on a pipe with no one to read it.
  • SIGQUIT — Default Action: A — Terminal quit signal.
  • SIGSEGV — Default Action: A — Invalid memory reference.
  • SIGSTOP — Default Action: S — Stop executing (cannot be caught or ignored).
  • SIGTERM — Default Action: T — Termination signal.
  • SIGTSTP — Default Action: S — Terminal stop signal.
  • SIGTTIN — Default Action: S — Background process attempting read.
  • SIGTTOU — Default Action: S — Background process attempting write.
  • SIGUSR1 — Default Action: T — User-defined signal 1.
  • SIGUSR2 — Default Action: T — User-defined signal 2.
  • SIGPOLL — Default Action: T — Pollable event.
  • SIGPROF — Default Action: T — Profiling timer expired.
  • SIGSYS — Default Action: A — Bad system call.
  • SIGTRAP — Default Action: A — Trace/breakpoint trap.
  • SIGURG — Default Action: I — High bandwidth data is available at a socket.
  • SIGVTALRM — Default Action: T — Virtual timer expired.
  • SIGXCPU — Default Action: A — CPU time limit exceeded.
  • SIGXFSZ — Default Action: A — File size limit exceeded.

The Default Actions, Decoded 💀

The default actions are as follows:

  • T — Abnormal termination of the process.
  • A — Abnormal termination of the process with additional actions.
  • I — Ignore the signal.
  • S — Stop the process.
  • C — Continue the process, if it is stopped; otherwise, ignore the signal.

The effects on the process in each case are described in the System Interfaces volume of POSIX.1-2017, Section 2.4.3, Signal Actions. 📖

The sigaction Structure 🦋

The <signal.h> header shall declare the sigaction structure, which shall include at least the following members:

void   (*sa_handler)(int)  Pointer to a signal-catching function
                           or one of the SIG_IGN or SIG_DFL.
sigset_t sa_mask           Set of signals to be blocked during execution
                           of the signal handling function.
int      sa_flags          Special flags.
void   (*sa_sigaction)(int, siginfo_t *, void *)
                           Pointer to a signal-catching function.
The storage occupied by sa_handler and sa_sigaction may overlap, and a conforming application shall not use both simultaneously. Pick a lane bestie, it's one or the other. 😭

Mask-Modifying Macros 🎀

The <signal.h> header shall define the following macros which shall expand to integer constant expressions that need not be usable in #if preprocessing directives:

  • SIG_BLOCK — The resulting set is the union of the current set and the signal set pointed to by the argument set.
  • SIG_UNBLOCK — The resulting set is the intersection of the current set and the complement of the signal set pointed to by the argument set.
  • SIG_SETMASK — The resulting set is the signal set pointed to by the argument set.

Symbolic Constants, The Whole Squad ✨

The <signal.h> header shall also define the following symbolic constants:

  • SA_NOCLDSTOP — Do not generate SIGCHLD when children stop or stopped children continue.
  • SA_ONSTACK — Causes signal delivery to occur on an alternate stack.
  • SA_RESETHAND — Causes signal dispositions to be set to SIG_DFL on entry to signal handlers.
  • SA_RESTART — Causes certain functions to become restartable.
  • SA_SIGINFO — Causes extra information to be passed to signal handlers at the time of receipt of a signal.
  • SA_NOCLDWAIT — Causes implementations not to create zombie processes or status information on child termination. See sigaction().
  • SA_NODEFER — Causes signal not to be automatically blocked on entry to signal handler.
  • SS_ONSTACK — Process is executing on an alternate signal stack.
  • SS_DISABLE — Alternate signal stack is disabled.
  • MINSIGSTKSZ — Minimum stack size for a signal handler.
  • SIGSTKSZ — Default size in bytes for the alternate signal stack.

One Last Type 💜

The <signal.h> header shall define the mcontext_t type through typedef.

and that's part 1, bestie — hold tight, there's more coming 🫶

okay so the <signal.h> header is also gonna define the ucontext_t type as a struct, and it shall include at least these members, no cap:

  • ucontext_t *uc_link — Pointer to the context that is resumed when this context returns.
  • sigset_t uc_sigmask — The set of signals that are blocked when this context is active.
  • stack_t uc_stack — The stack used by this context.
  • mcontext_t uc_mcontext — A machine-specific representation of the saved context.

next up, the <signal.h> header shall define the stack_t type as a struct, and it shall include at least these members:

  • void *ss_sp — Stack base or pointer.
  • size_t ss_size — Stack size.
  • int ss_flags — Flags.

and the <signal.h> header shall define the siginfo_t type as a struct, which shall include at least these members:

  • int si_signo — Signal number.
  • int si_code — Signal code.
  • int si_errno — If non-zero, an errno value associated with this signal, as described in <errno.h>.
  • pid_t si_pid — Sending process ID.
  • uid_t si_uid — Real user ID of sending process.
  • void *si_addr — Address of faulting instruction.
  • int si_status — Exit value or signal.
  • long si_band — Band event for SIGPOLL.
  • union sigval si_value — Signal value.

The <signal.h> header shall define the symbolic constants in the Code column of the following table for use as values of si_code that are signal-specific or non-signal-specific reasons why the signal was generated. periodt, every single one is here:

  • SIGILL — illegal opcode energy, we love a dramatic crash 💀
    • ILL_ILLOPC — Illegal opcode.
    • ILL_ILLOPN — Illegal operand.
    • ILL_ILLADR — Illegal addressing mode.
    • ILL_ILLTRP — Illegal trap.
    • ILL_PRVOPC — Privileged opcode.
    • ILL_PRVREG — Privileged register.
    • ILL_COPROC — Coprocessor error.
    • ILL_BADSTK — Internal stack error.
  • SIGFPE — math said nope, floating-point drama incoming 😭
    • FPE_INTDIV — Integer divide by zero.
    • FPE_INTOVF — Integer overflow.
    • FPE_FLTDIV — Floating-point divide by zero.
    • FPE_FLTOVF — Floating-point overflow.
    • FPE_FLTUND — Floating-point underflow.
    • FPE_FLTRES — Floating-point inexact result.
    • FPE_FLTINV — Invalid floating-point operation.
    • FPE_FLTSUB — Subscript out of range.
  • SIGSEGV — the classic segmentation fault, main character of bad vibes ✨
    • SEGV_MAPERR — Address not mapped to object.
    • SEGV_ACCERR — Invalid permissions for mapped object.
  • SIGBUS — bus error, lowkey chaotic 🦋
    • BUS_ADRALN — Invalid address alignment.
    • BUS_ADRERR — Nonexistent physical address.
    • BUS_OBJERR — Object-specific hardware error.
  • SIGTRAP — trap queen, literally 👑
    • TRAP_BRKPT — Process breakpoint.
    • TRAP_TRACE — Process trace trap.
  • SIGCHLD — child process updates, the group chat is active 🍼
    • CLD_EXITED — Child has exited.
    • CLD_KILLED — Child has terminated abnormally and did not create a core file.
    • CLD_DUMPED — Child has terminated abnormally and created a core file.
    • CLD_TRAPPED — Traced child has trapped.
    • CLD_STOPPED — Child has stopped.
    • CLD_CONTINUED — Stopped child has continued.
  • SIGPOLL — poll events, very I/O coded 💅
    • POLL_IN — Data input available.
    • POLL_OUT — Output buffers available.
    • POLL_MSG — Input message available.
    • POLL_ERR — I/O error.
    • POLL_PRI — High priority input available.
    • POLL_HUP — Device disconnected.
  • Any — these are the non-signal-specific ones, they can show up with any signal, fr fr 🌈
    • SI_USER — Signal sent by kill().
    • SI_QUEUE — Signal sent by sigqueue().
    • SI_TIMER — Signal generated by expiration of a timer set by timer_settime().
    • SI_ASYNCIO — Signal generated by completion of an asynchronous I/O request.
    • SI_MESGQ — Signal generated by arrival of a message on an empty message queue.

and that's a wrap on that table, bestie! 💅 Now, real talk: implementations can be extra. They may support additional si_code values not included in the list above, may generate values included in this list under circumstances other than those described in this list, and may contain extensions or limitations that prevent some values from being generated. But here's the tea: implementations do not generate a different value from the ones described in this list for circumstances described in this list. Periodt. ✨

Also, these signal-specific deets shall be available:

  • SIGILL / SIGFPE — member: void * si_addr; value: Address of faulting instruction. 💀
  • SIGSEGV / SIGBUS — member: void * si_addr; value: Address of faulting memory reference. 🫠
  • SIGCHLD —
    • member: pid_t si_pid; value: Child process ID.
    • member: int si_status; value: If si_code is equal to CLD_EXITED, then si_status holds the exit value of the process; otherwise, it is equal to the signal that caused the process to change state. The exit value in si_status shall be equal to the full exit value (that is, the value passed to _exit(), _Exit(), or exit(), or returned from main()); it shall not be limited to the least significant eight bits of the value.
    • member: uid_t si_uid; value: Real user ID of the process that sent the signal.
  • SIGPOLL — member: long si_band; value: Band event for POLL_IN, POLL_OUT, or POLL_MSG.

For some implementations, the value of si_addr may be inaccurate. 😭

The following shall be declared as functions and may also be defined as macros. Function prototypes shall be provided. 👑

           int    kill(pid_t, int);
           int    killpg(pid_t, int);
           void   psiginfo(const siginfo_t *, const char *);
           void   psignal(int, const char *);
           int    pthread_kill(pthread_t, int);
           int    pthread_sigmask(int, const sigset_t *restrict,
                      sigset_t *restrict);
           int    raise(int);
           int    sigaction(int, const struct sigaction *restrict,
                      struct sigaction *restrict);
           int    sigaddset(sigset_t *, int);
           int    sigaltstack(const stack_t *restrict, stack_t *restrict);
           int    sigdelset(sigset_t *, int);
           int    sigemptyset(sigset_t *);
           int    sigfillset(sigset_t *);
           int    sighold(int);
           int    sigignore(int);
           int    siginterrupt(int, int);
           int    sigismember(const sigset_t *, int);
           void (*signal(int, void (*)(int)))(int);
           int    sigpause(int);
           int    sigpending(sigset_t *);
           int    sigprocmask(int, const sigset_t *restrict, sigset_t *restrict);
           int    sigqueue(pid_t, int, union sigval);
           int    sigrelse(int);
           void (*sigset(int, void (*)(int)))(int);
           int    sigsuspend(const sigset_t *);
           int    sigtimedwait(const sigset_t *restrict, siginfo_t *restrict,
                      const struct timespec *restrict);
           int    sigwait(const sigset_t *restrict, int *restrict);
           int    sigwaitinfo(const sigset_t *restrict, siginfo_t *restrict);

Inclusion of the <signal.h> header may make visible all symbols from the <time.h> header. 👀

The following sections are informative. 📝

✨ APPLICATION USAGE ✨

So if your system isn't supporting the XSI option, the si_pid and si_uid members of siginfo_t are only required to be valid when si_code is SI_USER or SI_QUEUE. But on XSI-conforming systems? They're also valid for all si_code values less than or equal to 0. However — and this is the messy part — it's unspecified whether SI_USER and SI_QUEUE have values less than or equal to zero. So XSI applications should check whether si_code has the value SI_USER or SI_QUEUE or is less than or equal to 0 to tell whether si_pid and si_uid are valid. 💅

✨ RATIONALE ✨

None. That's it. Nothing. Nada. 🙈

✨ FUTURE DIRECTIONS ✨

The SIGPOLL and SIGPROF signals may be removed in a future version. 😭

✨ SEE ALSO ✨

  • <errno.h>
  • <stropts.h>
  • <sys_types.h>
  • <time.h>

The System Interfaces volume of POSIX.1‐2017, Section 2.2, The Compilation Environment, alarm(), ioctl(), kill(), killpg(), psiginfo(), pthread_kill(), pthread_sigmask(), raise(), sigaction(), sigaddset(), sigaltstack(), sigdelset(), sigemptyset(), sigfillset(), sighold(), siginterrupt(), sigismember(), signal(), sigpending(), sigqueue(), sigsuspend(), sigtimedwait(), sigwait(), timer_create(), wait(), waitid()

The Shell and Utilities volume of POSIX.1‐2017, kill

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 .

This gem is from IEEE/The Open Group, 2017, and it's the signal.h(7POSIX) page. Stay slaying, bestie! 💖✨

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