--- name: "sigaction" section: "2" platform: "freebsd" package: "base" version: "16.0-CURRENT-20261027-61c70dc16a79" release: "FreeBSD 16.0-CURRENT (2026-10-27)" platforms: ["linux", "freebsd", "macos"] url: "https://new.carta.tech/sigaction" --- # sigaction(2) software signal facilities ## NAME **sigaction** – software signal facilities ## LIBRARY Standard C Library (libc, -lc) ## SYNOPSIS **#include \** ``` struct sigaction { void (*sa_handler)(int); void (*sa_sigaction)(int, siginfo_t *, void *); int sa_flags; /* see signal options below */ sigset_t sa_mask; /* signal mask to apply */ }; ``` *int*\ **sigaction**(*int sig*, *const struct sigaction \* restrict act*, *struct sigaction \* restrict oact*); ## DESCRIPTION The system defines a set of signals that may be delivered to a process. Signal delivery resembles the occurrence of a hardware interrupt: the signal is normally blocked from further occurrence, the current thread context is saved, and a new one is built. A process may specify a *handler* to which a signal is delivered, or specify that a signal is to be *ignored*. A process may also specify that a default action is to be taken by the system when a signal occurs. A signal may also be *blocked* for a thread, in which case it will not be delivered to that thread until it is *unblocked*. The action to be taken on delivery is determined at the time of delivery. Normally, signal handlers execute on the current stack of the thread. This may be changed, on a per-handler basis, so that signals are taken on a special *signal stack*. Signal routines normally execute with the signal that caused their invocation *blocked*, but other signals may yet occur. A global *signal mask* defines the set of signals currently blocked from delivery to a thread. The signal mask for a thread is initialized from that of its parent (normally empty). It may be changed with a [sigprocmask(2)](/sigprocmask/2.md) or [pthread\_sigmask(3)](/pthread_sigmask/3.md) call, or when a signal is delivered to the thread. When a signal condition arises for a process or thread, the signal is added to a set of signals pending for the process or thread. Whether the signal is directed at the process in general or at a specific thread depends on how it is generated. For signals directed at a specific thread, if the signal is not currently *blocked* by the thread then it is delivered to the thread. For signals directed at the process, if the signal is not currently *blocked* by all threads then it is delivered to one thread that does not have it blocked (the selection of which is unspecified). Signals may be delivered any time a thread enters the operating system (e.g., during a system call, page fault or trap, or clock interrupt). If multiple signals are ready to be delivered at the same time, any signals that could be caused by traps are delivered first. Additional signals may be processed at the same time, with each appearing to interrupt the handlers for the previous signals before their first instructions. The set of pending signals is returned by the [sigpending(2)](/sigpending/2.md) system call. When a caught signal is delivered, the current state of the thread is saved, a new signal mask is calculated (as described below), and the signal handler is invoked. The call to the handler is arranged so that if the signal handling routine returns normally the thread will resume execution in the context from before the signal's delivery. If the thread wishes to resume in a different context, then it must arrange to restore the previous context itself. When a signal is delivered to a thread a new signal mask is installed for the duration of the process' signal handler (or until a [sigprocmask(2)](/sigprocmask/2.md) system call is made). This mask is formed by taking the union of the current signal mask set, the signal to be delivered, and the signal mask associated with the handler to be invoked. The **sigaction**() system call assigns an action for a signal specified by *sig*. If *act* is non-NULL, it specifies an action (SIG\_DFL, SIG\_IGN, or a handler routine) and mask to be used when delivering the specified signal. If *oact* is non-NULL, the previous handling information for the signal is returned to the user. The above declaration of *struct sigaction* is not literal. It is provided only to list the accessible members. See ⟨sys/signal.h⟩ for the actual definition. In particular, the storage occupied by *sa\_handler* and *sa\_sigaction* overlaps, and it is nonsensical for an application to attempt to use both simultaneously. Once a signal handler is installed, it normally remains installed until another **sigaction**() system call is made, or an [execve(2)](/execve/2.md) is performed. A signal-specific default action may be reset by setting *sa\_handler* to SIG\_DFL. The defaults are process termination, possibly with core dump; no action; stopping the process; or continuing the process. See the signal list below for each signal's default action. If *sa\_handler* is SIG\_DFL, the default action for the signal is to discard the signal, and if a signal is pending, the pending signal is discarded even if the signal is masked. If *sa\_handler* is set to SIG\_IGN current and pending instances of the signal are ignored and discarded. Options may be specified by setting *sa\_flags*. The meaning of the various bits is as follows: - SA\_NOCLDSTOP If this bit is set when installing a catching function for the SIGCHLD signal, the SIGCHLD signal will be generated only when a child process exits, not when a child process stops. - SA\_NOCLDWAIT If this bit is set when calling **sigaction**() for the SIGCHLD signal, the system will not create zombie processes when children of the calling process exit. If the calling process subsequently issues a [wait(2)](/wait/2.md) (or equivalent), it blocks until all of the calling process's child processes terminate, and then returns a value of -1 with *errno* set to ECHILD. The same effect of avoiding zombie creation can also be achieved by setting *sa\_handler* for SIGCHLD to SIG\_IGN. - SA\_ONSTACK If this bit is set, the system will deliver the signal to the process on a *signal stack*, specified by each thread with [sigaltstack(2)](/sigaltstack/2.md). - SA\_NODEFER If this bit is set, further occurrences of the delivered signal are not masked during the execution of the handler. - SA\_RESETHAND If this bit is set, the handler is reset back to SIG\_DFL at the moment the signal is delivered. - SA\_RESTART See paragraph below. - SA\_SIGINFO If this bit is set, the handler function is assumed to be pointed to by the *sa\_sigaction* member of *struct sigaction* and should match the prototype shown above or as below in EXAMPLES. This bit should not be set when assigning SIG\_DFL or SIG\_IGN. If a signal is caught during the system calls listed below, the call may be forced to terminate with the error EINTR, the call may return with a data transfer shorter than requested, or the call may be restarted. Restart of pending calls is requested by setting the SA\_RESTART bit in *sa\_flags*. The affected system calls include [open(2)](/open/2.md), [read(2)](/read/2.md), [write(2)](/write/2.md), [sendto(2)](/sendto/2.md), [recvfrom(2)](/recvfrom/2.md), [sendmsg(2)](/sendmsg/2.md) and [recvmsg(2)](/recvmsg/2.md) on a communications channel or a slow device (such as a terminal, but not a regular file) and during a [wait(2)](/wait/2.md) or [ioctl(2)](/ioctl/2.md). However, calls that have already committed are not restarted, but instead return a partial success (for example, a short read count). After a [pthread\_create(3)](/pthread_create/3.md) the signal mask is inherited by the new thread and the set of pending signals and the signal stack for the new thread are empty. After a [fork(2)](/fork/2.md) or [vfork(2)](/vfork/2.md) all signals, the signal mask, the signal stack, and the restart/interrupt flags are inherited by the child. The [execve(2)](/execve/2.md) system call reinstates the default action for all signals which were caught and resets all signals to be caught on the user stack. Ignored signals remain ignored; the signal mask remains the same; signals that restart pending system calls continue to do so. The following is a list of all signals with names as in the include file ⟨signal.h⟩: | | | | | --- | --- | --- | | **NAME** | **Default** **Action** | **Description** | | SIGHUP | terminate process | terminal line hangup | | SIGINT | terminate process | interrupt program | | SIGQUIT | create core image | quit program | | SIGILL | create core image | illegal instruction | | SIGTRAP | create core image | trace trap | | SIGABRT | create core image | [abort(3)](/abort/3.md) call (formerly SIGIOT) | | SIGEMT | create core image | emulate instruction executed | | SIGFPE | create core image | floating-point exception | | SIGKILL | terminate process | kill program | | SIGBUS | create core image | bus error | | SIGSEGV | create core image | segmentation violation | | SIGSYS | create core image | non-existent system call invoked | | SIGPIPE | terminate process | write on a pipe with no reader | | SIGALRM | terminate process | real-time timer expired | | SIGTERM | terminate process | software termination signal | | SIGURG | discard signal | urgent condition present on socket | | SIGSTOP | stop process | stop (cannot be caught or ignored) | | SIGTSTP | stop process | stop signal generated from keyboard | | SIGCONT | discard signal | continue after stop | | SIGCHLD | discard signal | child status has changed | | SIGTTIN | stop process | background read attempted from control terminal | | SIGTTOU | stop process | background write attempted to control terminal | | SIGIO | discard signal | I/O is possible on a descriptor (see [fcntl(2)](/fcntl/2.md)) | | SIGXCPU | terminate process | cpu time limit exceeded (see [setrlimit(2)](/setrlimit/2.md)) | | SIGXFSZ | terminate process | file size limit exceeded (see [setrlimit(2)](/setrlimit/2.md)) | | SIGVTALRM | terminate process | virtual time alarm (see [setitimer(2)](/setitimer/2.md)) | | SIGPROF | terminate process | profiling timer alarm (see [setitimer(2)](/setitimer/2.md)) | | SIGWINCH | discard signal | window size change | | SIGINFO | discard signal | status request from keyboard | | SIGUSR1 | terminate process | user defined signal 1 | | SIGUSR2 | terminate process | user defined signal 2 | ## NOTE The *sa\_mask* field specified in *act* is not allowed to block SIGKILL or SIGSTOP. Any attempt to do so will be silently ignored. The following functions are either reentrant or not interruptible by signals and are async-signal safe. Therefore applications may invoke them, without restriction, from signal-catching functions or from a child process after calling [fork(2)](/fork/2.md) in a multi-threaded process: Base Interfaces: **\_Exit**(), **\_exit**(), **accept**(), **access**(), **alarm**(), **bind**(), **cfgetispeed**(), **cfgetospeed**(), **cfsetispeed**(), **cfsetospeed**(), **chdir**(), **chmod**(), **chown**(), **close**(), **connect**(), **creat**(), **dup**(), **dup2**(), **execl**(), **execle**(), **execv**(), **execve**(), **faccessat**(), **fchdir**(), **fchmod**(), **fchmodat**(), **fchown**(), **fchownat**(), **fcntl**(), **\_Fork**(), **fstat**(), **fstatat**(), **fsync**(), **ftruncate**(), **getegid**(), **geteuid**(), **getgid**(), **getgroups**(), **getpeername**(), **getpgrp**(), **getpid**(), **getppid**(), **getsockname**(), **getsockopt**(), **getuid**(), **kill**(), **link**(), **linkat**(), **listen**(), **lseek**(), **lstat**(), **mkdir**(), **mkdirat**(), **mkfifo**(), **mkfifoat**(), **mknod**(), **mknodat**(), **open**(), **openat**(), **pause**(), **pipe**(), **poll**(), **pselect**(), **pthread\_sigmask**(), **raise**(), **read**(), **readlink**(), **readlinkat**(), **recv**(), **recvfrom**(), **recvmsg**(), **rename**(), **renameat**(), **rmdir**(), **select**(), **send**(), **sendmsg**(), **sendto**(), **setgid**(), **setpgid**(), **setsid**(), **setsockopt**(), **setuid**(), **shutdown**(), **sigaction**(), **sigaddset**(), **sigdelset**(), **sigemptyset**(), **sigfillset**(), **sigismember**(), **signal**(), **sigpending**(), **sigprocmask**(), **sigsuspend**(), **sleep**(), **sockatmark**(), **socket**(), **socketpair**(), **stat**(), **symlink**(), **symlinkat**(), **tcdrain**(), **tcflow**(), **tcflush**(), **tcgetattr**(), **tcgetpgrp**(), **tcsendbreak**(), **tcsetattr**(), **tcsetpgrp**(), **time**(), **times**(), **umask**(), **uname**(), **unlink**(), **unlinkat**(), **utime**(), **wait**(), **waitpid**(), **write**(). X/Open Systems Interfaces: **sigpause**(), **sigset**(), **utimes**(). Realtime Interfaces: **aio\_error**(), **clock\_gettime**(), **timer\_getoverrun**(), **aio\_return**(), **fdatasync**(), **sigqueue**(), **timer\_gettime**(), **aio\_suspend**(), **sem\_post**(), **timer\_settime**(). Base Interfaces not specified as async-signal safe by POSIX: **fpathconf**(), **pathconf**(), **sysconf**(). Base Interfaces not specified as async-signal safe by POSIX, but planned to be: **ffs**(), **htonl**(), **htons**(), **memccpy**(), **memchr**(), **memcmp**(), **memcpy**(), **memmove**(), **memset**(), **ntohl**(), **ntohs**(), **stpcpy**(), **stpncpy**(), **strcat**(), **strchr**(), **strcmp**(), **strcpy**(), **strcspn**(), **strlen**(), **strncat**(), **strncmp**(), **strncpy**(), **strnlen**(), **strpbrk**(), **strrchr**(), **strspn**(), **strstr**(), **strtok\_r**(), **wcpcpy**(), **wcpncpy**(), **wcscat**(), **wcschr**(), **wcscmp**(), **wcscpy**(), **wcscspn**(), **wcslen**(), **wcsncat**(), **wcsncmp**(), **wcsncpy**(), **wcsnlen**(), **wcspbrk**(), **wcsrchr**(), **wcsspn**(), **wcsstr**(), **wcstok**(), **wmemchr**(), **wmemcmp**(), **wmemcpy**(), **wmemmove**(), **wmemset**(). Extension Interfaces: **accept4**(), **bindat**(), **close\_range**(), **closefrom**(), **connectat**(), **eaccess**(), **ffsl**(), **ffsll**(), **flock**(), **fls**(), **flsl**(), **flsll**(), **futimesat**(), **pipe2**(), **strlcat**(). **strlcpy**(), **strsep**(). In addition, reading or writing *errno* is async-signal safe. All functions not in the above lists are considered to be unsafe with respect to signals. That is to say, the behaviour of such functions is undefined when they are called from a signal handler that interrupted an unsafe function. In general though, signal handlers should do little more than set a flag; most other actions are not safe. Also, it is good practice to make a copy of the global variable *errno* and restore it before returning from the signal handler. This protects against the side effect of *errno* being set by functions called from inside the signal handler. ## RETURN VALUES The **sigaction()** function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error. ## EXAMPLES There are three possible prototypes the handler may match: - ANSI C: *void* **handler**(*int*); - Traditional BSD style: *void* **handler**(*int*, *int code*, *struct sigcontext \*scp*); - POSIX SA\_SIGINFO: *void* **handler**(*int*, *siginfo\_t \*info*, *ucontext\_t \*uap*); The handler function should match the SA\_SIGINFO prototype if the SA\_SIGINFO bit is set in *sa\_flags*. It then should be pointed to by the *sa\_sigaction* member of *struct sigaction*. Note that you should not assign SIG\_DFL or SIG\_IGN this way. If the SA\_SIGINFO flag is not set, the handler function should match either the ANSI C or traditional BSD prototype and be pointed to by the *sa\_handler* member of *struct sigaction*. In practice, FreeBSD always sends the three arguments of the latter and since the ANSI C prototype is a subset, both will work. The *sa\_handler* member declaration in FreeBSD include files is that of ANSI C (as required by POSIX), so a function pointer of a BSD-style function needs to be casted to compile without warning. The traditional BSD style is not portable and since its capabilities are a full subset of a SA\_SIGINFO handler, its use is deprecated. The *sig* argument is the signal number, one of the SIG... values from ⟨signal.h⟩. The *code* argument of the BSD-style handler and the *si\_code* member of the *info* argument to a SA\_SIGINFO handler contain a numeric code explaining the cause of the signal, usually one of the SI\_... values from ⟨sys/signal.h⟩ or codes specific to a signal, i.e., one of the FPE\_... values for SIGFPE. The *scp* argument to a BSD-style handler points to an instance of *struct sigcontext*. The *uap* argument to a POSIX SA\_SIGINFO handler points to an instance of ucontext\_t. ## ERRORS The **sigaction**() system call will fail and no new signal handler will be installed if one of the following occurs: - \[EINVAL\] The *sig* argument is not a valid signal number. - \[EINVAL\] An attempt is made to ignore or supply a handler for SIGKILL or SIGSTOP. ## SEE ALSO [kill(1)](/kill/1.md), [kill(2)](/kill/2.md), [ptrace(2)](/ptrace/2.md), [setitimer(2)](/setitimer/2.md), [setrlimit(2)](/setrlimit/2.md), [sigaltstack(2)](/sigaltstack/2.md), [sigpending(2)](/sigpending/2.md), [sigprocmask(2)](/sigprocmask/2.md), [sigsuspend(2)](/sigsuspend/2.md), [wait(2)](/wait/2.md), [fpsetmask(3)](/fpsetmask/3.md), [setjmp(3)](/setjmp/3.md), [siginfo(3)](/siginfo/3.md), [siginterrupt(3)](/siginterrupt/3.md), [sigsetops(3)](/sigsetops/3.md), [ucontext(3)](/ucontext/3.md), [tty(4)](/tty/4.md) ## STANDARDS The **sigaction**() system call is expected to conform to ISO/IEC 9945-1:1990 (“POSIX.1”). The SA\_ONSTACK and SA\_RESTART flags are Berkeley extensions, as are the signals, SIGTRAP, SIGEMT, SIGBUS, SIGSYS, SIGURG, SIGIO, SIGXCPU, SIGXFSZ, SIGVTALRM, SIGPROF, SIGWINCH, and SIGINFO. Those signals are available on most BSD-derived systems. The SA\_NODEFER and SA\_RESETHAND flags are intended for backwards compatibility with other operating systems. The SA\_NOCLDSTOP, and SA\_NOCLDWAIT flags are featuring options commonly found in other operating systems. The flags are approved by Version 2 of the Single UNIX Specification (“SUSv2”), along with the option to avoid zombie creation by ignoring SIGCHLD.