Manual Pages  — SIGACTION

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) or pthread_sigmask(3) 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) 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) 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) 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) (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).
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), read(2), write(2), sendto(2), recvfrom(2), sendmsg(2) and recvmsg(2) on a communications channel or a slow device (such as a terminal, but not a regular file) and during a wait(2) or ioctl(2). 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) 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) or vfork(2) all signals, the signal mask, the signal stack, and the restart/interrupt flags are inherited by the child.

The execve(2) 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) 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 (seefcntl(2))
SIGXCPU	terminate process	cpu time limit exceeded (seesetrlimit(2))
SIGXFSZ	terminate process	file size limit exceeded (seesetrlimit(2))
SIGVTALRM	terminate process	virtual time alarm (seesetitimer(2))
SIGPROF	terminate process	profiling timer alarm (seesetitimer(2))
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

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) 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.