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NAME
sigaction — software signal facilities
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <signal.h>
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) 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 (see fcntl(2))
SIGXCPU terminate process cpu time limit exceeded (see setrlimit(2))
SIGXFSZ terminate process file size limit exceeded (see setrlimit(2))
SIGVTALRM terminate process virtual time alarm (see setitimer(2))
SIGPROF terminate process profiling timer alarm (see setitimer(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
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) 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(2), ptrace(2), setitimer(2), setrlimit(2), sigaltstack(2), sigpending(2), sigprocmask(2),
sigsuspend(2), wait(2), fpsetmask(3), setjmp(3), siginfo(3), siginterrupt(3), sigsetops(3), ucontext(3),
tty(4)
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.
Debian June 29, 2020 SIGACTION(2)