Name

sigaction — examine and change a signal action

Synopsis

#include <signal.h>
int sigaction( int   signum,
  const struct sigaction *  act,
  struct sigaction *  oldact);

DESCRIPTION

The sigaction() system call is used to change the action taken by a process on receipt of a specific signal.

signum specifies the signal and can be any valid signal except SIGKILL and SIGSTOP.

If act is non−null, the new action for signal signum is installed from act. If oldact is non−null, the previous action is saved in oldact.

The sigaction structure is defined as something like

struct sigaction {
};
  void (* sa_handler)(int);  
  void (* sa_sigaction)(int, siginfo_t *, void *);  
  sigset_t   sa_mask;  
  int   sa_flags;  
  void (* sa_restorer)(void);  

On some architectures a union is involved: do not assign to both sa_handler and sa_sigaction.

The sa_restorer element is obsolete and should not be used. POSIX does not specify a sa_restorer element.

sa_handler specifies the action to be associated with signum and may be SIG_DFL for the default action, SIG_IGN to ignore this signal, or a pointer to a signal handling function. This function receives the signal number as its only argument.

If SA_SIGINFO is specified in sa_flags, then sa_sigaction (instead of sa_handler) specifies the signal-handling function for signum. This function receives the signal number as its first argument, a pointer to a siginfo_t as its second argument and a pointer to a ucontext_t (cast to void *) as its third argument.

sa_mask gives a mask of signals which should be blocked during execution of the signal handler. In addition, the signal which triggered the handler will be blocked, unless the SA_NODEFER flag is used.

sa_flags specifies a set of flags which modify the behaviour of the signal handling process. It is formed by the bitwise OR of zero or more of the following:

SA_NOCLDSTOP

If signum is SIGCHLD, do not receive notification when child processes stop (i.e., when they receive one of SIGSTOP, SIGTSTP, SIGTTIN or SIGTTOU) or resume (i.e., they receive SIGCONT) (see wait(2)).

SA_NOCLDWAIT

(Linux 2.6 and later) If signum is SIGCHLD, do not transform children into zombies when they terminate. See also waitpid(2).

SA_RESETHAND

Restore the signal action to the default state once the signal handler has been called. SA_ONESHOT is an obsolete, non-standard synonym for this flag.

SA_ONSTACK

Call the signal handler on an alternate signal stack provided by sigaltstack(2). If an alternate stack is not available, the default stack will be used.

SA_RESTART

Provide behaviour compatible with BSD signal semantics by making certain system calls restartable across signals.

SA_NODEFER

Do not prevent the signal from being received from within its own signal handler. SA_NOMASK is an obsolete, non-standard synonym for this flag.

SA_SIGINFO

The signal handler takes 3 arguments, not one. In this case, sa_sigaction should be set instead of sa_handler. (The sa_sigaction field was added in Linux 2.1.86.)

The siginfo_t parameter to sa_sigaction is a struct with the following elements

siginfo_t {
    int      si_signo;    /* Signal number */
    int      si_errno;    /* An errno value */
    int      si_code;     /* Signal code */
    pid_t    si_pid;      /* Sending process ID */
    uid_t    si_uid;      /* Real user ID of sending process */
    int      si_status;   /* Exit value or signal */
    clock_t  si_utime;    /* User time consumed */
    clock_t  si_stime;    /* System time consumed */
    sigval_t si_value;    /* Signal value */
    int      si_int;      /* POSIX.1b signal */
    void    *si_ptr;      /* POSIX.1b signal */
    void    *si_addr;     /* Memory location which caused fault */
    int      si_band;     /* Band event */
    int      si_fd;       /* File descriptor */
}

si_signo, si_errno and si_code are defined for all signals. (si_signo is unused on Linux.) The rest of the struct may be a union, so that one should only read the fields that are meaningful for the given signal:

  • POSIX.1b signals and SIGCHLD fill in si_pid and si_uid.

  • SIGCHLD also fills in si_status, si_utime and si_stime.

  • si_int and si_ptr are specified by the sender of the POSIX.1b signal. See sigqueue(2) for more details.

  • SIGILL, SIGFPE, SIGSEGV, and SIGBUS fill in si_addr with the address of the fault. SIGPOLL fills in si_band and si_fd.

si_code indicates why this signal was sent. It is a value, not a bitmask. The values which are possible for any signal are listed in this table:

si_code
Value Signal origin
SI_USER kill(2), sigsend(2), or raise(3)
SI_KERNEL The kernel
SI_QUEUE sigqueue(2)
SI_TIMER POSIX timer expired
SI_MESGQ POSIX message queue state changed (since Linux 2.6.6)
SI_ASYNCIO AIO completed
SI_SIGIO queued SIGIO
SI_TKILL tkill(2) or tgkill(2) (since Linux 2.4.19)
SIGILL
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
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 floating point invalid operation
FPE_FLTSUB subscript out of range
SIGSEGV
SEGV_MAPERR address not mapped to object
SEGV_ACCERR invalid permissions for mapped object
SIGBUS
BUS_ADRALN invalid address alignment
BUS_ADRERR non-existent physical address
BUS_OBJERR object specific hardware error
SIGTRAP
TRAP_BRKPT process breakpoint
TRAP_TRACE process trace trap
SIGCHLD
CLD_EXITED child has exited
CLD_KILLED child was killed
CLD_DUMPED child terminated abnormally
CLD_TRAPPED traced child has trapped
CLD_STOPPED child has stopped
CLD_CONTINUED stopped child has continued (since Linux 2.6.9)
SIGPOLL
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

RETURN VALUE

sigaction() returns 0 on success and −1 on error.

ERRORS

EFAULT

act or oldact points to memory which is not a valid part of the process address space.

EINVAL

An invalid signal was specified. This will also be generated if an attempt is made to change the action for SIGKILL or SIGSTOP, which cannot be caught or ignored.

NOTES

According to POSIX, the behaviour of a process is undefined after it ignores a SIGFPE, SIGILL, or SIGSEGV signal that was not generated by kill(2) or raise(3). Integer division by zero has undefined result. On some architectures it will generate a SIGFPE signal. (Also dividing the most negative integer by −1 may generate SIGFPE.) Ignoring this signal might lead to an endless loop.

POSIX.1-1990 disallowed setting the action for SIGCHLD to SIG_IGN. POSIX.1-2001 allows this possibility, so that ignoring SIGCHLD can be used to prevent the creation of zombies (see wait(2)). Nevertheless, the historical BSD and System V behaviours for ignoring SIGCHLD differ, so that the only completely portable method of ensuring that terminated children do not become zombies is to catch the SIGCHLD signal and perform a wait(2) or similar.

POSIX.1-1990 only specified SA_NOCLDSTOP. POSIX.1-2001 added SA_NOCLDWAIT, SA_RESETHAND, SA_NODEFER, and SA_SIGINFO. Use of these latter values in sa_flags may be less portable in applications intended for older Unix implementations.

Support for SA_SIGINFO was added in Linux 2.2.

The SA_RESETHAND flag is compatible with the SVr4 flag of the same name.

The SA_NODEFER flag is compatible with the SVr4 flag of the same name under kernels 1.3.9 and newer. On older kernels the Linux implementation allowed the receipt of any signal, not just the one we are installing (effectively overriding any sa_mask settings).

sigaction() can be called with a null second argument to query the current signal handler. It can also be used to check whether a given signal is valid for the current machine by calling it with null second and third arguments.

It is not possible to block SIGKILL or SIGSTOP (by specifying them in sa_mask). Attempts to do so are silently ignored.

See sigsetops(3) for details on manipulating signal sets.

See signal(7) for a list of the async-signal-safe functions that can be safely called inside from inside a signal handler.

BUGS

In kernels up to and including 2.6.13, specifying SA_NODEFER in sa_flags preventing not only the delivered signal from being masked during execution of the handler, but also the signals specified in sa_mask. This bug is was fixed in kernel 2.6.14.

CONFORMING TO

POSIX.1-2001, SVr4.

UNDOCUMENTED

Before the introduction of SA_SIGINFO it was also possible to get some additional information, namely by using a sa_handler with second argument of type struct sigcontext. See the relevant kernel sources for details. This use is obsolete now.

SEE ALSO

kill(1), kill(2), pause(2), sigaltstack(2), signal(2), sigpending(2), sigprocmask(2), sigqueue(2), sigsuspend(2), wait(2), killpg(3), raise(3), siginterrupt(3), sigsetops(3), sigvec(3), core(5), signal(7)


  t
Copyright (c) 1994,1995 Mike Battersby <mib@deakin.edu.au>
and Copyright 2004, 2005 Michael Kerrisk <mtk-manpages@gmx.net>
based on work by faith@cs.unc.edu

Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Since the Linux kernel and libraries are constantly changing, this
manual page may be incorrect or out-of-date.  The author(s) assume no
responsibility for errors or omissions, or for damages resulting from
the use of the information contained herein.  The author(s) may not
have taken the same level of care in the production of this manual,
which is licensed free of charge, as they might when working
professionally.

Formatted or processed versions of this manual, if unaccompanied by
the source, must acknowledge the copyright and authors of this work.

Modified, aeb, 960424
Modified Fri Jan 31 17:31:20 1997 by Eric S. Raymond <esr@thyrsus.com>
Modified Thu Nov 26 02:12:45 1998 by aeb - add SIGCHLD stuff.
Modified Sat May  8 17:40:19 1999 by Matthew Wilcox
add POSIX.1b signals
Modified Sat Dec 29 01:44:52 2001 by Evan Jones <ejones@uwaterloo.ca>
SA_ONSTACK
Modified 2004-11-11 by Michael Kerrisk <mtk-manpages@gmx.net>
Added mention of SIGCONT under SA_NOCLDSTOP
Added SA_NOCLDWAIT
Modified 2004-11-17 by Michael Kerrisk <mtk-manpages@gmx.net>
Updated discussion for POSIX.1-2001 and SIGCHLD and sa_flags.
Formatting fixes
2004-12-09, mtk, added SI_TKILL + other minor changes
2005-09-15, mtk, split sigpending(), sigprocmask(), sigsuspend()
out of this page into separate pages.