NAME¶

sigaction - examine and change a signal action

SYNOPSIS¶

#include <signal.h>

int sigaction(int signum, const struct sigaction *act,
              struct sigaction *oldact);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

sigaction(): _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _POSIX_SOURCE

DESCRIPTION¶

The sigaction() system call is used to change the action taken by a process on receipt of a specific signal. (See signal(7) for an overview of signals.)

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The sa_restorer element is obsolete and should not be used.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 specifies a mask of signals which should be blocked (i.e., added to the signal mask of the thread in which the signal handler is invoked) 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 behavior of the signal. It is formed by the bitwise OR of zero or more of the following:

The siginfo_t argument 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 */


    int      si_trapno;   /* Trap number that caused


                             hardware-generated signal


                             (unused on most architectures) */


    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 */


    int      si_overrun;  /* Timer overrun count; POSIX.1b timers */


    int      si_timerid;  /* Timer ID; POSIX.1b timers */


    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_errno is generally 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.
• POSIX.1b timers (since Linux 2.6) fill in si_overrun and si_timerid. The si_timerid field is an internal ID used by the kernel to identify the timer; it is not the same as the timer ID returned by timer_create(2).
• SIGCHLD fills in si_status, si_utime and si_stime. The si_utime and si_stime fields do not include the times used by waited for children (unlike getrusage(2) and time(2). In kernels up to 2.6, and since 2.6.27, these fields report CPU time in units of sysconf(_SC_CLK_TCK). In 2.6 kernels before 2.6.27, a bug meant that these fields reported time in units of the (configurable) system jiffy (see time(7)).

• 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 is a value (not a bit mask) indicating why this signal was sent. The following list shows the values which can be placed in si_code for any signal, along with reason that the signal was generated.

The following values can be placed in si_code for a SIGILL signal:

The following values can be placed in si_code for a SIGFPE signal:

The following values can be placed in si_code for a SIGSEGV signal:

The following values can be placed in si_code for a SIGBUS signal:

The following values can be placed in si_code for a SIGTRAP signal:

The following values can be placed in si_code for a SIGCHLD signal:

The following values can be placed in si_code for a SIGPOLL signal:

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.

CONFORMING TO¶

POSIX.1-2001, SVr4.

NOTES¶

A child created via fork(2) inherits a copy of its parent's signal dispositions. During an execve(2), the dispositions of handled signals are reset to the default; the dispositions of ignored signals are left unchanged.

According to POSIX, the behavior 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 behaviors 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.

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 prevents not only the delivered signal from being masked during execution of the handler, but also the signals specified in sa_mask. This bug was fixed in kernel 2.6.14.

EXAMPLE¶

See mprotect(2).

SEE ALSO¶

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

COLOPHON¶

This page is part of release 3.22 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.