| sigaction(2) | System Calls Manual | sigaction(2) |
sigaction, rt_sigaction - examine and change a signal action
Standard C library (libc, -lc)
#include <signal.h>
int sigaction(int signum,
const struct sigaction *_Nullable restrict act,
struct sigaction *_Nullable restrict oldact);
sigaction():
_POSIX_C_SOURCE
siginfo_t:
_POSIX_C_SOURCE >= 199309L
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 field is not intended for application use. (POSIX does not specify a sa_restorer field.) Some further details of the purpose of this field can be found in sigreturn(2).
sa_handler specifies the action to be associated with signum and can be one of the following:
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 three arguments, as described below.
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:
When the SA_SIGINFO flag is specified in act.sa_flags, the signal handler address is passed via the act.sa_sigaction field. This handler takes three arguments, as follows:
void
handler(int sig, siginfo_t *info, void *ucontext)
{
...
}
These three arguments are as follows
The siginfo_t data type is a structure with the following fields:
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 */
union sigval 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 */
long si_band; /* Band event (was int in
glibc 2.3.2 and earlier) */
int si_fd; /* File descriptor */
short si_addr_lsb; /* Least significant bit of address
(since Linux 2.6.32) */
void *si_lower; /* Lower bound when address violation
occurred (since Linux 3.19) */
void *si_upper; /* Upper bound when address violation
occurred (since Linux 3.19) */
int si_pkey; /* Protection key on PTE that caused
fault (since Linux 4.6) */
void *si_call_addr; /* Address of system call instruction
(since Linux 3.5) */
int si_syscall; /* Number of attempted system call
(since Linux 3.5) */
unsigned int si_arch; /* Architecture of attempted system call
(since Linux 3.5) */
}
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 read only the fields that are meaningful for the given signal:
The si_code field inside the siginfo_t argument that is passed to a SA_SIGINFO signal handler is a value (not a bit mask) indicating why this signal was sent. For a ptrace(2) event, si_code will contain SIGTRAP and have the ptrace event in the high byte:
(SIGTRAP | PTRACE_EVENT_foo << 8).
For a non-ptrace(2) event, the values that can appear in si_code are described in the remainder of this section. Since glibc 2.20, the definitions of most of these symbols are obtained from <signal.h> by defining feature test macros (before including any header file) as follows:
For the TRAP_* constants, the symbol definitions are provided only in the first two cases. Before glibc 2.20, no feature test macros were required to obtain these symbols.
For a regular signal, the following list shows the values which can be placed in si_code for any signal, along with the 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 SIGIO/SIGPOLL signal:
The following value can be placed in si_code for a SIGSYS signal:
The sigaction() call on Linux accepts unknown bits set in act->sa_flags without error. The behavior of the kernel starting with Linux 5.11 is that a second sigaction() will clear unknown bits from oldact->sa_flags. However, historically, a second sigaction() call would typically leave those bits set in oldact->sa_flags.
This means that support for new flags cannot be detected simply by testing for a flag in sa_flags, and a program must test that SA_UNSUPPORTED has been cleared before relying on the contents of sa_flags.
Since the behavior of the signal handler cannot be guaranteed unless the check passes, it is wise to either block the affected signal while registering the handler and performing the check in this case, or where this is not possible, for example if the signal is synchronous, to issue the second sigaction() in the signal handler itself.
In kernels that do not support a specific flag, the kernel's behavior is as if the flag was not set, even if the flag was set in act->sa_flags.
The flags SA_NOCLDSTOP, SA_NOCLDWAIT, SA_SIGINFO, SA_ONSTACK, SA_RESTART, SA_NODEFER, SA_RESETHAND, and, if defined by the architecture, SA_RESTORER may not be reliably probed for using this mechanism, because they were introduced before Linux 5.11. However, in general, programs may assume that these flags are supported, since they have all been supported since Linux 2.6, which was released in the year 2003.
See EXAMPLES below for a demonstration of the use of SA_UNSUPPORTED.
sigaction() returns 0 on success; on error, -1 is returned, and errno is set to indicate the error.
The glibc wrapper function for sigaction() gives an error (EINVAL) on attempts to change the disposition of the two real-time signals used internally by the NPTL threading implementation. See nptl(7) for details.
On architectures where the signal trampoline resides in the C library, the glibc wrapper function for sigaction() places the address of the trampoline code in the act.sa_restorer field and sets the SA_RESTORER flag in the act.sa_flags field. See sigreturn(2).
The original Linux system call was named sigaction(). However, with the addition of real-time signals in Linux 2.2, the fixed-size, 32-bit sigset_t type supported by that system call was no longer fit for purpose. Consequently, a new system call, rt_sigaction(), was added to support an enlarged sigset_t type. The new system call takes a fourth argument, size_t sigsetsize, which specifies the size in bytes of the signal sets in act.sa_mask and oldact.sa_mask. This argument is currently required to have the value sizeof(sigset_t) (or the error EINVAL results). The glibc sigaction() wrapper function hides these details from us, transparently calling rt_sigaction() when the kernel provides it.
POSIX.1-2008.
POSIX.1-2001, SVr4.
POSIX.1-1990 disallowed setting the action for SIGCHLD to SIG_IGN. POSIX.1-2001 and later allow 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 specified only SA_NOCLDSTOP. POSIX.1-2001 added SA_NOCLDSTOP, SA_NOCLDWAIT, SA_NODEFER, SA_ONSTACK, SA_RESETHAND, SA_RESTART, 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 later. 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).
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.
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-safety(7) for a list of the async-signal-safe functions that can be safely called inside from inside a signal handler.
Before the introduction of SA_SIGINFO, it was also possible to get some additional information about the signal. This was done by providing an sa_handler signal handler with a second argument of type struct sigcontext, which is the same structure as the one that is passed in the uc_mcontext field of the ucontext structure that is passed (via a pointer) in the third argument of the sa_sigaction handler. See the relevant Linux kernel sources for details. This use is obsolete now.
When delivering a signal with a SA_SIGINFO handler, the kernel does not always provide meaningful values for all of the fields of the siginfo_t that are relevant for that signal.
Up to and including Linux 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 Linux 2.6.14.
See mprotect(2).
The following example program exits with status EXIT_SUCCESS if SA_EXPOSE_TAGBITS is determined to be supported, and EXIT_FAILURE otherwise.
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
void
handler(int signo, siginfo_t *info, void *context)
{
struct sigaction oldact;
if (sigaction(SIGSEGV, NULL, &oldact) == -1
|| (oldact.sa_flags & SA_UNSUPPORTED)
|| !(oldact.sa_flags & SA_EXPOSE_TAGBITS))
{
_exit(EXIT_FAILURE);
}
_exit(EXIT_SUCCESS);
}
int
main(void)
{
struct sigaction act = { 0 };
act.sa_flags = SA_SIGINFO | SA_UNSUPPORTED | SA_EXPOSE_TAGBITS;
act.sa_sigaction = &handler;
if (sigaction(SIGSEGV, &act, NULL) == -1) {
perror("sigaction");
exit(EXIT_FAILURE);
}
raise(SIGSEGV);
}
kill(1), kill(2), pause(2), pidfd_send_signal(2), restart_syscall(2), seccomp(2), sigaltstack(2), signal(2), signalfd(2), sigpending(2), sigprocmask(2), sigreturn(2), sigsuspend(2), wait(2), killpg(3), raise(3), siginterrupt(3), sigqueue(3), sigsetops(3), sigvec(3), core(5), signal(7)
| 2023-05-03 | Linux man-pages 6.05.01 |