rt_sigaction (2) Linux Manual Page
sigaction, rt_sigaction – examine and change a signal action
Synopsis
#include <signal.h>Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
int sigaction(int signum, const struct sigaction *act,
struct sigaction *oldact);
sigaction(): _POSIX_C_SOURCE
siginfo_t: _POSIX_C_SOURCE >= 199309L
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 {
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 is be one of the following:
- *
- SIG_DFL for the default action.
- *
- SIG_IGN to ignore this signal.
- *
- 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 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:
- 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)). This flag is meaningful only when establishing a handler for SIGCHLD.
- SA_NOCLDWAIT (since Linux 2.6)
- If signum is SIGCHLD, do not transform children into zombies when they terminate. See also waitpid(2). This flag is meaningful only when establishing a handler for SIGCHLD, or when setting that signal’s disposition to SIG_DFL.
- If the SA_NOCLDWAIT flag is set when establishing a handler for SIGCHLD, POSIX.1 leaves it unspecified whether a SIGCHLD signal is generated when a child process terminates. On Linux, a SIGCHLD signal is generated in this case; on some other implementations, it is not.
- If the SA_NOCLDWAIT flag is set when establishing a handler for SIGCHLD, POSIX.1 leaves it unspecified whether a SIGCHLD signal is generated when a child process terminates. On Linux, a SIGCHLD signal is generated in this case; on some other implementations, it is not.
- SA_NODEFER
- Do not add the signal to the thread’s signal mask while the handler is executing, unless the signal is specified in act.sa_mask. Consequently, a further instance of the signal may be delivered to the thread while it is executing the handler. This flag is meaningful only when establishing a signal handler.
- SA_NOMASK is an obsolete, nonstandard synonym for this flag.
- SA_NOMASK is an obsolete, nonstandard 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. This flag is meaningful only when establishing a signal handler.
- SA_RESETHAND
- Restore the signal action to the default upon entry to the signal handler. This flag is meaningful only when establishing a signal handler.
- SA_ONESHOT is an obsolete, nonstandard synonym for this flag.
- SA_ONESHOT is an obsolete, nonstandard synonym for this flag.
- SA_RESTART
- Provide behavior compatible with BSD signal semantics by making certain system calls restartable across signals. This flag is meaningful only when establishing a signal handler. See signal(7) for a discussion of system call restarting.
- SA_RESTORER
- Not intended for application use. This flag is used by C libraries to indicate that the sa_restorer field contains the address of a "signal trampoline". See sigreturn(2) for more details.
- SA_SIGINFO (since Linux 2.2)
- The signal handler takes three arguments, not one. In this case, sa_sigaction should be set instead of sa_handler. This flag is meaningful only when establishing a signal handler.
The siginfo_t argument to a SA_SIGINFO handler
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
- sig
- The number of the signal that caused invocation of the handler.
- info
- A pointer to a siginfo_t, which is a structure containing further information about the signal, as described below.
- ucontext
- This is a pointer to a ucontext_t structure, cast to void *. The structure pointed to by this field contains signal context information that was saved on the user-space stack by the kernel; for details, see sigreturn(2). Further information about the ucontext_t structure can be found in getcontext(3) and signal(7). Commonly, the handler function doesn’t make any use of the third argument.
The siginfo_t data type is a structure with the following fields:
siginfo_t {