sigaction() - software signal facilities
#include <signal.h>
int sigaction (int sig, const struct sigaction *act,
struct sigaction *oact)
The system defines a set of signals that may be delivered to a process. Upon receiving a signal, a process takes some action; the process may ignore the signal, it may deliver the signal to a specified handler function, it may block the signal (delivery is postponed until the signal is unblocked), or it may take the default action. The sigaction(2) call allows the calling process to examine or specify the action to be taken when a signal is received.
Sig specifies the signal. The action is described in a struct sigaction structure, defined in <signal.h>, which contains the following members:
| void | (*sa_handler)(int); |
| sigset_t | sa_mask |
| int | sa_flags |
Within the sigaction structure:
Signal routines 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 process. The signal mask for a process is initialized from that of its parent (normally empty). It may be changed with a sigprocmask(2) call, or when a signal is delivered to the process.
These signals are defined in the file <signal.h>:
| Symbol | Effect | Description |
|---|---|---|
| SIGABRT | terminate process | abort() call (formerly SIGIOT) |
| SIGALRM | terminate process | real-time timer expired |
| SIGBUS | terminate process | bus error |
| SIGCHLD | discard signal | child status has changed |
| SIGCONT | discard signal | continue after stop |
| SIGEXCEPT | terminate process | unrecognized Windows exception |
| SIGFPE | terminate process | floating-point exception |
| SIGHUP | terminate process | terminal line hangup |
| SIGILL | terminate process | illegal instruction |
| SIGINT | terminate process | interrupt program |
| SIGIO | discard signal | I/O possible on a descriptor (see fcntl()) |
| SIGKILL | terminate process | kill program |
| SIGPIPE | terminate process | write on a pipe with no reader |
| SIGPOLL | ||
| SIGPROF | terminate process | profiling timer alarm |
| SIGQUIT | terminate process | quit program |
| SIGSEGV | terminate process | segmentation violation |
| SIGSTOP | stop process | stop (cannot be caught or ignored) |
| SIGSYS | terminate process | invalid argument to system call |
| SIGTERM | terminate process | software termination signal |
| SIGTRAP | terminate process | trace trap |
| SIGTSTP | stop process | stop signal generated from keyboard |
| SIGTTIN | stop process | background read attempted from control terminal |
| SIGTTOU | stop process | background write attempted to control terminal |
| SIGURG | discard signal | urgent condition present on socket |
| SIGUSR1 | terminate process | User defined signal 1 |
| SIGUSR2 | terminate process | User defined signal 2 |
| SIGVTALRM | terminate process | virtual time alarm |
| SIGXCPU | terminate process | CPU time limit exceeded |
| SIGXFSZ | terminate process | file size limit exceeded |
| SIGWINCH | discard signal | window size change |
When a signal condition arises for a process, the signal is added to a set of signals pending for the process. If the signal is not currently blocked by the process then it is delivered to the process. Signals may be delivered any time a process enters the operating system (e.g., during a system call, 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) function. When a caught signal is delivered, the current state of the process 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 process will resume execution in the context from before the signal's delivery. If the process wishes to resume in a different context, then it must arrange to restore the previous context itself.
When a signal is delivered to a process a new signal mask is installed for the duration of the process' signal handler (or until a sigprocmask(1) 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.
Once a signal handler is installed, it remains installed until another sigaction(2) call is made, or an exec(2) is performed.
After a fork(2) all signals and the signal mask, are inherited by the child.
The exec(2) functions reinstate the default action for all signals which were caught and resets all signals to be caught on the user stack. Ignored signals remain ignored and the signal mask remains the same.
The mask specified in act is not allowed to block SIGKILL or SIGSTOP. This is ignored silently by the system.
The sigaction(2) funciton is the POSIX.1 replacement for the traditional signal(2) call and mechanism. The sigaction(2) mechanism is more reliable than old-style signals; for a discussion, see a reference such as Zlotnick's The POSIX.1 Standard: A Programmer's Guide.
A 0 value indicated that the call succeeded. A -1 return value indicates an error occurred and errno is set to indicated the reason.
The sigaction(2) call will fail and no new signal handler will be installed if one of the following occurs:
This implementation does not support the SA_RESTART bit for the sa_flags member of struct sigaction.
signal(2)
sigpending(2)
sigprocmask(2)
sigsuspend(2)
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