SIGNAL (3) | C library functions | Unix Manual Pages | :man▋
signal - simplified software signal facilities
.Fo signal "int sig" "void *funcint"
.Fc Ns "int";
.Fx Ap s equivalent but easier to read typedefd version: typedef "void *sig_t int"; sig_t signal "int sig" "sig_t func"
This signal facility is a simplified interface to the more general sigaction(2) facility.
Signals allow the manipulation of a process from outside its domain as well as allowing the process to manipulate itself or copies of itself (children). There are two general types of signals: those that cause termination of a process and those that do not. Signals which cause termination of a program might result from an irrecoverable error or might be the result of a user at a terminal typing the interrupt character. Signals are used when a process is stopped because it wishes to access its control terminal while in the background (see tty(4)). Signals are optionally generated when a process resumes after being stopped, when the status of child processes changes, or when input is ready at the control terminal. Most signals result in the termination of the process receiving them if no action is taken; some signals instead cause the process receiving them to be stopped, or are simply discarded if the process has not requested otherwise. Except for the SIGKILL and SIGSTOP signals, the signal function allows for a signal to be caught, to be ignored, or to generate an interrupt. These signals are defined in the file
.In signal.h :
| "No Name Default Action Description" |
1 SIGHUP "terminate process" "terminal line hangup"
2 SIGINT "terminate process" "interrupt program"
3 SIGQUIT "create core image" "quit program"
4 SIGILL "create core image" "illegal instruction"
5 SIGTRAP "create core image" "trace trap"
6 SIGABRT "create core image" "abort program"
| || (formerly SIGIOT) |
| 7 SIGEMT "create core image" "emulate instruction executed" |
8 SIGFPE "create core image" "floating-point exception"
9 SIGKILL "terminate process" "kill program"
10 SIGBUS "create core image" "bus error"
11 SIGSEGV "create core image" "segmentation violation"
12 SIGSYS "create core image" "non-existent system call invoked"
13 SIGPIPE "terminate process" "write on a pipe with no reader"
14 SIGALRM "terminate process" "real-time timer expired"
15 SIGTERM "terminate process" "software termination signal"
16 SIGURG "discard signal" "urgent condition present on socket"
17 SIGSTOP "stop process" "stop (cannot be caught or ignored)"
18 SIGTSTP "stop process" "stop signal generated from keyboard"
19 SIGCONT "discard signal" "continue after stop"
20 SIGCHLD "discard signal" "child status has changed"
21 SIGTTIN "stop process" "background read attempted from"
| || control terminal |
| 22 SIGTTOU "stop process" "background write attempted to" || control terminal |
| 23 SIGIO "discard signal" "I/O" || is possible on a descriptor (see fcntl(2)) |
| 24 SIGXCPU "terminate process" "cpu time limit exceeded (see" || setrlimit(2)) |
| 25 SIGXFSZ "terminate process" "file size limit exceeded (see" || setrlimit(2)) |
| 26 SIGVTALRM "terminate process" "virtual time alarm (see" || setitimer(2)) |
| 27 SIGPROF "terminate process" "profiling timer alarm (see" || setitimer(2)) |
| 28 SIGWINCH "discard signal" "Window size change" |
29 SIGINFO "discard signal" "status request from keyboard"
30 SIGUSR1 "terminate process" "User defined signal 1"
31 SIGUSR2 "terminate process" "User defined signal 2"
32 SIGTHR "terminate process" "thread interrupt"
| || |
The sig argument specifies which signal was received. The func procedure allows a user to choose the action upon receipt of a signal. To set the default action of the signal to occur as listed above, func should be SIG_DFL. A SIG_DFL resets the default action. To ignore the signal func should be SIG_IGN. This will cause subsequent instances of the signal to be ignored and pending instances to be discarded. If SIG_IGN is not used, further occurrences of the signal are automatically blocked and func is called.
The handled signal is unblocked when the function returns and the process continues from where it left off when the signal occurred.
.Bf -symbolic Unlike previous signal facilities, the handler func() remains installed after a signal has been delivered.
For some system calls, if a signal is caught while the call is executing and the call is prematurely terminated, the call is automatically restarted. (The handler is installed using the SA_RESTART flag with sigaction 2.) The affected system calls include read(2), write(2), sendto(2), recvfrom(2), sendmsg(2) and recvmsg(2) on a communications channel or a low speed device and during a ioctl(2) or wait(2). However, calls that have already committed are not restarted, but instead return a partial success (for example, a short read count). These semantics could be changed with siginterrupt(3).
When a process which has installed signal handlers forks, the child process inherits the signals. All caught signals may be reset to their default action by a call to the execve(2) function; ignored signals remain ignored.
If a process explicitly specifies SIG_IGN as the action for the signal SIGCHLD, the system will not create zombie processes when children of the calling process exit. As a consequence, the system will discard the exit status from the child processes. If the calling process subsequently issues a call to wait(2) or equivalent, it will block until all of the calling processs children terminate, and then return a value of -1 with errno set to ECHILD.
See sigaction(2) for a list of functions that are considered safe for use in signal handlers.
The previous action is returned on a successful call. Otherwise, SIG_ERR is returned and the global variable errno is set to indicate the error.
The signal function will fail and no action will take place if one of the following occur:
| || The sig argument is not a valid signal number. |
| || An attempt is made to ignore or supply a handler for SIGKILL or SIGSTOP. |
kill(1), kill(2), ptrace(2), sigaction(2), sigaltstack(2), sigprocmask(2), sigsuspend(2), wait(2), fpsetmask(3), setjmp(3), siginterrupt(3), tty(4)
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