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Man Pages


Manual Reference Pages  -  SIGNAL (3)

NAME

signal - simplified software signal facilities

CONTENTS

Library
Synopsis
Description
Return Values
Errors
See Also
History

LIBRARY


.Lb libc

SYNOPSIS


.In signal.h void *signal int sig void *funcintint

or in
.Fx Ap s equivalent but easier to read typedef’d version: typedef void *sig_t int;

sig_t signal int sig sig_t func

DESCRIPTION

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 :
Num      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
33      SIGLIBRT      terminate process      real-time library 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.
.Ef

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. Any handler installed with signal(3) will have the SA_RESTART flag set, meaning that any restartable system call will not return on receipt of a signal. 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 process’s 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.

RETURN VALUES

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.

ERRORS

The signal function will fail and no action will take place if one of the following occur:
[EINVAL]
  The sig argument is not a valid signal number.
[EINVAL]
  An attempt is made to ignore or supply a handler for SIGKILL or SIGSTOP.

SEE ALSO

kill(1), kill(2), ptrace(2), sigaction(2), sigaltstack(2), sigprocmask(2), sigsuspend(2), wait(2), fpsetmask(3), setjmp(3), siginterrupt(3), tty(4)

HISTORY

The signal facility appeared in BSD 4.0 . The option to avoid the creation of child zombies through ignoring SIGCHLD appeared in
.Fx 5.0 .
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