signal - simplified software signal facilities
void * Ns Fo signal Fa int sig Fa void *funcint Fc Ns int ;
or in Fx Ap s equivalent but easier to read typedef'd version: typedef void *sig_t int ;
signal (int sig sig_t func);
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
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
Except for the
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 :
The Fa sig argument specifies which signal was received. The Fa 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, Fa func should be SIG_DFL A SIG_DFL resets the default action. To ignore the signal Fa 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 Fa 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. (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 process's children terminate, and then return a value of -1 with errno set to Er ECHILD .
See sigaction(2) for a list of functions that are considered safe for use in signal handlers.
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Created 1996-2021 by Maxim Chirkov
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