NAME

signal − specify what to do upon receipt of a signal

SYNOPSIS

#include <sys/signal.h>

int (∗signal (sig, func))()
int sig;
int (∗func)();

DESCRIPTION

Signal allows the calling process to choose one of three ways in which it is possible to handle the receipt of a specific signal.  Sig specifies the signal and func specifies the choice. 

Sig can be assigned any one of the following except SIGKILL:

Func is assigned one of three values: SIG_DFL, SIG_IGN, or a function address. The actions prescribed by these values of are as follows:

SIG_DFL −  terminate process upon receipt of a signal

Upon receipt of the signal sig, the receiving process is to be terminated with the following consequences:

All of the receiving process’s open file descriptors will be closed. 

If the parent process of the receiving process is executing a wait, it will be notified of the termination of the receiving process and the terminating signal’s number will be made available to the parent process; see wait(2).

If the parent process of the receiving process is not executing a wait, the receiving process will be transformed into a zombie process (see exit(2) for definition of zombie process).

The parent process ID of each of the receiving process’s existing child processes and zombie processes will be set to 1.  This means the initialization process (see intro(2)) inherits each of these processes.

Each attached shared memory segment is detached and the value of shm_nattach in the data structure associated with its shared memory identifier is decremented by 1. 

For each semaphore for which the receiving process has set a semadj value (see semop(2)), that semadj value is added to the semval of the specified semaphore.

If the process has a process, text, or data lock, an unlock is performed (see plock(2)).

An accounting record will be written on the accounting file if the system’s accounting routine is enabled; see acct(2).

If the receiving process’s process ID, tty group ID, and process group ID are equal, the signal SIGHUP will be sent to all of the processes that have a process group ID equal to the process group ID of the receiving process. 

A ‘core image’ will be made in the current working directory of the receiving process if sig is one for which an asterisk appears in the above list and the following conditions are met:

The effective user ID and the real user ID of the receiving process are equal. 

An ordinary file named core exists and is writable or can be created.  If the file must be created, it will have the following properties:

a mode of 0666 modified by the file creation mask (see umask(2))

a file owner ID that is the same as the effective user ID of the receiving process

a file group ID that is the same as the effective group ID of the receiving process

SIG_IGN −  ignore signal

The signal sig is to be ignored. 

Note: the signal SIGKILL cannot be ignored. 

function address −  catch signal

Upon receipt of the signal sig, the receiving process is to execute the signal-catching function pointed to by func. The signal number sig will be passed as the only argument to the signal-catching function.  Before entering the signal-catching function, the value of func for the caught signal will be set to SIG_DFL unless the signal is SIGILL, SIGTRAP, or SIGPWR. 

Upon return from the signal-catching function, the receiving process will resume execution at the point it was interrupted. 

When a signal that is to be caught occurs during a read, a write, an open, or an ioctl system call on a slow device (like a terminal; but not a file), during a pause system call, or during a wait system call that does not return immediately due to the existence of a previously stopped or zombie process, the signal-catching function will be executed and then the interrupted system call will return a −1 to the calling process with errno set to EINTR. 

Note: the signal SIGKILL cannot be caught. 

A call to signal cancels a pending signal sig except for a pending SIGKILL signal. 

Signal will fail if one or more of the following are true:

Sig is an illegal signal number, including SIGKILL.  ­[EINVAL]

Func points to an illegal address.  ­[EFAULT]

RETURN VALUE

Upon successful completion, signal returns the previous value of func for the specified signal sig. Otherwise, a value of −1 is returned and errno is set to indicate the error. 

SEE ALSO

kill(1), kill(2), pause(2), ptrace(2), wait(2), setjmp(3C). 

WARNING

Two other signals that behave differently than the signals described above exist in this release of the system; they are:

SIGCLD18death of a child (reset when caught)
SIGPWR19power fail (not reset when caught)

There is no guarantee that, in future releases of the UNIX System, these signals will continue to behave as described below; they are included only for compatibility with other versions of the UNIX System.  Their use in new programs is strongly discouraged. 

For these signals, func is assigned one of three values: SIG_DFL, SIG_IGN, or a function address. The actions prescribed by these values of are as follows:

SIG_DFL - ignore signal

The signal is to be ignored. 

SIG_IGN - ignore signal

The signal is to be ignored.  Also, if sig is SIGCLD, the calling process’s child processes will not create zombie processes when they terminate; see exit(2).

function address - catch signal

If the signal is SIGPWR, the action to be taken is the same as that described above for func equal to function address. The same is true if the signal is SIGCLD except, that while the process is executing the signal-catching function any received SIGCLD signals will be queued and the signal-catching function will be continually reentered until the queue is empty. 

The SIGCLD affects two other system calls (wait(2), and exit(2)) in the following ways:

wait If the func value of SIGCLD is set to SIG_IGN and a wait is executed, the wait will block until all of the calling process’s child processes terminate; it will then return a value of −1 with errno set to ECHILD. 

exit If in the exiting process’s parent process the func value of SIGCLD is set to SIG_IGN, the exiting process will not create a zombie process. 

When processing a pipeline, the shell makes the last process in the pipeline the parent of the proceeding processes.  A process that may be piped into in this manner (and thus become the parent of other processes) should take care not to set SIGCLD to be caught. 

BUGS

If a repeated signal arrives before the last one can be reset, there is no chance to catch it. 

The type specification of the routine and its func argument are problematical. 

The symbols sighnd and sigtrap are globally defined symbols used by signal(2) and are reserved words.

Version 2.1  —  January 02, 1985