SIGACTION(2) System Calls Manual SIGACTION(2)
NAME
sigaction, signal - manage signal state and handlers
SYNOPSIS
#include <signal.h>
int sigaction(int sig, const struct sigaction *act, struct sigaction *oact)
void (*signal(int sig, void (*handler)(int)))(int);
DESCRIPTION
Sigaction() is used to examine, set, or modify the attributes of a signal. The argument sig is the signal in question. The act argument
points to a structure containing the new attributes of the signal, the structure pointed to by oact will receive the old attributes that
were in effect before the call.
The act and oact arguments may be NULL to indicate that either no new attributes are to be set, or that the old attributes are not of
interest.
The structure containing the signal attributes is defined in <signal.h> and looks like this:
struct sigaction {
void (*sa_handler)(int sig);
sigset_t sa_mask;
int sa_flags;
};
The sa_handler field contains the address of a signal handler, a function that is called when the process is signalled, or one of these
special constants:
SIG_DFL Default signal handling is to be performed. This usually means that the process is killed, but some signals may be ignored by
default.
SIG_IGN Ignore the signal.
The sa_mask field indicates a set of signals that must be blocked when the signal is being handled. Whether the signal sig itself is
blocked when being handled is not controlled by this mask. The mask is of a "signal set" type that is to be manipulated by the sigset(3)
functions.
How the signal is handled precisely is specified by bits in sa_flags. If none of the flags is set then the handler is called when the sig-
nal arrives. The signal is blocked during the call to the handler, and unblocked when the handler returns. A system call that is inter-
rupted returns -1 with errno set to EINTR. The following bit flags can be set to modify this behaviour:
SA_RESETHAND Reset the signal handler to SIG_DFL when the signal is caught.
SA_NODEFER Do not block the signal on entry to the handler.
SA_COMPAT Handle the signal in a way that is compatible with the the old signal() call.
The old signal() signal system call sets a signal handler for a given signal and returns the old signal handler. No signals are blocked,
the flags are SA_RESETHAND | SA_NODEFER | SA_COMPAT. New code should not use signal(). Note that signal() and all of the SA_* flags are
Minix extensions.
Signal handlers are reset to SIG_DFL on an execve(2). Signals that are ignored stay ignored.
Signals
Minix knows about the following signals:
signal num notes description
SIGHUP 1 k Hangup
SIGINT 2 k Interrupt (usually DEL or CTRL-C)
SIGQUIT 3 kc Quit (usually CTRL-)
SIGILL 4 kc Illegal instruction
SIGTRAP 5 xkc Trace trap
SIGABRT 6 kc Abort program
SIGFPE 8 k Floating point exception
SIGKILL 9 k Kill
SIGUSR1 10 k User defined signal #1
SIGSEGV 11 kc Segmentation fault
SIGUSR2 12 k User defined signal #2
SIGPIPE 13 k Write to a pipe with no reader
SIGALRM 14 k Alarm clock
SIGTERM 15 k Terminate (default for kill(1))
SIGCHLD 17 pvi Child process terminated
SIGCONT 18 p Continue if stopped
SIGSTOP 19 ps Stop signal
SIGTSTP 20 ps Interactive stop signal
SIGTTIN 21 ps Background read
SIGTTOU 22 ps Background write
SIGWINCH 23 xvi Window size change
The letters in the notes column indicate:
k The process is killed if the signal is not caught.
c The signal causes a core dump.
i The signal is ignored if not caught.
v Only Minix-vmd implements this signal.
x Minix extension, not defined by POSIX.
p These signals are not implemented, but POSIX requires that they are defined.
s The process should be stopped, but is killed instead.
The SIGKILL signal cannot be caught or ignored. The SIGILL and SIGTRAP signals cannot be automatically reset. The system silently
enforces these restrictions. This may or may not be reflected by the attributes of these signals and the signal masks.
Types
POSIX prescribes that <sys/types.h> has the following definition:
typedef int (*sighandler_t)(int)
With this type the following declarations can be made:
sighandler_t sa_handler;
sighandler_t signal(int sig, sighandler_t handler);
This may help you to understand the earlier declarations better. The sighandler_t type is also very useful in old style C code that is
compiled by a compiler for standard C.
SEE ALSO
kill(1), kill(2), pause(2), sigprocmask(2), sigsuspend(2), sigpending(2), sigset(3).
DIAGNOSTICS
Sigaction() returns 0 on success or -1 on error. Signal() returns the old handler on success or SIG_ERR on error. The error code may be:
EINVAL Bad signal number.
EFAULT Bad act or oact addresses.
AUTHOR
Kees J. Bot (kjb@cs.vu.nl)
SIGACTION(2)