.TH SIGNAL 2
.UC 4
.SH NAME
signal, sigaction, sigsuspend, sigpending, sgetmask, ssetmask \- signal handling functions.
.SH SYNOPSIS
.nf
.B #include <signal.h>
.B #include <unistd.h>
.B int sigaction(int signum, const struct sigaction *new,
.ti 21
.B struct sigaction *old);
.B void (*signal(int signum, (void *handler)(int))))(int)
.B sigsuspend(int restart, sigset_t old_mask, sigset_t set);
.B int sigpending(sigset_t *buf);
.B sigset_t sgetmask (void);
.B sigset_t ssetmask (sigset_t mask);
.fi
.SH DESCRIPTION
.B sgetmask()
and 
.B ssetmask()
get and set the mask of blocked signals.
Signals are bitmapped into this mask as 1 << (signum - 1).
Both return the old value of the mask.  Note that 
.B SIGKILL 
and 
.B SIGSTOP
cannot be blocked.
.PP
.B sigpending()
fills in 
.I *buf
with which signals are pending but blocked. 0 is 
returned.
.PP
.B sigsuspend()
is used to atomically swap out the signal mask,
while waiting for a signal.
If the 
.I restart
flag is set, then the current mask is replaced with 
old_mask, and sigsuspend exists with a 
.B -EINTR.  
Otherwise, the current signal mask is set to set, a 
.B pause()
system call performed, and 
.B -ERESTARTNOINTR 
is returned.
.PP
.B signal() 
is used to set up a signal handler for 
.I signum.
The signal handler is set to 
.I handler,
which may be a user specified function or one of these from signal.h:
.br
.B SIG_DFL     
- use the default handler.
.br
.B SIG_IGN
- ignore signal.  
.PP
Note that 
.B signal() 
sets up signals to be SA_ONESHOT and SA_NOMASK.
This may be changed by a call to 
.B sigaction().
.PP
Also note that SIG_KILL and SIG_STOP cannot be changed.
.PP
The address of the handler is returned on success, a negative value on error.
.PP
.B sigaction()
is similar to a more general version of 
.B signal().
It can be used to  set the entire 
.B sigaction 
structure for a signal.  
.PP
If 
.I old
is non null, the old action will be stored there.
The action will then be set to
.I new.
.bp
.PP
The 
.BB sigaction 
structure is as defined in /usr/include/signal.h :
.br
.nf
struct sigaction {
.in 24
void (*sa_handler)(int);
sigset_t sa_mask;
int sa_flags;
void (*sa_restorer)(void);
};
.in 10
.fi
.PP
0 is returned on success.  Note that 
.B SIGKILL 
and 
.B SIGSTOP
may not have signals set for them.
.SH ERRORS
.B -EINVAL 
is returned when an invalid signum is passed to the functions.
.SH FILES
linux/signal.h
.br
/usr/include/linux/sys.h
.br
/usr/include/unistd.h
.SH SEE ALSO
kill(2), raise(3)
.SH BUGS
.B signal() 
and the code in sched.c,
signal.c cannot correctly handle a SIG_DFL handler.