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 attri-
butes of a signal. The argument sig is the signal in ques-
tion. 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 sig-
nalled, 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 sig-
nal sig itself is blocked when being handled is not con-
trolled 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 signal arrives. The signal is blocked dur-
ing the call to the handler, and unblocked when the handler
returns. A system call that is interrupted 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 3 extensions.
Signal handlers are reset to SIG_DFL on an execve(2). Sig-
nals that are ignored stay ignored.
Signals
MINIX 3 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 3 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 sys-
tem 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 defin-
ition:
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)