SIGACTION(2)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);
DESCRIPTIONSigaction() is used to examine, set, or modify the attributes of a sig‐
nal. 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 inter‐
est.
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 func‐
tion that is called when the process is signalled, or one of these spe‐
cial 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 signal
arrives. The signal is blocked during 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). Signals that are
ignored stay ignored.
Signals
MINIX 3 knows about the following signals:
signal num notes description
SIGHUP 1 km Hangup
SIGINT 2 k Interrupt (usually DEL or CTRL-C)
SIGQUIT 3 kcm Quit (usually CTRL-\)
SIGILL 4 Kc Illegal instruction
SIGTRAP 5 Kc Trace trap
SIGABRT 6 kcm Abort program
SIGBUS 7 Kc Bus error
SIGFPE 8 Kc 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 km Terminate (default for kill(1))
SIGEMT 16 xKc Emulator trap
SIGCHLD 17 pi Child process terminated
SIGCONT 18 pi Continue if stopped
SIGSTOP 19 ps Stop signal
SIGTSTP 20 ps Interactive stop signal
SIGWINCH 21 xi Window size change
SIGTTIN 22 ps Background read
SIGTTOU 23 ps Background write
SIGVTALRM 24 k Virtual alarm clock
SIGPROF 25 k Profiler alarm clock
The letters in the notes column indicate:
k The process is killed if the signal is not caught.
K The process is killed if the signal is not caught. If the signal
is received due to an exception while ignored or masked, the
process is killed even if a handler is defined to catch the sig‐
nal.
c The signal causes a core dump.
i The signal is ignored if not caught.
m The signal is converted to a message for system processes.
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 and SIGSTOP signals cannot be caught or ignored. The SIG‐
ILL 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 com‐
piled by a compiler for standard C.
SEE ALSOkill(1), kill(2), pause(2), sigprocmask(2), sigsuspend(2), sigpend‐
ing(2), sigset(3).
DIAGNOSTICSSigaction() 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)
