SIGNAL(7) Linux Programmer’s Manual SIGNAL(7)
NAME
signal - overview of signals
DESCRIPTION
Linux supports both POSIX reliable signals (hereinafter "standard signals") and POSIX real-time signals.
Signal Dispositions
Each signal has a current disposition, which determines how the process behaves when it is delivered the signal.
The entries in the "Action" column of the tables below specify the default disposition for each signal, as follows:
Term Default action is to terminate the process.
Ign Default action is to ignore the signal.
Core Default action is to terminate the process and dump core (see core(5)).
Stop Default action is to stop the process.
Cont Default action is to continue the process if it is currently stopped.
A process can change the disposition of a signal using sigaction(2) or (less portably) signal(2). Using these system calls, a process
can elect one of the following behaviors to occur on delivery of the signal: perform the default action; ignore the signal; or catch
the signal with a signal handler, a programmer-defined function that is automatically invoked when the signal is delivered. (By
default, the signal handler is invoked on the normal process stack. It is possible to arrange that the signal handler uses an alter-
nate stack; see sigaltstack(2) for a discussion of how to do this and when it might be useful.)
The signal disposition is a per-process attribute: in a multithreaded application, the disposition of a particular signal is the same
for all threads.
A child created via fork(2) inherits a copy of its parent’s signal dispositions. During an execve(2), the dispositions of handled
signals are reset to the default; the dispositions of ignored signals are left unchanged.
Sending a Signal
The following system calls and library functions allow the caller to send a signal:
raise(3) Sends a signal to the calling thread.
kill(2) Sends a signal to a specified process, to all members of a specified process group, or to all processes on the system.
killpg(2) Sends a signal to all of the members of a specified process group.
pthread_kill(3) Sends a signal to a specified POSIX thread in the same process as the caller.
tgkill(2) Sends a signal to a specified thread within a specific process. (This is the system call used to implement
pthread_kill(3).)
sigqueue(2) Sends a real-time signal with accompanying data to a specified process.
Waiting for a Signal to be Caught
The following system calls suspend execution of the calling process or thread until a signal is caught (or an unhandled signal termi-
nates the process):
pause(2) Suspends execution until any signal is caught.
sigsuspend(2) Temporarily changes the signal mask (see below) and suspends execution until one of the unmasked signals is caught.
Synchronously Accepting a Signal
Rather than asynchronously catching a signal via a signal handler, it is possible to synchronously accept the signal, that is, to
block execution until the signal is delivered, at which point the kernel returns information about the signal to the caller. There
are two general ways to do this:
* sigwaitinfo(2), sigtimedwait(2), and sigwait(3) suspend execution until one of the signals in a specified set is delivered. Each of
these calls returns information about the delivered signal.
* signalfd(2) returns a file descriptor that can be used to read information about signals that are delivered to the caller. Each
read(2) from this file descriptor blocks until one of the signals in the set specified in the signalfd(2) call is delivered to the
caller. The buffer returned by read(2) contains a structure describing the signal.
Signal Mask and Pending Signals
A signal may be blocked, which means that it will not be delivered until it is later unblocked. Between the time when it is generated
and when it is delivered a signal is said to be pending.
Each thread in a process has an independent signal mask, which indicates the set of signals that the thread is currently blocking. A
thread can manipulate its signal mask using pthread_sigmask(3). In a traditional single-threaded application, sigprocmask(2) can be
used to manipulate the signal mask.
A child created via fork(2) inherits a copy of its parent’s signal mask; the signal mask is preserved across execve(2).
A signal may be generated (and thus pending) for a process as a whole (e.g., when sent using kill(2)) or for a specific thread (e.g.,
certain signals, such as SIGSEGV and SIGFPE, generated as a consequence of executing a specific machine-language instruction are
thread directed, as are signals targeted at a specific thread using pthread_kill(3)). A process-directed signal may be delivered to
any one of the threads that does not currently have the signal blocked. If more than one of the threads has the signal unblocked,
then the kernel chooses an arbitrary thread to which to deliver the signal.
A thread can obtain the set of signals that it currently has pending using sigpending(2). This set will consist of the union of the
set of pending process-directed signals and the set of signals pending for the calling thread.
A child created via fork(2) initially has an empty pending signal set; the pending signal set is preserved across an execve(2).
Standard Signals
Linux supports the standard signals listed below. Several signal numbers are architecture-dependent, as indicated in the "Value" col-
umn. (Where three values are given, the first one is usually valid for alpha and sparc, the middle one for ix86, ia64, ppc, s390, arm
and sh, and the last one for mips. A - denotes that a signal is absent on the corresponding architecture.)
First the signals described in the original POSIX.1-1990 standard.
Signal Value Action Comment
----------------------------------------------------------------------
SIGHUP 1 Term Hangup detected on controlling terminal
or death of controlling process
SIGINT 2 Term Interrupt from keyboard
SIGQUIT 3 Core Quit from keyboard
SIGILL 4 Core Illegal Instruction
SIGABRT 6 Core Abort signal from abort(3)
SIGFPE 8 Core Floating point exception
SIGKILL 9 Term Kill signal
SIGSEGV 11 Core Invalid memory reference
SIGPIPE 13 Term Broken pipe: write to pipe with no
readers
SIGALRM 14 Term Timer signal from alarm(2)
SIGTERM 15 Term Termination signal
SIGUSR1 30,10,16 Term User-defined signal 1
SIGUSR2 31,12,17 Term User-defined signal 2
SIGCHLD 20,17,18 Ign Child stopped or terminated
SIGCONT 19,18,25 Cont Continue if stopped
SIGSTOP 17,19,23 Stop Stop process
SIGTSTP 18,20,24 Stop Stop typed at tty
SIGTTIN 21,21,26 Stop tty input for background process
SIGTTOU 22,22,27 Stop tty output for background process
The signals SIGKILL and SIGSTOP cannot be caught, blocked, or ignored.
<snip>
CONFORMING TO
POSIX.1, except as noted.
BUGS
SIGIO and SIGLOST have the same value. The latter is commented out in the kernel source, but the build process of some software still
thinks that signal 29 is SIGLOST.
SEE ALSO
kill(1), getrlimit(2), kill(2), killpg(2), setitimer(2), setrlimit(2), sgetmask(2), sigaction(2), sigaltstack(2), signal(2), sig-
nalfd(2), sigpending(2), sigprocmask(2), sigqueue(2), sigsuspend(2), sigwaitinfo(2), abort(3), bsd_signal(3), longjmp(3), raise(3),
sigset(3), sigsetops(3), sigvec(3), sigwait(3), strsignal(3), sysv_signal(3), core(5), proc(5), pthreads(7)
COLOPHON
This page is part of release 3.22 of the Linux man-pages project. A description of the project, and information about reporting bugs,
can be found at http://www.kernel.org/doc/man-pages/.
Linux 2008-10-15 SIGNAL(7)
Author
Topic: HG612 modem stats - multiple instances v2 - halts DSLstats (Read 13672 times)