signal(4)signal(4)NAME
signal, signal.h - Contains definitions and variables used by signal
functions
DESCRIPTION
The /usr/include/signal.h file defines the signals described in the
following table.
───────────────────────────────────────────────────────────────────
Signal Number Meaning
───────────────────────────────────────────────────────────────────
SIGHUP 1 Hangup.
SIGINT 2 Interrupt.
SIGQUIT 3 Quit. (1)
SIGILL 4 Invalid instruction (not reset when caught).
(1)
SIGTRAP 5 Trace trap (not reset when caught). (1)
SIGABRT 6 End process (see the abort() function). (1)
SIGEMT 7 EMT instruction.
SIGFPE 8 Arithmetic exception, integer divide by 0
(zero), or floating-point exception. (1)
SIGKILL 9 Kill (cannot be caught or ignored).
SIGBUS 10 Specification exception. (1)
SIGSEGV 11 Segmentation violation. (1)
SIGSYS 12 Invalid parameter to system call. (1)
SIGPIPE 13 Write on a pipe when there is no process to
read it.
SIGALRM 14 Alarm clock.
SIGTERM 15 Software termination signal.
SIGURG 16 Urgent condition on I/O channel. (2)
SIGSTOP 17 Stop (cannot be caught or ignored). (3)
SIGTSTP 18 Interactive stop. (3)
SIGCONT 19 Continue if stopped. (4)
SIGCHLD 20 To parent on child stop or exit. (2)
SIGTTIN 21 Background read attempted from control termi‐
nal. (3)
SIGTTOU 22 Background write attempted from control ter‐
minal. (3)
SIGIO 23 Input/Output possible or completed. (2)
SIGXCPU 24 CPU time limit exceeded (see the setrlimit()
function).
SIGXFSZ 25 File size limit exceeded (see the setrlimit()
function).
SIGVTALRM 26 Virtual time alarm (see the setitimer() func‐
tion).
SIGPROF 27 Profiling time alarm (see the setitimer()
function).
SIGWINCH 28 Window size change. (2)
SIGINFO 29 Information request. (2)
SIGUSR1 30 User-defined signal 1.
SIGUSR2 31 User-defined signal 2.
───────────────────────────────────────────────────────────────────
Notes to table:
Default action includes creating a core dump file. Default action is
to ignore these signals. Default action is to stop the process receiv‐
ing these signals. Default action is to restart or continue the
process receiving these signals.
The three types of actions that can be associated with a signal:
SIG_DFL, SIG_IGN, or a pointer to a function are described as follows:
Default action: signal-specific default action.
Except for those signal numbers marked with a (2), (3), or (4),
the default action for a signal is to end the receiving process
with all of the consequences described in the _exit() system
call. In addition, a memory image file is created in the current
directory of the receiving process if the signal parameter is
one for which a superscript 1 appears in the preceding list and
the following conditions are met: The effective user ID and the
real user ID of the receiving process are equal. An ordinary
file named core exists in the current directory and is writable,
or it can be created. If the file must be created, it will have
the following properties: The access permission code 0600, modi‐
fied by the file creation mask (see the umask() function). A
file owner ID that is the same as the effective user ID of the
receiving process. A file group ID that is inherited from the
containing directory (if the file system is mounted grpid) or
from the owning process (if the file system is mounted nogrpid).
For signal numbers marked with a superscript 4, the default
action is to restart the receiving process if it is stopped, or
to continue execution of the receiving process.
For signal numbers marked with a superscript 3, the default
action is to stop the execution of the receiving process tempo‐
rarily. When a process stops, a SIGCHLD signal is sent to its
parent process, unless the parent process has set the SA_NOCLD‐
STOP bit. While a process is stopped, any additional signals
that are sent to the process are not delivered until the process
is continued. An exception to this is SIGKILL, which always
terminates the receiving process. Another exception is SIGCONT,
which always causes the receiving process to restart or continue
running. A process whose parent has ended shall be sent a
SIGKILL signal if the SIGTSTP, SIGTTIN, or SIGTTOU signals are
generated for that process.
For signal numbers marked with a superscript 2, the default
action is to ignore the signal. In this case, delivery of the
signal has no effect on the receiving process.
If a signal action is set to SIG_DFL while the signal is pend‐
ing, the signal remains pending. Ignore signal.
Delivery of the signal has no effect on the receiving process.
If a signal action is set to SIG_IGN while the signal is pend‐
ing, the pending signal is discarded.
Note that the SIGKILL and SIGSTOP signals cannot be ignored.
Catch signal.
Upon delivery of the signal, the receiving process is to run the
signal-catching function specified by the pointer to function.
The signal-handler subroutine can be declared as follows:
void handler(signal) int signal;
The signal parameter is the signal number.
A new signal mask is calculated and installed for the duration of the
signal-catching function (or until sigprocmask() or sigsuspend() system
calls are made). This mask is formed by taking the union of the process
signal mask, the mask associated with the action for the signal being
delivered, and a mask corresponding to the signal being delivered. The
mask associated with the signal-catching function is not allowed to
block those signals that cannot be ignored. This is enforced by the
kernel without causing an error to be indicated. If and when the sig‐
nal-catching function returns, the original signal mask is restored
(modified by any sigprocmask() calls that were made since the signal-
catching function was called) and the receiving process resumes execu‐
tion at the point it was interrupted.
The signal-catching function can cause the process to resume in a dif‐
ferent context by calling the longjmp() subroutine. When the longjmp()
subroutine is called, the process leaves the signal stack, if it is
currently on it, and restores the process signal mask to the state when
the corresponding setjmp() call was made.
Once an action is installed for a specific signal, it remains installed
until another action is explicitly requested (by another call to the
sigaction() system call), or until one of the exec system calls is
called.
If a signal action is set to a pointer to a function while the signal
is pending, the signal remains pending.
When signal-catching functions are invoked asynchronously with process
execution, the behavior of some of the functions defined by this stan‐
dard is unspecified if they are called from a signal-catching function.
The following set of functions are reentrant with respect to signals
(that is, applications can invoke them, without restriction, from sig‐
nal-catching functions):
_exit()access()alarm()chdir()chmod()chown()close()creat()dup2()dup()exec()fcntl()fork()fstat()getegid()geteuid()getgid()getgroups()getpgrp()getpid()getppid()getuid()kill()link()lseek()mkdir()mkfifo()open()pause()pipe()read()rename()rmdir()sem_post()setgid()setpgrp()setuid()sigaction()sigaddset()sigdelset()sigfillset()siginitset()sigismember()signal()sigpending()sigprocmask()sigsuspend()sleep()statx()tcdrain()tcflow()tcflush()tcgetattr()tcgetprgp()tcsendbreak()tcsetattr()tcsetpgrp()time()times()umask()uname()unlink()ustat()utime()wait2()wait()write()
All other system calls should not be called from signal-catching func‐
tions since their behavior is undefined.
SEE ALSO
Functions: sigaction(2), sigblock(2), sigemptyset(3), siginterrupt(3),
siglongjmp(3), sigpause(3), sigpending(2), sigprocmask(2), sigre‐
turn(2), sigset(3), sigsetjmp(3), sigstack(2), sigsuspend(2),
sigvec(2), sigwait(3)signal(4)
