signal(2)

1SIGNAL(2)                  Linux Programmer's Manual                 SIGNAL(2)
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NAME

6       signal - ANSI C signal handling
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SYNOPSIS

9       #include <signal.h>
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11       typedef void (*sighandler_t)(int);
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13       sighandler_t signal(int signum, sighandler_t handler);
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DESCRIPTION

16       The  signal()  system call installs a new signal handler for the signal
17       with number signum.  The signal handler is set to handler which may  be
18       a user specified function, or either SIG_IGN or SIG_DFL.
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20       Upon  arrival of a signal with number signum the following happens.  If
21       the corresponding handler  is  set  to  SIG_IGN,  then  the  signal  is
22       ignored.   If  the  handler  is set to SIG_DFL, then the default action
23       associated with the signal (see signal(7))  occurs.   Finally,  if  the
24       handler  is  set to a function handler then first either the handler is
25       reset to SIG_DFL or an implementation-dependent blocking of the  signal
26       is performed, and then handler is called with argument signum.
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28       Using  a  signal  handler function for a signal is called "catching the
29       signal".  The signals SIGKILL and SIGSTOP cannot be caught or ignored.
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RETURN VALUE

32       The signal() function returns the previous value of the signal handler,
33       or SIG_ERR on error.
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PORTABILITY

36       The original Unix signal() would reset the handler to SIG_DFL, and Sys‐
37       tem V (and the Linux kernel and libc4,5) does the same.  On  the  other
38       hand,  BSD does not reset the handler, but blocks new instances of this
39       signal from occurring during a call of the handler.  The glibc2 library
40       follows the BSD behaviour.
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42       If  one on a libc5 system includes <bsd/signal.h> instead of <signal.h>
43       then signal() is redefined as  __bsd_signal  and  signal  has  the  BSD
44       semantics. This is not recommended.
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46       If  one  on  a  glibc2  system  defines  a  feature  test macro such as
47       _XOPEN_SOURCE or uses a  separate  sysv_signal  function,  one  obtains
48       classical behaviour. This is not recommended.
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50       Trying  to change the semantics of this call using defines and includes
51       is not a good idea. It is better to avoid signal() altogether, and  use
52       sigaction(2) instead.
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NOTES

55       The effects of this call in a multi-threaded process are unspecified.
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57       The  routine  handler  must be very careful, since processing elsewhere
58       was interrupted at some arbitrary point. POSIX has the concept of "safe
59       function".   If  a  signal  interrupts  an unsafe function, and handler
60       calls an unsafe function, then the behavior is  undefined.  Safe  func‐
61       tions are listed explicitly in the various standards.  The POSIX.1-2003
62       list is
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64       _Exit() _exit()  abort()  accept()  access()  aio_error()  aio_return()
65       aio_suspend()  alarm() bind() cfgetispeed() cfgetospeed() cfsetispeed()
66       cfsetospeed() chdir() chmod() chown() clock_gettime() close() connect()
67       creat() dup() dup2() execle() execve() fchmod() fchown() fcntl() fdata‐
68       sync()  fork()  fpathconf()  fstat()  fsync()   ftruncate()   getegid()
69       geteuid()  getgid()  getgroups() getpeername() getpgrp() getpid() getp‐
70       pid()  getsockname()  getsockopt()  getuid()  kill()  link()   listen()
71       lseek()  lstat()  mkdir()  mkfifo()  open()  pathconf()  pause() pipe()
72       poll() posix_trace_event() pselect() raise() read()  readlink()  recv()
73       recvfrom()   recvmsg()  rename()  rmdir()  select()  sem_post()  send()
74       sendmsg() sendto() setgid() setpgid()  setsid()  setsockopt()  setuid()
75       shutdown()   sigaction()  sigaddset()  sigdelset()  sigemptyset()  sig‐
76       fillset() sigismember() signal() sigpause() sigpending()  sigprocmask()
77       sigqueue()  sigset()  sigsuspend() sleep() socket() socketpair() stat()
78       symlink() sysconf() tcdrain() tcflow()  tcflush()  tcgetattr()  tcgetp‐
79       grp()  tcsendbreak()  tcsetattr() tcsetpgrp() time() timer_getoverrun()
80       timer_gettime()  timer_settime()  times()  umask()   uname()   unlink()
81       utime() wait() waitpid() write().
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83       According  to  POSIX,  the behaviour of a process is undefined after it
84       ignores a SIGFPE, SIGILL, or SIGSEGV signal that was not  generated  by
85       the  kill(2)  or  the raise(3) functions.  Integer division by zero has
86       undefined result.  On some architectures it will generate a SIGFPE sig‐
87       nal.   (Also  dividing  the  most  negative  integer by -1 may generate
88       SIGFPE.)  Ignoring this signal might lead to an endless loop.
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90       See sigaction(2) for details on what happens when  SIGCHLD  is  set  to
91       SIG_IGN.
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93       The  use  of sighandler_t is a GNU extension.  Various versions of libc
94       predefine this  type;  libc4  and  libc5  define  SignalHandler,  glibc
95       defines sig_t and, when _GNU_SOURCE is defined, also sighandler_t.
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CONFORMING TO

98       C89, C99, POSIX.1-2001.
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