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When a signal occurs, the normal flow of control of a program is interrupted. If a signal occurs that is being trapped by a signal handler, that handler is invoked. When it is finished, execution continues at the point at which the signal occurred. This arrangement could cause problems if the signal handler invokes a library function that was being executed at the time of the signal. Since library functions are not guaranteed to be reentrant, they should not be called from a signal handler that returns.

Implementation Details

The And according to the OpenBSD signal() man page

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identifies functions that are either reentrant or not interruptible by

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signals and are

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asynchronous-signal safe.

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Applications may therefore invoke

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them, without restriction, from signal-catching functions

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Base Interfaces:

_exit(), access(), alarm(), cfgetispeed(), cfgetospeed(), cfsetispeed(),
cfsetospeed(), chdir(), chmod(), chown(), close(), creat(), dup(),
dup2(), execle(), execve(), fcntl(), fork(), fpathconf(), fstat(),
fsync(), getegid(), geteuid(), getgid(), getgroups(), getpgrp(),
getpid(), getppid(), getuid(), kill(), link(), lseek(), mkdir(),
mkfifo(), open(), pathconf(), pause(), pipe(), raise(), read(), rename(),
rmdir(), setgid(), setpgid(), setsid(), setuid(), sigaction(),
sigaddset(), sigdelset(), sigemptyset(), sigfillset(), sigismember(),
signal(), sigpending(), sigprocmask(), sigsuspend(), sleep(), stat(),
sysconf(), tcdrain(), tcflow(), tcflush(), tcgetattr(), tcgetpgrp(),
tcsendbreak(), tcsetattr(), tcsetpgrp(), time(), times(), umask(),
uname(), unlink(), utime(), wait(), waitpid(), write().

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Non-Compliant Code Example

This non-compliant code example invokes the longjmp function to transfer control from the signal handler int_handler() back into main(). Once control returns to main(), a potentially non-reentrant system call is made to strcpy().

Non-Compliant Coding Example

Using the longjmp function inside a signal handler is particularly dangerous, as it could call any part of your code.

#include <setjmp.h> 
#include <signal.h> 
 
static jmp_buf env; 
 
void int_handler() { 
  longjmp(env, 1); 
  return;
} 
 
int main(void) { 
  char *foo; 

  signal(SIGINT, int_handler); 
 
  if (setjmp(env) == 0) { 
    foo = malloc(15);
    strcpy(foo, "Nothing yet.");
  }
  else {
    strcpy(foo, "Signal caught.");
  }

  /* main loop which displays foo */

  return 0;
}

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#include <signal.h> 

int interrupted = 0;
 
void int_handler() { 
  interrupted = 1;
  return;
} 
 
int main(void) { 
  char *foo; 

  signal(SIGINT, int_handler); 
    
  foo = malloc(15);
  strcpy(foo, "Nothing yet.");
  
  /* main loop which displays foo */
  if (interrupted == 1) {
    strcpy(foo, "Signal caught.");
  }

  return 0;
}

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