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Signal Handling/Catching using C

+2 votes

Signals are software interrupts i.e. ctrl+c ctrl+z etc.

By hitting ctrl+c or somethign similar, a process sends a signal to kill another process etc are all such cases where a process needs to do signal handling. So in summary a robust program need to handle signals because signal deliver asynchronous events to the application.

List of Signals

   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            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 

Note: SIGKILL and SIGSTOP can not be trapped

Trapping a Signal in C

If a process wishes to handle a certain signal(s), the process has to register a signal handling function to the kernel.


void (*signal(int signo, void (*func )(int)))(int);

The signal handler function has void return type and accepts a signal number corresponding to the signal that needs to be handled.

To get the signal handler function registered to the kernel, the signal handler function pointer is passed as second argument to the ‘signal’ function.

Sample Code

#include <stdio.h>
#include <stdlib.h>
#include <signal.h>

// Define the function to be called when ctrl-c (SIGINT) signal is sent to process
signal_callback_handler(int signum)
   printf("Caught signal %d\n",signum);
   // Your Code 

int main()
   // Registering signal 
   if (signal(SIGINT, sig_handler) == SIG_ERR)
     printf("\ncan't catch SIGINT\n");

      printf("Program processing....\n");

Important Point while Handling the Signal
1. Signal Catching Functions should be Reentrant to avoid the signal generation by the different thread when one thread is handling the signal.
2. There may be a need to different signal handling behavior for different thread i.e. For example, a thread A can choose to ignore a particular signal but a thread B can choose to catch the same signal. In this case the request made by thread A gets overruled by thread B’s request as signals are delivered only to a single thread in any process. (exception: hardware exceptions or the timer expiry). Use posix APIs like pthread_sigmask() to counter this shortcoming.

Comments are welcome

posted May 27, 2014 by Salil Agrawal

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