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Accessing With one exception accessing or modifying shared objects in signal handlers can lead to race conditions, opening up security holes.

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The C99 standard dictates the use of volatile sig_atomic_t. The type of sig_atomic_t is implementation defined, although there are bounding constraints. To be fully compliant, you one can only assign values from 0 to 127, inclusive, to a sig_atomic_t.

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Signal handlers should only unconditionally set and flaga flag of type volatile sig_atomic_t, and then return.

#include <signal.h> 
#include <stdlib.h>
#include <string.h>
 
char *err_msg; 
volatile sig_atomic_t e_flag = 0;
 
void handler() { 
  e_flag = 1;
} 
 
int main() { 
  signal(SIGINT, handler); 

  err_msg = malloc(24);
  strcpy(err_msg, "No errors yet.");
 
  /* main code loop */
  if(e_flag)
    strcpy(err_msg, "SIGINT received.");


  return 0;
}

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