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

According to the C99 Rationale:

The C89 Committee concluded that about the only thing a strictly conforming program can do in a signal handler is to assign a value to a volatile static variable which can be written uninterruptedly and promptly return.

Non-Compliant Coding Example

err_msg is updated to reflect the SIGINT signal that was encountered. Issues will occur if a SIGINT is generated prior to the malloc of err_msg finishing.

#include <signal.h> 
 
char *err_msg; 
 
void handler() { 
  strcpy(err_msg, "SIGSEGVSIGINT encountered.");
} 
 
int main() { 
  signal(SIGSEGVSIGINT, handler); 

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

  return 0;
}

Compliant Solution

Signal handlers

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

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


  return 0;
}

Risk Assessment

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