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It is possible to safely use the same handler for multiple signals, but doing so increases the likelihood of a security vulnerability. The delivered signal is masked and is not delivered until the registered signal handler exits. However, if this same handler is registered to handle a different signal, execution of the handler may be interrupted by this new signal. If a signal handler is constructed with the expectation that it cannot be interrupted, a vulnerability might exist. To eliminate this attack vector, each signal handler should only be registered to handle only one type of signal.

Non-Compliant Coding Example

...

#include <signal.h> 
#include <stdlib.h>
#include <string.h>
 
volatile sig_atomic_t sig1 = 0;
volatile sig_atomic_t sig2 = 0;
 
void handler(int signum) { 
  if (sig1) {
     sig2 = 1;
  }
  if (signum == SIGUSR1) {
    sig1 = 1;
  }
} 
 
int main(void) {
  signal(SIGUSR1, handler);
  signal(SIGUSR2, handler);

  while (1) {
    if (sig2) break;
    sleep(SLEEP_TIME);
  }
 
  /* ... */

  return 0;
}

The problem with this code is that there is a race condition in the implementation of handler(). If handler() is called to handle SIGUSR1, and is interrupted to handle SIGUSR2, it is possible that sig2 will not be set. This non-compliant code example also violates SIG31-C. Do not access or modify shared objects in signal handlers.

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This compliant solution registers two separate signal handlers to process SIGUSR1 and SIGUSR2. The sig1_handler() handler waits for SIGUSER1. After this signal occurs, the sig2_handler() is registered to handle SIGUSER2. This solution is fully compliant and accomplishing accomplishes the goal of detecting whether one or more SIGUSR1 signals are followed by SIGUSR2.

#include <signal.h> 
#include <stdlib.h>
#include <string.h>
 
volatile sig_atomic_t sig1 = 0;
volatile sig_atomic_t sig2 = 0;
 
void sig1_handler(int signum) { 
  sig1 = 1;
} 
 
void sig2_handler(int signum) { 
  sig2 = 1;
} 

int main(void) {
  signal(SIGUSR1, handler);

  while (1) {
    if (sig1) break;
    sleep(SLEEP_TIME);
  }

  signal(SIGUSR2, handler);
  while (1) {
    if (sig2) break;
    sleep(SLEEP_TIME);
  }

  /* ... */

  return 0;
}

...