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This non-compliant program registers a single signal handler to process both SIGUSR1 and SIGUSR2. The variable sig2 should be set to one if one or more SIGUSR1 signals are followed by SIGUSR2. a Note that this 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;
  }
  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.

Compliant Solution

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 the goal of detecting 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) { 
  ifsig1 (sig1) {
     sig2 = 1;
  }
  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;
}

Risk Assessment

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