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Exceptions

SIG31-EX1:  The consensus formed at the April 2008 meeting of ISO/IEC WG14 was that it is acceptable to read a variable of type volatile sig_atomic_t as well as write it, and that doing so was not a problem on any known implementation. In this compliant solution, a variable of type volatile sig_atomic_t is read and then written before returning from the signal handler.

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

volatile sig_atomic_t e_flag = 0;

void handler(int signum) {
  if (0 == e_flag) {
    e_flag = 1;
  }
}

int main(void) {
  enum { MAX_MSG_SIZE = 24 }; 
  char *err_msg = (char *)malloc(MAX_MSG_SIZE);
  if (err_msg == NULL) {
    /* Handle error condition */
  }

  signal(SIGINT, handler);
  strcpy(err_msg, "No errors yet.");

  /* Main code loop */

  if (e_flag) {
    strcpy(err_msg, "SIGINT received.");
  }
  return 0;
}

SIG31-EX2:  The C Standard in subclause 7.14.1.1 paragraph 5 makes a special exception for errno when a valid call to the signal() function results in a SIG_ERR return, allowing errno to take an indeterminate value. See ERR32-C. Do not rely on indeterminate values of errno.

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