Fork child process terminates with signal 0 - c

Question

I have a small function that should give me information about how a child process finished.
int debug_wait() {
int status;
wait(&status);
if (WIFSIGNALED(status)) {
int sig = WSTOPSIG(status);
printf("failed with signal %d (%s)\n", sig, strsignal(sig));
return 1;
}
else if (!WIFEXITED(status)) {
printf("ended in an unexpected way\n");
return 1;
}
return 0;
}
But I get the following result:
double free or corruption (out)
tests failed with signal 0 (Unknown signal 0)
I know I should fix my code, but why do I get signal no. 0? Is there a mistake in my function or does it have an other meaning?
Since people asked, here is a example program:
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <string.h>
int debug_wait() {
int status;
wait(&status);
if (WIFSIGNALED(status)) {
int sig = WSTOPSIG(status);
printf("tests failed with signal %d (%s)\n", sig, strsignal(sig));
return 1;
}
else if (!WIFEXITED(status)) {
printf("tests ended in an unexpected way\n");
return 1;
}
return 0;
}
int main() {
void* ptr = malloc(10);
pid_t pid = fork();
if (pid == 0) {
free(ptr);
free(ptr);
}
else {
debug_wait();
}
}

Answer 1

You are decoding the wrong thing. If WIFSIGNALED is true you can use WTERMSIG and WCOREDUMP (check #ifdef WCOREDUMP first). To decude the WSTOPSIG then WIFSTOPPED must be true.
Example:
int status;
pid_t pid = wait(&status);
if (pid == -1) return 1;
if (WIFEXITED(status)) {
printf("normal exit %d\n", WEXITSTATUS(status));
} else if (WIFSIGNALED(status)) {
printf("termsig %d core dump=%c\n", WTERMSIG(status),
WCOREDUMP(status) ? 'Y' : 'N');
} else if (WIFSTOPPED(status)) {
printf("stop %d cont=%c\n", WSTOPSIG(status),
WIFCONTINUED(status) ? 'Y' : 'N');
}
Without using the feature test macros:
if (WIFEXITED(status)) {
printf("normal exit %d\n", WEXITSTATUS(status));
} else if (WIFSIGNALED(status)) {
printf("termsig %d\n", WTERMSIG(status));
} else if (WIFSTOPPED(status)) {
printf("stop %d\n", WSTOPSIG(status));
}