fork - Can I exit from parent? - c

Question

I have written a code which will call exec as shown below. But if I call isValid from main, both child and parent process are returning and I am getting the output twice.
I want to get the out of the exec and want to check the return value in main only once. which process I need to exit inorder to make this work properly?
int isValid(void)
{
int number, statval;
int child_pid;
child_pid = fork();
if(child_pid == -1) { printf("Could not fork! \n"); exit( 1 ); }
else if(child_pid == 0)
{
execl(...); // Child
}
else
{
// Parent
waitpid( child_pid, &statval, WUNTRACED );
if(WIFEXITED(statval))
{
if (WEXITSTATUS(statval) == 0)
return 1;
else
return 0;
}
else
printf("Child did not terminate with exit\n");
}
return 0;
}
int main(void)
{
if (isValid())
{
printf("Valid\n");
}
else
{
printf("Invalid\n");
}
}

Answer 1

The doubled output may occur when the execl fails. The code above does not check the execl return value. Actually the execl returns only in case of failure so there is no need to check the returned value but it is necessary to handle error anyway.
EDIT:
If the exec fails then:
Child's isValid() returns 0 at the end of the function and the main prints "Invalid"
Parent waits for the child exit and then WIFEXITED is true because child exits and WEXITSTATUS is 0 because the child exits normally. Parent's isValid returns 1 and "Valid" is printed.

Answer 2

#include <sys/shm.h>
int *tabPID;
int isValid(void)
{
int number, statval;
if(fork() == 0){
tabPID[1] = getpid();
execl(...); // Child
return -1;
}
// Parent
tabPID[0]=getpid();
usleep(10);//as Basile Starynkevitch suggests
waitpid(tabPID[1], &statval, WUNTRACED );
if(WIFEXITED(statval))
{
if (WEXITSTATUS(statval) == 0)
return 1;
else
return 0;
}
else
printf("Child did not terminate with exit\n");
return 0;
}
int main(void)
{
shmId = shmget(1234, 2*sizeof(int), IPC_CREAT|0666);
tabPID = shmat(shmId, NULL, 0);
if (isValid())
{
printf("Valid\n");
}
else
{
printf("Invalid\n");
}
}

Answer 3

You probably should do some tiny things (perhaps some usleep(3) for a few milliseconds) after the fork but before the waitpid to have some real chance to get the waitpid(2) call succeed. Otherwise, it could happen that the other process won't be scheduled to run.
BTW, you should test the return value from waitpid, .e.g. code
statval = 0;
pid_t wpid = waitpid(child_pid, &statval, WUNTRACED );
if (wpid>0) {
assert (wpid == child_pid);
if (WIFEXITED(statval)) {
if (WEXITSTATUS(statval) == 0)
return 1;
At last, regarding the title of your question, read job control wikipage, read several chapters of Advanced Linux Programming, and consider using daemon(3)
Also, just after the execl be sure to put
perror("execl");
exit(EXIT_FAILURE);
to handle the rare case of execl failure.