How to get the return value of a program ran via calling a member of the exec family of functions?

Question

I know that it is possible to read commands output with a pipe? But what about getting return value ? For example i want to execute:

execl(\"/bin/ping\", \"/         


        

Answer 1

You can wait on the child process and get its exit status. The system call is wait(pid), try to read about it.

Answer 2

Had trouble understanding and applying the existing answers.

In AraK's answer, if the application has more than one child process running, it is not possible to know which specific child process produced the exit status obtained. According the man page,

wait() and waitpid()

The wait() system call suspends execution of the calling process until one of its children terminates. The call wait(&status) is equivalent to:

       waitpid(-1, &status, 0);

   The **waitpid()** system call suspends execution of the calling process until a **child specified by pid** argument has changed state.

So, to obtain the exit status of a specific child process we should rewrite the answer as :

#include <stdio.h>
#include <unistd.h>
#include <sys/wait.h>
int main()
{
    int number, statval;
    int child_pid;
    printf("%d: I'm the parent !\n", getpid());
    child_pid = fork();
    if(child_pid == -1)
    { 
        printf("could not fork! \n");
        exit( 1 );
    }
    else if(child_pid == 0)
    {
        execl("/bin/ping", "/bin/ping" , "-c", "1", "-t", "1", ip_addr, NULL);
    }
    else
    {
        printf("PID %d: waiting for child\n", getpid());
        waitpid( child_pid, &statval, WUNTRACED
                    #ifdef WCONTINUED       /* Not all implementations support this */
                            | WCONTINUED
                    #endif
                    );
        if(WIFEXITED(statval))
            printf("Child's exit code %d\n", WEXITSTATUS(statval));
        else
            printf("Child did not terminate with exit\n");
    }
    return 0;
}

Feel free to turn this answer to an edit of AraK's answer.

Answer 3

exec function familly does not return, the return int is here only when an error occurs at launch time (like not finding file to exec).

You have to catch return value from the signal sent to the process that forked before calling exec.

call wait() or waitpid() in your signal handler (well, you can also call wait() in your process without using any signal handler if it has nothing else to do).

Answer 4

You can use waitpid to get the exit status of you child process as:

int childExitStatus;
waitpid( pID, &childExitStatus, 0); // where pID is the process ID of the child.

Answer 5

Here is an example I wrote long time ago. Basically, after you fork a child process and you wait its exit status, you check the status using two Macros. WIFEXITED is used to check if the process exited normally, and WEXITSTATUS checks what the returned number is in case it returned normally:

#include <stdio.h>
#include <unistd.h>
#include <sys/wait.h>
int main()
{
    int number, statval;
    printf("%d: I'm the parent !\n", getpid());
    if(fork() == 0)
    {
        number = 10;
        printf("PID %d: exiting with number %d\n", getpid(), number);
        exit(number) ;
    }
    else
    {
        printf("PID %d: waiting for child\n", getpid());
        wait(&statval);
        if(WIFEXITED(statval))
            printf("Child's exit code %d\n", WEXITSTATUS(statval));
        else
            printf("Child did not terminate with exit\n");
    }
    return 0;
}