c - System call fork() and execv function

Question

I'm trying to run two executables consecutively using this c code:

#include <stdio.h>
#include <unistd.h>

int main (int argc, char *argv[])
{
    fork();
    execv("./prcs1", &argv[1]); // GIVE ADDRESS OF 2nd element as starting point to skip source.txt
    fork();
    execv("./prcs2", argv);
    printf("EXECV Failed
");
}

The program exits after the first execv() call despite the fork, it never gets to the second execv(). I've tried calling wait() after the first fork but I'm not sure that's what it's missing.

Any ideas why control doesn't return to the parent after the child exits?

Answer 1

You need to understand how fork and execv work together.

• fork() makes a duplicate of the current process, returning 0 to child, childpid to parent
• fork() can fail, and returns -1 on failure, check for that
• execv() replaces the duplicated parent process with a new process
• typical fork/exec pairing replaces the child process with a new process
• often you fork more than one child, and want them to run simultaneously,
• however, you asked for them to run consecutively, that is one after another
• thus, you need to wait for the first to complete before starting the second
• thus you need to use some variant of wait(), example below uses waitpid() to wait for specific child

You need stdlib for exit (in case execv fails), and errno, to print the reason,

//I'm trying to run two executables consecutively using this c code:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>

You may want to examine the reason your child exited (core dump, signal, normal exit), thus I have added this function,

#include <sys/types.h>
#include <sys/wait.h>

//WIFEXITED(status) returns true if the child terminated normally, that is, by calling exit(3) or _exit(2), or by returning from main().
//WEXITSTATUS(status) returns the exit status of the child.  This consists of the least significant 8 bits of the status argument that the child specified in a call to exit(3) or _exit(2) or as the argument for a return statement in main().  This macro should only be employed if WIFEXITED returned true.
//WIFSIGNALED(status) returns true if the child process was terminated by a signal.
//WTERMSIG(status) returns the number of the signal that caused the child process to terminate.  This macro should only be employed if WIFSIGNALED returned true.
//WCOREDUMP(status) returns true if the child produced a core dump.  This macro should only be employed if WIFSIGNALED returned true.  This macro is not specified in POSIX.1-2001 and is not available on some UNIX implementations (e.g., AIX, SunOS).  Only use this enclosed in #ifdef WCOREDUMP ... #endif.
//WIFSTOPPED(status) returns true if the child process was stopped by delivery of a signal; this is only possible if the call was done using WUNTRACED or when the child is being traced (see ptrace(2)).
//WSTOPSIG(status) returns the number of the signal which caused the child to stop.  This macro should only be employed if WIFSTOPPED returned true.
//WIFCONTINUED(status) (since Linux 2.6.10) returns true if the child process was resumed by delivery of SIGCONT.
int
exitreason(pid_t cid, int status)
{
    if( WIFEXITED(status) )
    {
        printf("child %d terminated normally, that is, by calling exit(3) or _exit(2), or by returning from main().
",cid);
        if( WEXITSTATUS(status) )
        {
        printf("child %d exit status %d.  This consists of the least significant 8 bits of the status argument that the child specified in a call to exit(3) or _exit(2) or as the argument for a return statement in main().
",cid,WEXITSTATUS(status));
        }
    }
    if( WIFSIGNALED(status) )
    {
        printf("child %d process was terminated by a signal.
",cid);
        if( WTERMSIG(status) )
        {
        printf("child %d signal %d that caused the child process to terminate.
",cid,WTERMSIG(status));
        }
        if( WCOREDUMP(status) )
        {
        printf("child %d produced a core dump.  WCOREDUMP() is not specified in POSIX.1-2001 and is not available on some UNIX implementations (e.g., AIX, SunOS).  Only use this enclosed in #ifdef WCOREDUMP ... #endif.
",cid);
        }
    }
    if( WIFSTOPPED(status) )
    {
        printf("child %d process was stopped by delivery of a signal; this is only possible if the call was done using WUNTRACED or when the child is being traced (see ptrace(2)).
",cid);
        if( WSTOPSIG(status) )
        {
        printf("child %d number of the signal which caused the child to stop.
",cid);
        }
    }
    if( WIFCONTINUED(status) )
    {
        printf("child %d process was resumed by delivery of SIGCONT.
");
    }
}

And here is your program annotated with comments explaining which sections of code are processed by the parent, and which by the child(ren).

int main (int argc, char *argv[])
{
    char proc1[] = "/bin/echo"; //"./prcs1";
    char proc2[] = "/bin/echo"; //"./prcs2";
    pid_t cid1, cid2, cidX;
    int status=0;
    int waitoptions = 0;
    //WNOHANG    return immediately if no child has exited.
    //WUNTRACED  also return if a child has stopped (but not traced via ptrace(2)).  Status for traced children which have stopped is provided even if this option is not specified.
    //WCONTINUED also return if a stopped child has been resumed by delivery of SIGCONT.
    int res;

    if( (cid1 = fork()) == 0 ) //child1
    {
        printf("in child1
");
        if( (res = execv(proc1, &argv[1])) < 0 ) // GIVE ADDRESS OF 2nd element as starting point to skip source.txt
        {
        printf("error: child1: %d exec failed %d
", cid1, errno);
        printf("error: cannot execv %s
",proc1);
        exit(91); //must exit child
        }
    }
    else if( cid1 > 0 ) //cid>0, parent, waitfor child
    {
        cidX = waitpid(cid1, &status, waitoptions);
        printf("child1: %d res %d
", cid1, res);
        exitreason(cid1, status);
    }
    else //cid1 < 0, error
    {
        printf("error: child1 fork failed
");
    }

    if( (cid2 = fork()) == 0 ) //child2
    {
        printf("in child2
");
        if( (res = execv(proc2, &argv[1])) < 0 ) // GIVE ADDRESS OF 2nd element as starting point to skip source.txt
        {
        printf("error: child2: %d exec failed %d
", cid2, errno);
        printf("error: cannot execv %s
",proc2);
        exit(92); //must exit child
        }
    }
    else if( cid2 > 0 ) //cid>0, parent, waitfor child
    {
        cidX = waitpid(cid2, &status, waitoptions);
        printf("child2: %d res %d
", cid2, res);
        exitreason(cid2, status);
    }
    else //cid2 < 0, error
    {
        printf("error: child2 fork failed
");
    }
}