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Top Forums Programming C function to start process but to return right away Post 302428030 by Corona688 on Tuesday 8th of June 2010 03:13:25 PM
Old 06-08-2010
fork and exec. fork() copies your process but returns a different value to the copy so it can take a new direction. exec() replaces it with a different program. the original program can do other things and/or wait for the child to complete. see man fork, man execv, man wait.

Code:
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>

int main(void)
{
  pid_t pid=fork();
  if(pid < 0)
  {
    perror("Couldn't fork");
    exit(1);
  }
  else if(pid == 0) // child code
  {
    execv("/bin/echo", "/bin/echo", "asdf", NULL);
    // this should never happen since execv REPLACES the process that calls it
    exit(1);
  }
  else // parent code
  {
    int status;
    // do other stuff.  child will keep running.
    sleep(5);

    // wait for child to complete
    wait(&status);
    printf("child returned %d\n", WEXITSTATUS(status));
    return(0);
  }
}

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WAIT(2) 							System Calls Manual							   WAIT(2)

NAME
wait - wait for a process to exit SYNOPSIS
#include <u.h> #include <libc.h> int wait(Waitmsg *w) DESCRIPTION
Wait causes a process to wait for any child process (see fork(2)) to exit. It returns the pid of a child that has exited and fills in w with more information about the child. W points to a Waitmsg, which has this structure: typedef struct Waitmsg { char pid[12]; /* of loved one */ char time[3*12]; /* of loved one & descendants */ char msg[ERRLEN]; } Waitmsg; Pid is the child's process id. The time array contains the time the child and its descendants spent in user code, the time spent in system calls, and the child's elapsed real time, all in units of milliseconds. All integers in a Waitmsg are formatted as right-justified textual numbers in 11-byte fields followed by a blank. Msg contains the message that the child specified in exits(2). For a normal exit, msg[0] is zero, otherwise msg is prefixed by the process name, a blank, the process id, and a colon. If there are no more children to wait for, wait returns immediately, with return value -1. SOURCE
/sys/src/libc/9syscall SEE ALSO
fork(2), exits(2), the wait file in proc(3) DIAGNOSTICS
Sets errstr. WAIT(2)
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