very urgent!details about execve()???????

vrdhananjay · 02-08-2005, 10:06 AM

hello,
my app needs gnuplot as a backend.someone suggested that i use execve()...
i was reading the man pages and noticed that execve() does not return on success...
i dont know how to use it yet.....my argument is going to be an array of integers....
i dint get what env[] was for?

can someone please explain me how to use this?in a really simple way...
also i want something that returns on sucess cos' i want to display the graph and then continue with my app?

pls help...project due day after...
thanks
dhananjay

itsme86 · 02-08-2005, 10:13 AM

You'll need to fork() first. Call execve() in the child process and in the parent process use one of the wait() functions to wait for the child to exit.

itsme86 · 02-08-2005, 10:19 AM

More information: You can't pass an array of integers to execve(). It needs to be an array of strings like it says in the man page. You'll need to do some conversion on the array it sounds like.

If you don't want to pass env[] to the program then use execv() instead of exeve().

vrdhananjay · 02-08-2005, 10:19 AM

hey,
thanks for that ....
would you happen to have a snippert of code with both fork() and execve() used....cos i dont know much aboutr these...
thanks
djhananjay

itsme86 · 02-08-2005, 10:26 AM

Here's a simple example:

Code:

itsme@dreams:~/C$ cat p1.c
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>

int main(void)
{
  char *args[] = { "./p2", "beefy", "meat", "is", "good", NULL };

  if(!fork())
    execv("./p2", args);

  puts("Waiting for child to exit...");
  wait(NULL);
  puts("Child exited.");

  return 0;
}

Code:

itsme@dreams:~/C$ cat p2.c
#include <stdio.h>

int main(int argc, char **argv)
{
  puts("Program started. Arguments are:");
  while(*argv)
  {
    printf("  %s\n", *argv);
    argv++;
  }

  return 0;
}

Code:

itsme@dreams:~/C$ ./p1
Waiting for child to exit...
Program started. Arguments are:
  ./p2
  beefy
  meat
  is
  good
Child exited.

AngryLlama · 02-08-2005, 01:10 PM

Just so you understand how fork works: Fork creates a copy of your process then returns zero (0) to the child process and returns nonzero (the child's pid) to the parent. This is how all processes are created in Linux (everything forks off of init pretty much).

I thought it was important to put an exit in the child code to prevent zombies. No?

Code:

if( !fork()) {
  // Child Code
  execv("./p2",args);
  exit(0);
}
 
// Parent Code
...

itsme86 · 02-08-2005, 01:26 PM

Quote:

I thought it was important to put an exit in the child code to prevent zombies. No?

Not really in this case anyway:

Code:

DESCRIPTION
       The exec family of functions replaces the current  process
       image  with  a new process image.  The functions described
       in this  manual  page  are  front-ends  for  the  function
       execve(2).   (See  the manual page for execve for detailed
       information about the replacement of the current process.)

Since the process is replaced with the new image, the exit() call is never reached anyway.

Usually, I think it's wait()'ing for the child to exit that reaps the zombies.

itsme86 · 02-08-2005, 01:39 PM

Quote:

Usually, I think it's wait()'ing for the child to exit that reaps the zombies.

My suspicions are confirmed. Here's an excerpt from man wait:

Code:

DESCRIPTION
       The  wait  function suspends execution of the current pro-
       cess until a child has exited, or until a signal is deliv-
       ered  whose  action is to terminate the current process or
       to call a  signal  handling  function.   If  a  child  has
       already  exited by the time of the call (a so-called "zom-
       bie" process), the function returns immediately.  Any sys-
       tem resources used by the child are freed.

AngryLlama · 02-08-2005, 02:07 PM

Ok that makes sence. The exec() exits the process automatically (assuming the replacing process exits correctly)

However, rdhananjay, just for reference: you will want the exit() if you are forking and not using an exec (or similiar function) in the child code.

Peace

acid_kewpie · 02-08-2005, 03:00 PM

your question is *not* urgent for any of our members who kindly donate their time to LQ.org, plese do not mark threads as urgent in the future. thanks

vrdhananjay · 02-08-2005, 05:04 PM

ll keep that in mind...

vrdhananjay · 02-08-2005, 05:07 PM

This is what i used eventually,seems to work okay....
will replace the return 0; with exit(0) and try it...

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

int main(void)
{ pid_t me, a;
char *args[] = {"/usr/bin/gnuplot","gpl.txt",NULL};

me = fork();
if(!me)
execv("/usr/bin/gnuplot", args);

puts("Waiting for child to exit...");
wait(NULL);
puts("Child exited.");
return 0;
}

thanks guys
dhananjay