Internal working of system() and evecvp()

Aquarius_Girl · 02-17-2010, 07:11 AM

system() and execvp() both work by creating a new process.

Question:
Do they create new processes by internally calling fork() or there is some other way ?

Kindly guide !

wje_lq · 02-17-2010, 10:27 AM

Quote:

Originally Posted by anishakaul

system() and execvp() both work by creating a new process.

Question:
Do they create new processes by internally calling fork() or there is some other way ?

system() calls fork(). execvp() doesn't call fork(), but that's not because execvp() creates a new process by another way. It's because execvp() doesn't create a new process at all. It uses the old process, taking place of the program which did the execvp().

Aquarius_Girl · 02-17-2010, 10:53 PM

Thanks for the enlightening reply!

Aquarius_Girl · 02-18-2010, 12:23 AM

Well then does it mean execvp() is less costly than system() ?

And does it mean fork() and system() cost the same ?

Then why to use system() when it calls fork() internally, we can use fork() directly ?

wje_lq · 02-18-2010, 04:08 AM

Quote:

Originally Posted by anishakaul

Well then does it mean execvp() is less costly than system() ?

And does it mean fork() and system() cost the same ?

Then why to use system() when it calls fork() internally, we can use fork() directly ?

First, let me correct myself. You may get slightly better performance by using vfork() instead of fork(), but be sure you read the fine print in the man page.

Ignoring that issue, when you want to create a subprocess and have it run something, you can either:

use system(); or
use fork(), and in the child process use one of the members of the exec*() family, such as execvp.

This means that if you're going to use execvp(), you had better use fork() (or vfork()) with it, and not system(), not for performance reasons, but for bug avoidance reasons.

bigearsbilly · 02-18-2010, 05:08 AM

the exec family are the process starters.

everything winds up being exec'd

wje_lq · 02-18-2010, 05:16 AM

Quote:

Originally Posted by bigearsbilly

everything winds up being exec'd

True.

Quote:

Originally Posted by bigearsbilly

the exec family are the process starters.

False.

You can run a program for a while, and then use something from the exec*() family to run a different program, replacing the one you're running, in the same process. You want a different process? Use fork() or vfork() first.