These are the first two results when I search "fork vs exec" in google.


http://www.yolinux.com/TUTORIALS/ForkExecProcesses.html
http://en.wikipedia.org/wiki/Fork-exec

I found very good explanations in both the links(yolinux is a bit better)

-Sri

On Tue, Feb 7, 2012 at 12:38 AM, Vijay Chauhan [off-list ref]wrote:

Hi List,

I am learning Linux and trying to understand exec and fork function.
execl says that it overlays the running address space. What does it mean?

I created the following program and used top command with
intentionally wrong arguments:

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

int main(){
       int a = -1;
       if(fork()==0){
               printf("Inside child\n");
               printf("child pid=%d, parentid=%d\n", getpid(), getppid());
               execl("/usr/bin/top", "/usr/bin/top", ">/dev/null"
,(char*)0 );
               scanf("inside child provide a %d", &a);
               printf("Inside child a=%d\n", a);
               exit(1);
       } else {
               printf("Inside parent, going to wait\n");
               printf("my pid=%d, parentid=%d\n", getpid(), getppid());
               scanf("input parent %d\n", &a);
               wait(NULL);
               printf("Wait over\n");
               printf("Inside parent a=%d\n", a);
       }
       return 0;
}

When i run this program, it gives following output:
[vijay at localhost]$ ./a.out
Inside parent, going to wait
Inside child
child pid=2775, parentid=2774
my pid=2774, parentid=2681
       top: unknown argument '>'
usage:  top -hv | -bcisSHM -d delay -n iterations [-u user | -U user]
-p pid [,pid ...]

10
Wait over
Inside parent a=-1
[vijay at localhost]$

Why the child scanf and printf not executed?
In the parent program, i expected input parent should be printed. But
it doesnt and just wait for some input. When i entered 10 it resumes
but printing -1 as a value.

Could anyone please help me to understand this behavior? or any good
tutorial or book

Thanks.
Vijay

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SK Malik
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