fork() and exec()

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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:


int main(){
	int a = -1;
		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);
	} else {
		printf("Inside parent, going to wait\n");
		printf("my pid=%d, parentid=%d\n", getpid(), getppid());
		scanf("input parent %d\n", &a);
		printf("Wait over\n");
		printf("Inside parent a=%d\n", a);
	return 0;

When i run this program, it gives following output:
[vijay@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 ...]

Wait over
Inside parent a=-1

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


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