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Execvp function

Posted on 2009-07-06
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Last Modified: 2013-12-26
I have written  a program to parse user commands and send them to execvp to execute, which it does, but after the execvp nothing happens. I want it to print out a message, but it just gets stuck. Am I missing something after the execvp call?
#include <stdio.h>
#include <string.h>
 
int main(int argc, char *argv[])
{
char comm[100];
char *pnt[100];
int i=0;
int pid;
int status;
 
if (argc==1 ||  argc >2){
printf("Incorrect number of arguments. Exiting\n");
exit(0);
}
printf("%s ", argv[1]);
gets(comm);
 
pnt[i] = strtok (comm," ");
 
while (pnt[i] != NULL){
pnt[++i]=strtok(NULL," ");
}
if ((pid = fork()) < 0){
printf("Error unable to fork");
exit(1);
}
else if (pid == 0){
int count=1;
printf("Executing command no:%d\n",count);
execvp(*pnt,pnt);
printf("Command no. %d was executed\n",count);
count++;}
else{ while (waitpid(pid));
printf("Would you like to enter another command?");
exit(0);
}
}

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Question by:wannabe_cpgeek
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6 Comments
 
LVL 40

Accepted Solution

by:
mrjoltcola earned 250 total points
ID: 24791319
One thing you must understand about the exec*() functions. When you call it, the program you call actually _replaces_ the running program, in memory, so no lines of code after the exec() call will be executed, unless exec() failed.

To kick off a shell command like that you can use popen() or fork() + exec()

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Author Comment

by:wannabe_cpgeek
ID: 24791338
So is there a way I can go back to my program and run the other commands?
0
 
LVL 40

Expert Comment

by:mrjoltcola
ID: 24791371
use popen() to run the commands. That opens the called program as a process and you can read from the "stream" just like reading from a FILE * (actually popen returns a FILE*).

Read the man page on popen or google for a sample.

0
 

Author Comment

by:wannabe_cpgeek
ID: 24791466
Thank you, but my assignment saids I have to use execvp, so is there a way using it?
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LVL 12

Assisted Solution

by:Let_Me_Be
Let_Me_Be earned 250 total points
ID: 24792067
The answer was already supplied in the first reply. If you want to execute another process running in parallel with the original one, you must use a combination of fork and exec. Fork will create a copy of your running program, exec with replace the running program (use the child for this) with a new one.

Here is a more detailed overview: http://www.yolinux.com/TUTORIALS/ForkExecProcesses.html
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