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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
 
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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?
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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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