Executing a program using fork and exec

Question

I am new to C programming. I am trying to run a program given by the path specified by the user using the fork(), exec(), and waitpid() commands. I have been trying to get this to run correctly for hours now and I keep getting errors I am not sure how to troubleshoot, as soon as I solve one error, new ones arise. I was wondering if someone can help me understand why my implementation does not work smoothly?

Many thanks

#include <stdio.h>
#include <string.h>
#include <stdlib.h>


int main(void)  {
    char command1[256], command2[256], path[556];
    printf("# ");
    scanf("%s", command1);
    scanf("%s", command2);
    scanf("%s", path);
    if(strcmp(command1,"quit")==0)
        exit(0);
    else if(strcmp(command1, "run")==0 && strcmp(command2, "command")==0){

            printf("%s", path);

            pid_t process;

            process = fork();

            //fork error
            if (process < 0){
               perror("fork");
               exit(0);
            }
            else if (process > 0){  //this is the parent process
               execl(path, "sh" , "-c", ":ls -l *.c", 0);
            }
            else {//this is the child process
               waitpid(process); //waits until the program terminates 
            }

    }




return 0;

}

1. #include <unistd.h>, #include <sys/wait.h> and #include <sys/types.h>, then pid_t waitpid(pid_t pid, int *status, int options);. 2. waitpid waits for children to terminate, so you should exec in the child and wait in the parent. — Daniel Fischer
– Daniel Fischer, Commented Feb 8, 2013 at 23:21
@DanielFischer, when I follow step 1, I get an error saying that there is no such file or directory. — AnchovyLegend
– AnchovyLegend, Commented Feb 8, 2013 at 23:24
@MHZ What's your OS? Unless Windows, check the man pages for fork and waitpid. If Windows, iirc, there is no fork. — Daniel Fischer
– Daniel Fischer, Commented Feb 8, 2013 at 23:26

Bob Murphy · Accepted Answer · 2013-02-08 23:22:34Z

2

It looks to me like you have things swapped. With fork/exec, you generally do the exec in the child process, and the waitpid in the parent process.

answered Feb 8, 2013 at 23:22

Bob Murphy

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2 Comments

AnchovyLegend Over a year ago

Thanks for the reply. After swapping, I am still getting an 'Linker Error' error that says that I have an undefined reference to _fork.

Martin R Over a year ago

@MHZ: Windows does not have a fork() function, see my comment above.

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