Simple Fork()/execvp()/waitpid() Example

#include <sys/types.h>
#include <sys/wait.h>
#include <stdio.h>
#include <errno.h>

#include <unistd.h>

#define EXEC_FAILED 1
    
    int main(int argc, char *argv[])
    {
      int status;
      int pid;
      char *prog_arv[4];

      /* 
       * Build argument list
       */

       /* Remove the path or make the path correct for your system, as needed */
      prog_argv[0] = "/usr/local/bin/ls";
      prog_argv[1] = "-l";
      prog_argv[2] = "/";
      prog_argv[3] = NULL;

      /*
       * Create a process space for the ls  
       */
      if ((pid=fork()) < 0)
      {
        perror ("Fork failed");
        exit(errno);
      }

      if (!pid)
      {
        /* This is the child, so execute the ls */ 
        execvp (prog_argv[0], prog_argv);
        exit(EXEC_FAILED);
      }

      if (pid)
      {
        /* 
         * We're in the parent; let's wait for the child to finish
         */
        waitpid (pid, NULL, 0); /* Could also be wait(NULL); */
      }
    }
  

Simple Fork()/execlp()/wait() Example

#include <sys/types.h>
#include <sys/wait.h>
#include <stdio.h>
#include <errno.h>
#include <unistd.h>

#define EXEC_FAILED 1
    
    int main(int argc, char *argv[])
    {
      int status;
      int pid;
      char *prog_arv[4];

      /* 
       * Build argument list
       */

       /* Remove the path or make the path correct for your system, as needed */
      prog_argv[0] = "/usr/local/bin/ls";
      prog_argv[1] = "-l";
      prog_argv[2] = "/";
      prog_argv[3] = NULL;

      /*
       * Create a process space for the ls  
       */
      if ((pid=fork()) < 0)
      {
        perror ("Fork failed");
        exit(errno);
      }

      if (!pid)
      {
        /* This is the child, so execute the ls */ 
        execlp (prog_argv[0], prog_argv[0], prog_argv[1], prog_argv[2], NULL);
        exit(EXEC_FAILED);
      }

      if (pid)
      {
        /* 
         * We're in the parent; let's wait for the child to finish
         */
        waitpid (NULL); /* Could also be waitpid(pid, NULL, 0); */
      }
    }
  

Important Things To Remember

• The most common error for programmers new to fork-exec code is fogetting to NULL terminate the argument list within exec_(). Remember: For execv_(), the last element of the argument array needs to be a NULL pointer. For execl_(), the last argument to the target program passed into execl_() needs to be NULL (for certain forms, such as execlp(), there will be additional arguments to the exec, itself, after that.
  
  
• Upon success, exce blows away the current process image -- so it never returns. As a result, if there is code after the exec -- it should be part of the response to the error -- not "general case got this far" code.
  
  
• For the purpose of this course, --always-- call exit() after an exec. Always. Always. Always. When we get to the lab, it'll ensure that you don't "Fork bomb" the cycle servers.
  
  
• The most common reasons for exec to fail are an absent or incorrect path, the wrong file name, the file not being present where expected, and the permissions on the file not allowing for execution by the user running the program.