execvp and fork problems

Posted on 2011-03-20
Last Modified: 2013-12-17
Hi all,
Can anyone explain the use of execvp, fork and dup2?
As i need to understand how this func works first in order to write a program that will output each 'command' (separated by pipes) with a separate process along with its process id.
we'll need two codes to get this compiled, one is given which is the makeargv.c (to make the commands into token form) and we should write another one which i've stated above.

the makeargv.c (given)
#include "makeargv.h"
#include <string.h>
#include <stdlib.h>
* Make argv array (*arvp) for tokens in s which are separated by
* delimiters.   Return -1 on error or the number of tokens otherwise.
int makeargv(char *s, char *delimiters, char ***argvp)
  char *t;
  char *snew;
  int numtokens;
  int i;
   /* snew is real start of string after skipping leading delimiters */
  snew = s + strspn(s, delimiters);
                             /* create space for a copy of snew in t */
  if ((t = calloc(strlen(snew) + 1, sizeof(char))) == NULL) {
     *argvp = NULL;
     numtokens = -1;
  } else {                     /* count the number of tokens in snew */
     strcpy(t, snew);
     if (strtok(t, delimiters) == NULL)
        numtokens = 0;
        for (numtokens = 1; strtok(NULL, delimiters) != NULL;
               /* create an argument array to contain ptrs to tokens */
     if ((*argvp = calloc(numtokens + 1, sizeof(char *))) == NULL) {
        numtokens = -1;
     } else {            /* insert pointers to tokens into the array */
        if (numtokens > 0) {
           strcpy(t, snew);
           **argvp = strtok(t, delimiters);
           for (i = 1; i < numtokens + 1; i++)
              *((*argvp) + i) = strtok(NULL, delimiters);
        } else {
          **argvp = NULL;
  return numtokens;

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and this should what i should be writing to output each 'command' (separated by pipes) with a separate process along with its process id. I'm not sure how should i write this code, this is just a start for my code.
#include <sys/types.h>
#include <fcntl.h>
#include <stdlib.h>
#include <stdio.h>

extern makeargv (char *, char *, char ***)

int main ()
          char buf[80];
          int i, j;
          char **argp;
          int pid;
          write(1,"prompt ",7);
          fgets(buf, 80, stdin);
          j = makeargv(buf, "|\n", &argp);
          pid = fork();
          for (i=0, i<j, i++)
                printf("%s process ID: %d",argp[i], pid);
          execvp(argp[i], &argp[i]);


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eg: ls -l | cmd1 | cmd2

output should be something like that:
ls -l process ID 41
cmd1 process ID 42
cmd2 process ID 43

Any help will be greatly appreciated:)
Question by:crazy4s
  • 2
LVL 11

Accepted Solution

DeepuAbrahamK earned 500 total points
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Author Comment

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I've edited some of my codes for the  output each 'command' (separated by pipes) with a separate process along with its process id.

#include <string.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <unistd.h>
#include "ourhdr.h"

extern int makeargv(char *, char * , char ***);

int main()
        int i,j,vpret;
        char buf[80];   // command line
        pid_t pid;      // process ids
        while(strcmp(fgets(buf, 80, stdin), "exit\n")!=0)  // USER COMMAND
                j=makeargv(buf,"|",&argp); // J = NUMBER OF TOKENS
                for (i=0;i<j;i++)
                pid = fork();
                switch (pid)
                        case -1:
                                err_sys("fork failed");
                        case 0: // child
                                printf("%s process ID: %d\n", argp[i], pid);
                        default: // parent
                                waitpid(pid, NULL, 0);
        printf(" make argv test exited\n"); //successfully exit

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however my output is just like this:

bronco:~> gcc test.c makeargv.c error.c
bronco:~> ./a.out
ls -l | cat test.c | cat test2.c
ls -l
 cat test.c
 cat test2.c

Prompt>ls -l | cat test.c | cat test2.c
ls -l
 cat test.c
 cat test2.c

 make argv test exited

Can somebody help me what should i do in order to output the process along its ids???
thanks in advance:)

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