run_inferior.c

// gcc -o run_inferior run_inferior.c -W -Wall

/* A function to execute a child program and feed the input line-by-line back
 * to the calling program. The function mst be called with a callback function
 * that will receive each line as it is read in, a program to execute and the
 * command line arguments for that program.
 *
 * There are two versions of this function: one that takes the command line
 * arguments as variadic parameters delimited with a NULL pointer and one that
 * takes an array of parameters with a length specified for the array.
 */

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

#define MAX_LINE_LEN    ((1024 * 1024) * 10)    // 10MB lines

typedef int (run_inferior_func_t) (const char *, void *);

int run_inferior (run_inferior_func_t *callback,
                  void                *argp,
                  const char          *program,
                  size_t               nargs,
                  char               **argv)
{
   int ret = EXIT_FAILURE;
   char *cmd = NULL;

   char *line = NULL;

   FILE *inf = NULL;

   size_t len = strlen (program);

   if (!(cmd = malloc (len+1)))
      goto errorexit;

   strcpy (cmd, program);

   for (size_t i=0; i<nargs; i++) {
      size_t index = len;
      len += strlen (argv[i]);
      char *tmp = realloc (cmd, len + 2);
      if (!tmp)
         goto errorexit;
      cmd = tmp;
      cmd[index] = ' ';
      len++;
      strcpy (&cmd[index+1], argv[i]);
   }

   // printf ("Executing [%s]\n", cmd);

   if (!(inf = popen (cmd, "r")))
      goto errorexit;

   if (!(line = calloc (MAX_LINE_LEN, 1)))
      goto errorexit;

   size_t index = 0;
   int c = 0;

   while (!feof (inf) && !ferror (inf)  && (c = fgetc (inf))!=EOF) {
      if (c=='\n' || index>=MAX_LINE_LEN) {
         line[index++] = c;
         line[index] = 0;
         if (callback (line, argp) < 0)
            goto errorexit;
         index = 0;
      } else {
         line[index++] = c;
      }
   }

errorexit:
   if (inf)
      pclose (inf);
   free (cmd);
   free (line);
   return ret;
}

int run_inferior_v (run_inferior_func_t *callback,
                  void                  *argp,
                  const char            *program,
                  ...)
{
   int ret = EXIT_FAILURE;
   char **argv = NULL;
   size_t nargs = 0;
   va_list ap, ap_count;

   char *tmp = NULL;
   size_t i = 0;

   va_start (ap, program);
   va_start (ap_count, program);

   do {
      if ((tmp = va_arg (ap_count, char *)))
         nargs++;
   } while (tmp);

   if (!(argv = calloc (nargs + 1, sizeof *argv)))
      goto errorexit;

   do {
      if ((tmp = va_arg (ap, char *)))
         argv[i++] = tmp;
   } while (tmp);

   ret = run_inferior (callback, argp, program, nargs, argv);

   free (argv);

   va_end (ap);
   va_end (ap_count);

errorexit:
   return ret;
}

int main (void)
{
   int ret = EXIT_FAILURE;

   int result = run_inferior_v ((run_inferior_func_t *)fputs, stdout,
                                 "ls", "-l", "-a", "..", NULL);
   printf ("%i\n", result);

   return ret;
}