#include <stdio.h>      /* FILE, sprintf, fgets, puts */
#include <stdlib.h>     /* atoi, exit, EXIT_SUCCESS */
#include <string.h>     /* strtok, strcmp */
#include <sys/types.h>  /* pid_t */
#include <sys/wait.h>   /* WIFEXITED, WEXITSTATUS */

char *procname(pid_t pid)
{
   static char line[133], command[80], *linep, *token, *cmd;
   FILE *fp;
   int status;

   if (0 == pid) return (char *)0;

   sprintf(command, "ps -p %d 2>/dev/null", pid);
   fp = popen(command, "r");
   if ((FILE *)0 == fp) return (char *)0;

   /* read the header line */
   if ((char *)0 == fgets(line, sizeof line, fp))
   {
      pclose(fp);
      return (char *)0;
   }

   /* figure out where the command name is from the column headings.
    * (BSD-ish machines put the COMMAND in the 5th column, while SysV
    * seems to put CMD or COMMAND in the 4th column.)
    */
   for (linep = line; ; linep = (char *)0)
   {
      if ((char *)0 == (token = strtok(linep, " \t\n")))
      {
         pclose(fp);
         return (char *)0;
      }
      if (0 == strcmp("COMMAND", token) || 0 == strcmp("CMD", token))
      { /*  we found the COMMAND column */
         cmd = token;
         break;
      }
   }

   /* read the ps(1) output line */
   if ((char *)0 == fgets(line, sizeof line, fp))
   {
      pclose(fp);
      return (char *)0;
   }

   /* grab the "word" underneath the command heading... */
   if ((char *)0 == (token = strtok(cmd, " \t\n")))
   {
      pclose(fp);
      return (char *)0;
   }

   status = pclose(fp);
   if (!WIFEXITED(status) || 0 != WEXITSTATUS(status)) 
     return (char *)0;

   return token;
}

int main(int argc, char *argv[])
{
   puts(procname(atoi(argv[1])));
   exit(EXIT_SUCCESS);
}