why wrong file descriptor ??

hi friends,
its all code, but not getting why it is telling wrong descriptor.

Code:

      1 //redirecting output of ls -l command to wc, with help of tmp file
      2
      3 #include<stdio.h>
      4 #include<fcntl.h>
      5
      6
      7 void main()
      8     {
      9         int fd ;
     10         switch(fork())
     11             {
     12                 default : break ;
     13
     14                 case -1 : fprintf(stderr, "system error\n") ;
     15                           exit(1) ;
     16                 case 0 :  if((fd = open("tmp", O_CREAT|O_WRONLY,0600\
     17                               )) == -1)
     18                             {
     19                                 perror("open") ;
     20                                 printf("Couldn't create tmp file\n") ;
     21                                 exit(2) ;
     22                             }
     23
     24                          close(1) ; // closing 1 row
     25                          if(dup(fd) != 1)
     26                             {
     27                                 fprintf(stderr, " Dup error...");
     28                                 exit(3) ;
     29
     30                             }
     31                         close(fd) ; // closing the extra file des
     32                         execl("/bin/ls","ls","-l", NULL) ;
     33                         fprintf(stderr,"error exit 4") ;
     34             } // end of switch
     35
     36         switch(fork()) // this fork for wc
     37             {
     38                 case -1 : // if not able to fork
     39                          break;
     40                 case 0 :if((fd = open("tmp", O_CREAT|O_WRONLY,0600\
     41                               )) == -1)
     42                             {
     43                                 perror("open") ;
     44                                 printf("Couldn't create tmp file\n") ;
     45                                 exit(2) ;
     46                             }
     47
     48                         close(0); // freeing the 0th row in file table
     49
     50                         //printf("fd = %d\n", dup(fd)) ;
     51                         //close(0) ;
     52                         if(dup(fd) != 0)
     53                             {
     54                                 fprintf(stderr," wc input error(dup)");
     55                                 exit(4) ;
     56                             }
     57
     58                 //      close(fd) ;
     59                         execl("/usr/bin/wc", "wc",NULL) ;
     60                         fprintf(stderr, "error wc exit 5");
     61                         exit(5) ;
     62             }// end of wc switch
     63
     64
     65
     66         wait(NULL) ;
     67         exit(0) ;
     68     }// end of main

> void main()
http://faq.cprogramming.com/cgi-bin/...&id=1043284376

> fd = open("tmp", O_CREAT|O_WRONLY
Both of them try and create a write_only file.
Unfortunately, one of them is expecting to be able to read the file.

Your understanding of fork() is fundamentally broken. Everything is asynchronous after you call a fork(), so it's quite easy for the "wc" to happen before the "ls" and for you to end up with a problem.

man 3 popen


Quzah.