Thread: why wrong file descriptor ??

  1. #1
    Registered User
    Join Date
    Aug 2006
    Posts
    17

    Question 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

  2. #2
    and the hat of int overfl Salem's Avatar
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    > 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.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.

  3. #3
    ATH0 quzah's Avatar
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    man 3 popen


    Quzah.
    Hope is the first step on the road to disappointment.

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