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This is a discussion on pipe within the C Programming forums, part of the General Programming Boards category; Hey guys, I had to make some changes and how come this always returns 0 characters if I do like ...

  1. #16
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    Hey guys, I had to make some changes and how come this always returns 0 characters if I do like this in the parent:

    [C] #include <stdio.h> #include <stdlib.h> #include <unistd.h> #include <sys/ - Pastebin.com

  2. #17
    and the hat of wrongness Salem's Avatar
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    Please post the code on this site, not some link.
    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.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  3. #18
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    Code:
    #include <stdio.h>
    #include <stdlib.h>
    #include <unistd.h>
    #include <sys/types.h>
    #include <sys/wait.h>
    #include <sys/stat.h>
    #include <fcntl.h>
    #include <string.h>
    
    
    main(int argc, char* argv[])
    {
      int fds[2], pid, i, fd;
      char *pipeBuffer;
      fd = open(argv[1], O_RDWR | O_APPEND);
    
      if(argc != 2)
      {
        printf("Usage: my_program <filname of textfile>\n");
        exit(0);
      }
    
      //create a pipe
      if(pipe(fds) == -1)
      {
        perror("pipe");
        exit(EXIT_FAILURE);
      }
    
      //create a fork
      if((pid = fork()) == -1) 
      {
        perror("fork");
        exit(EXIT_FAILURE);
      }
    
    
      if(pid == 0) 
      {
        close(fds[1]); 
    
    
        int letterCounter = 0;
        int counter = 0;
        printf("\n");
    
    
        //read letters from the pipe
        while(read(fds[0], &pipeBuffer[counter], 1) > 0)
        {
          if(isalpha(pipeBuffer[counter]))
          {
            letterCounter++;
          }
    
    
          counter++;
        }
    
    
        //write number of letters to end of file
        char tmp[1]={0x0};
        sprintf(tmp,"%d", letterCounter);
        write(fd, tmp, strlen(tmp));
    
    
        printf("Letters: %d\n\n", letterCounter);
    
    
        close(fd);
        close(fds[0]);
        _exit(EXIT_SUCCESS);
      }
      else 
      {
        close(fds[0]); 
    
    
        int c;
        FILE *fdx, *fdy;
        fdx = fdopen(fd, "r");
        fdy = fdopen(fds[1], "w");
    
    
        while((c = fgetc(fdx)) != EOF)
        {
          printf("%c", c);
          fputc(c, fdy);
        }
    
    
        printf("\n\n");
    
    
        fclose(fdx);
        fclose(fdy);
    
    
        close(fds[1]); 
        wait(NULL);
        exit(EXIT_SUCCESS);
      }
    }
    Last edited by Gatsu; 03-25-2013 at 01:57 AM.

  4. #19
    Registered User
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    1,066
    Compiling with warnings I get:
    Code:
    $ make foo
    cc -Wall -Wextra -ggdb3    foo.c   -o foo
    foo.c:11:1: warning: return type defaults to ‘int’ [-Wreturn-type]
    foo.c: In function ‘main’:
    foo.c:51:7: warning: implicit declaration of function ‘isalpha’ [-Wimplicit-function-declaration]
    foo.c:13:20: warning: unused variable ‘i’ [-Wunused-variable]
    foo.c:51:28: warning: ‘pipeBuffer’ may be used uninitialized in this function [-Wuninitialized]
    Code:
    char *pipeBuffer;
    ...
    while(read(fds[0], &pipeBuffer[counter], 1) > 0)
    "pipeBuffer" points to anywhere. And indexing it will likely result in a crash with a bigger file.
    But since you just want to read one character after another you don't need to use a buffer. Just do
    Code:
    char c;
    read(fds[0], &c, 1);

    Here's another buffer overrun:
    Code:
    char tmp[1]={0x0};
    sprintf(tmp,"%d", letterCounter);
    How many characters can you store in "tmp"?

    Bye, Andreas
    Salem likes this.

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