"ls | sort" with pipes and forks?

This is a discussion on "ls | sort" with pipes and forks? within the C Programming forums, part of the General Programming Boards category; I'm practicing with exec and pipes and forks but I can't seem to understand how to have "sort" sort the ...

  1. #1
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    "ls | sort" with pipes and forks?

    I'm practicing with exec and pipes and forks but I can't seem to understand how to have "sort" sort the output of "ls". I'm actually unsure of how to do this. I have my code below.

    Code:
    #include <stdio.h>
    #include <unistd.h>
    #include <stdlib.h>
    #include <sys/wait.h>
    
    int main(int argc, char** argv) {
    
        int fd[2];
        int pid;
        char read_buf[READ_SIZE + 1];
        int chars_read;
    
        /*
         * Check for adequate args.
         */
        if (argc < 1) {
            fprintf(stderr, "usage: grab-stdout prog arg ...");
            exit(-1);
        }
    
        /*
         * Create a pipe that will have its output written to by the executed
         * program.
         */
        pipe(fd);
    
        /*
         * Fork off a process to exec the program.
         */
        if ((pid = fork()) < 0) {
            perror("fork");
      exit(-1);
        }
    
        /*
         * Forked child has its stdout be the write end of the pipe.
         */
        else if (pid == 0) {
    
            /*
             * Don't need read end of pipe in child.
             */
            close(fd[0]);
    
            /*
             * This use of dup2 makes the output end of the pipe be stdout.
             */
            dup2(fd[1], STDOUT_FILENO);
    
            /*
             * Don't need fd[1] after the dup2.
             */
            close(fd[1]);
    
            /*
             * Exec the program given in the command line, including any args.
             */
    
            execlp("ls", "ls", 0);
            
            perror("exec1");
            exit(-1);
        }
    
        /*
         * Parent takes its input from the read end of the pipe.
         */
        else {
            /*
             * Don't need write end of pipe in parent.
             */
            close(fd[1]);
    
            //I'm unsure of what to do here but this is all I could think of
            execlp("sort", "sort", "-r", fd[0] );
    
            perror("exec2");
            exit(-1);
    
        }
    
        exit(0);
    }
    Thanks for any input.

  2. #2
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    * Parent takes its input from the read end of the pipe.
    . . .
    * Don't need write end of pipe in parent.
    OMG! Thanks for the laugh!

  3. #3
    and the hat of wrongness Salem's Avatar
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    > execlp("sort", "sort", "-r", fd[0] );
    Well you use the same dup2 (for stdin of course) and then
    execlp("sort", "sort", "-r" );
    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.

  4. #4
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    ok i got it, Thanks Salem!
    Last edited by alwut; 01-09-2008 at 02:08 AM. Reason: not up to date

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