/* Pipe example
**      This program prints out 10 random numbers in sorted order.
**      The sorting is performed by the /bin/sort program - we pipe 
**      the numbers to this program (with the -n option for sorting numbers). 
**      To do this, we create a child (so that the pipe can be shared) 
**      which becomes (execs) the sort program.
*/

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main(void)
{
/* Two file descriptors - one for reading, one for writing */
    int fd[2];  
    int pid, i, num;
    FILE* output;

    /* Create pipe */
    /* Needs to be before we fork! */
    if(pipe(fd) == -1) {
        perror("Can't create pipe");
        exit(1);
    }
    /* Fork - child and parent now share the same descriptors */
    pid = fork();

    if(pid) {
         /* Parent */
            /* Close the reading end of the pipe */
        close(fd[0]);

        /* Duplicate - stdout  now goes to pipe */
        dup2(fd[1], STDOUT_FILENO);

        /* Don't need two file descriptors - close the duplicate copy */
        close(fd[1]);

        /* Print out some random numbers - output goes to pipe now */
        for(i = 0; i< 10; i++) { 
            num = rand();
            printf("%d\n", num);
        }

        /* Finished writing, close stdout */
        fclose(stdout);

        /* Wait for child to finish */
        waitpid(pid, NULL, 0);

    } else {
            /* Child */
            /* Close the writing end of the pipe */
        close(fd[1]);

        /* Make standard input come from pipe */
        dup2(fd[0], STDIN_FILENO);

        /* Don't need the second file descriptor - they both point the same end of the pipe */
        close(fd[0]);

        /* Exec the sort program... reads from standard input, which is now the pipe, outputs to standard output, which is still the terminal */
        execl("/usr/bin/sort", "sort", "-n", NULL);
    }
}