How to execute the command ls|sort -r in C using pipe and fork()?

Question

I was trying to solve this problem for school project but I couldn't understand how to open and close the read and write within the pipe: the question is: Create a C program that executes the following command ls| sort -r using pipe and creation of a fork() and dup()

Answer 1

I don't normally condone the posting of solutions to homework problems, but there's enough bad code available on the internet that I thought I would post what I consider to be not terrible code. Perhaps I'm being presumptuous to think that, but:

/* Execute ls | sort -r */

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

static void xpipe(int *fd) { if( pipe(fd) == -1 ){ err(1, "pipe"); } }
static void xdup2(int a, int b) { if( dup2(a, b) == -1 ){ err(1, "dup2"); } }
static void xclose(int fd) { if( close(fd) == -1 ){ err(1, "close"); } }

static void
execute(int *fd, char **cmd, int w)
{
        switch( fork() ){
        case -1:
                err(EXIT_FAILURE, "fork");
        case 0:
                xdup2(fd[w], w);
                xclose(fd[0]);
                xclose(fd[1]);
                execvp(cmd[0], cmd);
                perror("execvp");
                exit(EXIT_FAILURE);
        }
}

int
main(void)
{
        int rv = EXIT_SUCCESS;
        char *ls[] = { "ls", NULL };
        char *sort[] = { "sort", "-r", NULL };
        int fd[2];
        xpipe(fd);
        execute(fd, ls, 1);
        execute(fd, sort, 0);
        xclose(fd[0]);
        xclose(fd[1]);
        for( int i = 0; i < 2; i++ ){
                int status;
                wait(&status);
                if( ! WIFEXITED(status) || WEXITSTATUS(status) ){
                        rv = EXIT_FAILURE;
                }
        }
        return rv;
}

Answer 2

int main(int argc, char*argv[])
{
int fd[2];
pid_t p1,p2;
pipe(fd);
if (pipe(fd)==-1){
perror("Erreur pipe");
exit();
}

p1=fork();
if (p1==-1){
perror("Erreur fork");
exit();
}
else if (p1==0) {
close(fd[0]);
dup2(fd[1],1);
execlp("ls","ls",argv[1],0);
}

p2=fork();
if (p2==-1){
perror("Erreur fork");
exit();
}
else if (p2==0){
close(fd[1]);
dup2(fd[0],0);
execlp("sort","-r",NULL);
}
waitpid(p1,nullptr,0);
close(fd[1]);
waitpid(p2,nullptr,0);
return 0;
}
// can you have a look at this and tell me if it would work?