Father-child process use pipe to talk, hangs after "execlp", why? - linux

Question

I've got a simple text file called "tmp" under current directory, I wish to "cat" this file and then "sort" it, I want to use a c program to act like pipe "|" so I tried to use a father/child talk to do this.
Unexpectedly, the program hangs after "cat", like below:
#include<stdlib.h>
#include<stdio.h>
#include<unistd.h>
int main(){
int pipefd[2];
pipe(pipefd);
int& readfd=pipefd[0];
int& writefd=pipefd[1];
pid_t pid=fork();
if(pid==0){//child
dup2(STDIN_FILENO,writefd);
close(readfd);
execlp("cat","cat","tmp",NULL);
printf("child cat ends\n");
exit(0);
}else{//father
dup2(STDOUT_FILENO,readfd);
close(writefd);
execlp("sort","sort",NULL);
printf("father sort ends\n");
}
int status;
wait(&status);
printf("father exists\n");
return 0;
}
g++ to compile and run this file, after "cat" tihis file, I don't even see "child cat ends", it just hangs.
Where's the problem, how to fix it?
Thanks

Answer 1

1) The order of arguments in dup2 is incorrect. Look at dup2
2) parameters (stdin/stdout) to dup2 are incorrect.
3) The exec() family of functions replace the process image with a new one. So the code after that call does not get to run (unless the exec() fails), so I removed those.
Here is the code:
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
int main(){
int pipefd[2];
pipe(pipefd);
int& readfd = pipefd[0];
int& writefd = pipefd[1];
pid_t pid = fork();
if(pid == 0){ //child
dup2(writefd, 1); // 1 is STDOUT_FILENO -- cat already has input -- needs output
close(readfd);
execlp("cat","cat","tmp.txt", NULL);
perror("execlp() failed in child");
}else{ //father
dup2(readfd, 0); // 0 is STDIN_FILENO -- because sort needs input!
close(writefd);
execlp("sort","sort", NULL);
perror("execlp() failed in parent");
}
return 0;
}