Writing to a pipe in a child process called by execl - c

Question

I don't understand how to use the created pipe from mp.c in the child process sp.c. I (think I) can't seem to access proper file descriptor when using execl for outside process.
/***************mp.c*****************/
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>
int main(int argc, char *argv[]) {
char *procpath = "/mypath/sp";
char *procname = "sp";
pid_t pid;
int fd[2];
int ret;
char buf[20];
memset(&buf[0], 0, sizeof(buf));
ret = pipe(fd);
if(ret == -1){
perror("pipe");
exit(1);
}
pid = fork();
printf("%d\n",pid);
if (pid == 0){
//dup2(mypipefd[1],STDOUT_FILENO);
ret = execl(procpath, procpath, "1","2",NULL);
perror("execl failed to run slave program");
exit(1);
}
else if (pid > 0){
/* Parent process*/
printf("execl ret val = %d",ret);
printf("Parent process \n");
close(fd[1]);
read(fd[0],buf,15);
// close(fd[1]);
close(fd[0]);
printf("buf: %s TEST\n", buf);
printf("buf: %s TEST\n", buf);
}
else{
printf("call to fork failed, no child\n");
exit(-1);
}
exit(0);
}
and the created process...
/***************sp.c*****************/
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
int main(int argc, char *argv[]){
int ret;
//printf("Child process \n");
int fd[2];
pipe(fd);
//dup2(fd[1],1);
//int out;
/*ret = dup2(fd[1],1);
if (ret = -1){
printf("%s\n", strerror(errno));
};*/
//sprintf()
//printf("%d\n", ret);
//mypipefd = argv[1];
printf("Child process \n");
//close(fd[0]);
write(fd[1], "Hello there!",12);
close(fd[1]);
exit(0);
}

Answer 1

The problem is that you are creating a different pipe in each application. In order to correctly communicate over a pipe, two program should share the same pipe ( one of the file descriptor create by the pipe function).
Basically to solve this problem, you must create the pipe in one application and send the file descriptor to the other program without calling again the system call pipe. A file descriptor can be sent to another process by using a socket unix domain. Look a this post Can I share a file descriptor to another process on linux or are they local to the process?.