if ((pid = fork()) < 0) {
        perror("fork error");
    } else if (pid > 0) {     
        /*parent, I want to send cmd like "ls" to clild */
    } else {                /* child */
        execl("/usr/bin/sh","sh","-c","ssh user@host",NULL);
        /*clild,I want to read cmd like "ls" from parent as the input for ssh, something like `expect` */
    }

#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#include <signal.h>
#include <sys/wait.h>


int main(int argc, char *argv[])
{
	if (argc < 2)
	{
	fprintf(stderr, "%s [exec args ...]\n", argv[0]);
	return 1;
	}

	//create a pipe with separate input and output descriptors

	int pipes[2] = { -1, -1 };
	if (pipe(pipes) != 0)
	{
	fprintf(stderr, "%s: could not create pipes: %s\n", argv[0], strerror(errno));
	return 1;
	}

	//fork in order to exec the new process

	pid_t new_process = -1;
	if ((new_process = fork()) < 0)
	{
	fprintf(stderr, "%s: fork error: %s\n", argv[0], strerror(errno));
	return 1;
	}


	if (!new_process)
	//forked process
	{
	//replace standard input with the input descriptor
	dup2(pipes[0], STDIN_FILENO);

	//close the other descriptors
	//NOTE: with other than standard ..., check to make sure you aren't
	//closing what you just 'dup2'ed over!
	close(pipes[0]);
	close(pipes[1]);

	//pause until signaled
	raise(SIGSTOP);

	//if 'execvp' returns then that's an error
	int outcome = execvp(argv[1], argv + 1);
	fprintf(stderr, "%s: 'execvp' error: %s\n", argv[0], strerror(errno));
	_exit(outcome);
	}


	//parent process

	//wait for the fork(s) to pause

	int status = 0;
	while (waitpid(new_process, &status, WUNTRACED) == 0 && !WIFSTOPPED(status));
	if (!WIFSTOPPED(status))
	{
	fprintf(stderr, "%s: error synchronizing with fork: %s\n", argv[0], strerror(errno));
	kill(new_process, SIGCONT);
	return 1;
	}


	//put the fork(s) in a new process group for group control
	setpgid(new_process, new_process);

	//the input descriptor isn't needed
	close(pipes[0]);

	//open a stream for output to the fork

	FILE *output = fdopen(pipes[1], "w");
	if (!output)
	{
	fprintf(stderr, "%s: internal communication error: %s\n", argv[0], strerror(errno));
	kill(new_process, SIGCONT);
	return 1;
	}

	//resume the process group all at once (will continue all in the group)
	killpg(new_process, SIGCONT);

	char buffer[256];
	setlinebuf(output);

	//read from standard input until the child exits or input is closed

	while (!ferror(output) && !ferror(stdin) && fgets(buffer, sizeof buffer, stdin))
	{
	fprintf(stderr, "forwarding: %s", buffer);
	fprintf(output, "%s", buffer);
	}


	return 0;
}