gistfile1.c

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

int
main(void) {
	int p[2], status;
	pid_t child;
	char *argv[2] = { "/usr/bin/wc", NULL };

	if (pipe(p) < 0) {
		err(errno, "pipe");
	}

	if ((child = fork()) < 0) {
		err(errno, "fork");
	}

	if (child) {
		close(p[1]);
		if (dup2(p[0], STDIN_FILENO) < 0) {
			err(errno, "dup2");
		}
		if (execv(argv[0], argv) < 0) {
			err(errno, "execv");
		}
	} else {
		close(p[0]);
		/* Here, you'd read from stdin, filter, then write to p[1] */
		if (write(p[1], "foo\n", 4) < 0) {
			err(errno, "write");
		}
		wait(&status);
		fprintf(stderr, "child exited with status %d\n", status);
	}

	return 0;
}