An example of writing to a child process' stdin, in C.

write_to_pipe.c

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

// A minimal-ish example of writing data to a child process' stdin.
int main(int argc, char **argv) {
  char print_buffer[256];
  int stdin_pipe[2];
  int result;
  pid_t child_pid;

  // First, create the pipe.
  result = pipe(stdin_pipe);
  if (result != 0) {
    printf("Failed creating communication pipe: %s\n", strerror(errno));
    return 1;
  }

  // Next, fork.
  child_pid = fork();
  if (child_pid < 0) {
    printf("Fork error: %s\n", strerror(errno));
    return 1;
  }

  if (child_pid == 0) {
    // Before exec'ing, the child must set the receiving end of the pipe to
    // stdin.
    result = dup2(stdin_pipe[0], STDIN_FILENO);
    if (result < 0) {
      printf("Failed setting pipe to stdin descriptor: %s\n", strerror(errno));
      return 1;
    }
    close(stdin_pipe[1]);
    result = execlp("cat", "cat", NULL);
    if (result < 0) {
      printf("Failed exec'ing a process: %s\n", strerror(errno));
    }
  }

  // Now, the parent can write data to its end of the pipe. The child `cat`
  // process should print out this string.
  close(stdin_pipe[0]);
  memset(print_buffer, 0, sizeof(print_buffer));
  snprintf(print_buffer, sizeof(print_buffer), "Hello! %s %f\n", "whatever",
    -1337.12);
  if (write(stdin_pipe[1], print_buffer, strlen(print_buffer) + 1) <= 0) {
    printf("Failed writing to pipe: %s\n", strerror(errno));
    return 1;
  }
  close(stdin_pipe[1]);
  return 0;
}