tee stdin into a process and stdout

ptee.c

#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>

#ifndef BUFSIZ
#define BUFSIZ 1024
#endif

int main(int argc, char *argv[]) {
  if (argc < 2)
    return 1;

  int p[2];
  pipe(p);

  pid_t child = fork();
  if (child == 0) {
    close(p[1]);
    dup2(p[0], STDIN_FILENO);
    execvp(argv[1], argv + 1);
    _exit(127);
  }

  char buf[BUFSIZ];
  ssize_t bytes;
  while ((bytes = read(STDIN_FILENO, buf, BUFSIZ)) != 0) {
    write(STDOUT_FILENO, buf, bytes);
    write(p[1], buf, bytes);
  }

  close(p[1]);

  int status;
  waitpid(child, &status, 0);

  return status;
}