• The tee user program copies everything from standard input to standard output, making zero or more extra copies in the named files supplied as user program arguments. For example, if the file alphabet.txt contains 27 bytes—the 26 letters of the English alphabet followed by a newline character, then the following would print the alphabet to standard output and to three files named one.txt, two.txt, and three.txt.

myth4> cat alphabet.txt | tee one.txt two.txt three.txt
abcdefghijklmnopqrstuvwxyz
myth4> cat one.txt 
abcdefghijklmnopqrstuvwxyz
myth4> cat two.txt
abcdefghijklmnopqrstuvwxyz
myth4> diff one.txt two.txt
myth4> diff one.txt three.txt
myth4>

• If the file vowels.txt contains the five vowels and the newline character, and tee is invoked as follows, one.txt would be rewritten to contain only the English vowels, but two.txt and three.txt would be left alone.

myth4> more vowels.txt | tee one.txt
aeiou
myth4> more one.txt 
aeiou
myth4> more two.txt 
abcdefghijklmnopqrstuvwxyz
myth4>

• Full implementation of our own t executable is right here.
• Implementation here replicates much of what copy.c does, but it illustrates how you can use low-level I/O to manage many sessions with multiple files. The implementation here is low on error checking, because I want you to focus on the low-level I/O and how it succeeds, not on how it fails.

Implementation of t

static void writeall(int fd, const char buffer[], size_t len) {
  size_t numWritten = 0;
  while (numWritten < len) {
    numWritten += write(fd, buffer + numWritten, len - numWritten);
  }
}

int main(int argc, char *argv[]) {
  int fds[argc];
  fds[0] = STDOUT_FILENO;
  for (size_t i = 1; i < argc; i++)
    fds[i] = open(argv[i], O_WRONLY | O_CREAT | O_TRUNC, 0644);

  char buffer[2048];
  while (true) {
    ssize_t numRead = read(STDIN_FILENO, buffer, sizeof(buffer));
    if (numRead == 0) break;
    for (size_t i = 0; i < argc; i++)
      writeall(fds[i], buffer, numRead);
  }

  for (size_t i = 1; i < argc; i++) close(fds[i]);
  return 0;
}

• Feature 1: Note that argc incidentally provides a count on the number of descriptors that need to be written to.
• Feature 2: STDIN_FILENO is a built-in constant for the number 0, which is the descriptor normally attached to standard input. STDOUT_FILENO is a constant for the number 1, which is the default descriptor bound to standard output.
• Feature 3: I assume all system calls succeed here. I'm not being lazy, I promise. I'm just trying to keep the examples as clear and compact as possible.