A copy of the command "tee" implemented using unix system calls.

tee.c

#include <unistd.h>
#include <fcntl.h>
#include <string.h>

#define MAX_READ 1024
#define MAX_OPEN 128

void printerr(char*);

int main(int argc, char *argv[]) { 
	int opt, i;
	ssize_t bytes_read;
	char buff[MAX_READ];
	int fds[MAX_OPEN]; 
	char* fnames[MAX_OPEN];

	int flags = O_WRONLY | O_CREAT | O_TRUNC;
	int opened = 0;
	const mode_t perms = S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH;

	while ((opt = getopt(argc, argv, "a")) != -1)
		switch (opt) {
		case 'a':
			flags ^= O_TRUNC;
			flags |= O_APPEND;
		}

	/* open the files */
	for (i = optind; i < argc; i++)
		if (opened < MAX_OPEN) {
			fnames[opened] = argv[i];
			fds[opened] = open(fnames[opened], flags, perms);
			if (fds[opened] == -1) 
				printerr(strcat(fnames[opened], 
					": can't be opened or created"));
			else
				opened++;
		} else {
			printerr(strcat(argv[i], 
			": too many files, skipping this and the rest"));
			break;
		}

	/* read and write */
	while((bytes_read = read(STDIN_FILENO, buff, MAX_READ)) > 0) {
		if (write(STDOUT_FILENO, buff, bytes_read) < bytes_read)
			printerr("error while writing to standard output");
		for (i = 0; i < opened; i++)
			if (write(fds[i], &buff, bytes_read) < bytes_read)
				printerr(strcat(fnames[i], 
				": couldn't write some data"));
	}

	/* close the files */
	for (i = 0; i < opened; i++)
		if (close(fds[i]) == -1)
			printerr(strcat(fnames[i], 
				": couldn't close, some data may be lost"));

	if (bytes_read == -1)
		return 1;
	return 0;
}

void printerr(char* msg)
{
	msg = strcat(msg, "\n");
	write(STDERR_FILENO, msg, strlen(msg));
}