banan-os/userspace/tee/main.cpp

#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <sys/stat.h>

#define MAX_FILES 20
#define BUF_SIZE 1024

int main(int argc, char** argv)
{
    int files[MAX_FILES] {};
    size_t file_count = 0;

    int arg = 1;

    int oflag = O_WRONLY | O_CREAT;
    if (arg < argc && strcmp(argv[arg], "-a") == 0)
    {
        oflag |= O_APPEND;
        arg++;
    }
    else
    {
        oflag |= O_TRUNC;
    }

    for (int i = arg; i < argc; i++)
    {
        files[file_count] = open(argv[i], oflag, 0644);
        if (files[file_count] == -1)
            perror(argv[i]);
        else
            file_count++;
        
        if (file_count >= MAX_FILES)
        {
            fprintf(stderr, "only up to %d files are supported\n", MAX_FILES);
            break;
        }
    }

    char* buffer = (char*)malloc(BUF_SIZE);
    for (;;)
    {
        ssize_t nread = read(STDIN_FILENO, buffer, BUF_SIZE);
        if (nread == -1)
            perror("stdin");
        if (nread <= 0)
            break;
        write(STDOUT_FILENO, buffer, nread);
        for (size_t i = 0; i < file_count; i++)
            write(files[i], buffer, nread);
    }
    free(buffer);

    if (ferror(stdin)) 
        perror("stdin");

    for (size_t i = 0; i < file_count; i++)
        close(files[i]);

    return 0;
}