serenity/Userland/tail.cpp

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <errno.h>
#include <getopt.h>
#include <unistd.h>
#include <AK/Assertions.h>
#include <LibCore/CFile.h>

static void print_usage_and_exit()
{
    printf("usage: tail [-f, -n <linecount>] <filename>\n");
    exit(1);
}

int tail_from_pos(CFile& file, off_t startline, bool want_follow)
{
    if (!file.seek(startline + 1))
        return -1;

    while (true) {
        const auto& b = file.read(4096);
        if (b.is_empty()) {
            if (!want_follow) {
                break;
            } else {
                while (!file.can_read()) {
                    // FIXME: would be nice to have access to can_read_from_fd with an infinite timeout
                    usleep(100);
                }
                continue;
            }
        }

        if (write(STDOUT_FILENO, b.pointer(), b.size()) < 0) {
            return -1;
        }
    }

    return 0;
}

off_t find_seek_pos(CFile& file, int wanted_lines)
{
    // Rather than reading the whole file, start at the end and work backwards,
    // stopping when we've found the number of lines we want.
    off_t pos = 0;
    if (!file.seek(0, CIODevice::SeekMode::FromEndPosition, &pos)) {
        fprintf(stderr, "Failed to find end of file: %s\n", file.error_string());
        return -1;
    }

    off_t end = pos;
    int lines = 0;

    // FIXME: Reading char-by-char is only OK if CIODevice's read buffer
    // is smart enough to not read char-by-char. Fix it there, or fix it here :)
    for(; pos >= 0; pos--) {
        file.seek(pos);
        const auto& ch = file.read(1);
        if (ch.is_empty()) {
            // Presumably the file got truncated?
            // Keep trying to read backwards...
        } else {
            if (*ch.pointer() == '\n' && (end - pos) > 1) {
                lines++;
                if (lines == wanted_lines)
                    break;
            }
        }
    }

    return pos;
}

static void exit_because_we_wanted_lines()
{
    fprintf(stderr, "Expected a line count after -n");
    exit(-1);
}

int main(int argc, char *argv[])
{
    int line_count = 0;
    bool flag_follow = false;
    int opt;
    while ((opt = getopt(argc, argv, "fn:")) != -1) {
        switch (opt) {
        case 'f':
            flag_follow = true;
            break;
        case 'n':
            line_count = strtol(optarg, NULL, 10);
            if (errno == EINVAL) {
                exit_because_we_wanted_lines();
            }
            break;
        default:
            print_usage_and_exit();
        }
    }
    const char *path = nullptr;
    if (optind >= argc) {
        print_usage_and_exit();
    }

    path = argv[optind];

    CFile f(path);
    if (!f.open(CIODevice::ReadOnly)) {
        fprintf(stderr, "Error opening file %s: %s\n", path, strerror(errno));
        exit(-1);
    }

    auto pos = find_seek_pos(f, line_count);
    return tail_from_pos(f, pos, flag_follow);
}