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

/*
 * Exercises lseek() with offsets that span the 16-bit boundary.
 * big.txt is 102400 bytes (100 KB); every 256-byte block opens with a
 * "[off=NNNNNNN] block #BBBB " label, so any seek aligned to 256 lands
 * on a self-identifying marker.
 */

static void show_marker(int fd, long off)
{
    long pos = lseek(fd, off, SEEK_SET);
    printf("\nseek %ld -> pos=%ld\n", off, pos);

    char buf[28];
    int n = read(fd, buf, 27);
    if (n <= 0) {
        printf("  read failed: %d\n", n);
        return;
    }
    buf[n] = 0;
    printf("  bytes: \"%s\"\n", buf);
}

int main(void)
{
    int fd = open("BIG.TXT", O_RDONLY);
    if (fd < 0) {
        printf("open failed: %d\n", fd);
        (void)getchar();
        return 1;
    }

    /* Total file size. */
    long size = lseek(fd, 0L, SEEK_END);
    printf("file size = %ld bytes\n", size);

    /* Seek inside the first 64 KB (high16 = 0). */
    show_marker(fd, 0L);
    show_marker(fd, 256L);
    show_marker(fd, 32768L);

    /* Right at the 64 KB boundary (offset 0x00010000, high16 = 1). */
    show_marker(fd, 65536L);

    /* Past 64 KB (high16 still 1, low16 != 0). */
    show_marker(fd, 65536L + 256L);
    show_marker(fd, 81920L);

    /* Near end of file (size = 102400 = 0x19000, high16 = 1). */
    show_marker(fd, 102400L - 256L);

    /* SEEK_CUR: from current position, +256. After the previous SEEK_SET
     * to 102144, then read of 27 bytes, pos is 102171. +256 -> 102427,
     * which is past EOF — ESTEX should clamp or error. */
    long after_cur = lseek(fd, 256L, SEEK_CUR);
    printf("\nSEEK_CUR +256 -> pos=%ld (past EOF)\n", after_cur);

    close(fd);
    puts("");
    puts("Press any key to exit...");
    (void)getchar();
    return 0;
}
