Function read_header

win/share/gifread.c:190–287

* Read gif header, including colormaps. We expect only one image per * file, so if that image has a local colormap, overwrite the global one. */

 * file, so if that image has a local colormap, overwrite the global one.
 */
static void
read_header(FILE *fd)
{
    unsigned char buf[16];
    unsigned char c;
    char version[4];

    if (!ReadOK(fd, buf, 6)) {
        Fprintf(stderr, "error reading magic number\n");
        exit(EXIT_FAILURE);
    }

    if (strncmp((genericptr_t) buf, "GIF", 3) != 0) {
        Fprintf(stderr, "not a GIF file\n");
        exit(EXIT_FAILURE);
    }

    (void) strncpy(version, (char *) buf + 3, 3);
    version[3] = '\0';

    if ((strcmp(version, "87a") != 0) && (strcmp(version, "89a") != 0)) {
        Fprintf(stderr, "bad version number, not '87a' or '89a'\n");
        exit(EXIT_FAILURE);
    }

    if (!ReadOK(fd, buf, 7)) {
        Fprintf(stderr, "failed to read screen descriptor\n");
        exit(EXIT_FAILURE);
    }

    GifScreen.Width = LM_to_uint(buf[0], buf[1]);
    GifScreen.Height = LM_to_uint(buf[2], buf[3]);
    GifScreen.Colors = 2 << (buf[4] & 0x07);
    GifScreen.ColorResolution = (((buf[4] & 0x70) >> 3) + 1);
    GifScreen.Background = buf[5];
    GifScreen.AspectRatio = buf[6];

    if (BitSet(buf[4], LOCALCOLORMAP)) { /* Global Colormap */
        if (!ReadColorMap(fd, GifScreen.Colors)) {
            Fprintf(stderr, "error reading global colormap\n");
            exit(EXIT_FAILURE);
        }
    }

    if (GifScreen.AspectRatio != 0 && GifScreen.AspectRatio != 49) {
        Fprintf(stderr, "warning - non-square pixels\n");
    }

    for (;;) {
        if (!ReadOK(fd, &c, 1)) {
            Fprintf(stderr, "EOF / read error on image data\n");
            exit(EXIT_FAILURE);
        }

        if (c == ';') { /* GIF terminator */
            return;
        }