GIF

#include <stdint.h>

#pragma pack(push, 1)
typedef struct {
    char    signature[3];   /* 'G','I','F'                    */
    char    version[3];     /* '8','7','a' oder '8','9','a'    */
} gif_header_t;

typedef struct {
    uint16_t width;         /* Leinwandbreite  (Little-Endian)*/
    uint16_t height;        /* Leinwandhöhe    (Little-Endian)*/
    uint8_t  packed;        /* siehe Bitfeld unten            */
    uint8_t  bg_color_index;/* Index in globale Palette       */
    uint8_t  pixel_aspect;  /* Seitenverhältnis (meist 0)     */
} gif_lsd_t;
#pragma pack(pop)

/* Bitmasken für das packed-Feld */
#define GIF_HAS_GCT(p)   (((p) & 0x80) != 0)
#define GIF_GCT_SIZE(p)  (1 << (((p) & 0x07) + 1))  /* Farbanzahl */

#pragma pack(push, 1)
typedef struct {
    uint8_t  separator;     /* immer 0x2C                     */
    uint16_t left, top;     /* Position auf der Leinwand      */
    uint16_t width, height; /* Bildgröße                      */
    uint8_t  packed;        /* lokale Palette?, Interlace     */
} gif_image_descriptor_t;

typedef struct {
    uint8_t  introducer;    /* 0x21 Extension                 */
    uint8_t  label;         /* 0xF9 Graphic Control           */
    uint8_t  block_size;    /* immer 4                        */
    uint8_t  packed;        /* Disposal + Transparenz-Flag    */
    uint16_t delay_cs;      /* Verzögerung in 1/100 Sekunden  */
    uint8_t  transp_index;  /* transparenter Paletten-Index   */
    uint8_t  terminator;    /* 0x00                           */
} gif_gce_t;
#pragma pack(pop)

#include <stdio.h>
#include <string.h>

int read_gif_header(FILE *f, gif_lsd_t *lsd) {
    gif_header_t h;
    if (fread(&h, 1, sizeof h, f) != sizeof h) return -1;
    if (memcmp(h.signature, "GIF", 3) != 0) return -1;     /* kein GIF */

    int v89 = (memcmp(h.version, "89a", 3) == 0);
    int v87 = (memcmp(h.version, "87a", 3) == 0);
    if (!v89 && !v87) return -1;                            /* unbekannt */

    fread(lsd, 1, sizeof *lsd, f);
    printf("GIF%.3s  %ux%u  Palette: %s (%d Farben)\n",
           h.version, lsd->width, lsd->height,
           GIF_HAS_GCT(lsd->packed) ? "global" : "keine",
           GIF_GCT_SIZE(lsd->packed));
    return 0;
}