JPEG

#include <stdint.h>

#define JPEG_SOI  0xD8   /* Start of Image */
#define JPEG_EOI  0xD9   /* End of Image   */
#define JPEG_SOF0 0xC0   /* Baseline DCT   */
#define JPEG_DHT  0xC4   /* Huffman-Tabelle*/
#define JPEG_DQT  0xDB   /* Quant.-Tabelle */
#define JPEG_SOS  0xDA   /* Start of Scan  */
#define JPEG_APP0 0xE0   /* JFIF           */

/* Ein Marker mit Länge: FF <code> <length(2)> <payload...> */
typedef struct {
    uint8_t  prefix;   /* immer 0xFF                          */
    uint8_t  code;     /* Markercode, z. B. 0xC0              */
    uint16_t length;   /* Big-Endian, inkl. dieser 2 Bytes    */
    /* uint8_t payload[length - 2]; */
} jpeg_marker_t;

#pragma pack(push, 1)
typedef struct {
    uint8_t  precision;       /* meist 8 Bit pro Sample        */
    uint16_t height;          /* Bildhöhe   (Big-Endian)       */
    uint16_t width;           /* Bildbreite (Big-Endian)       */
    uint8_t  num_components;  /* 1 = Graustufen, 3 = YCbCr     */
} jpeg_sof0_t;

typedef struct {
    uint8_t id;               /* Komponenten-ID (1=Y,2=Cb,3=Cr)*/
    uint8_t sampling;         /* hi nibble = H, lo nibble = V  */
    uint8_t quant_table_id;   /* Index der DQT-Tabelle         */
} jpeg_component_t;
#pragma pack(pop)

#include <stdio.h>
#include <arpa/inet.h>  /* ntohs */

int jpeg_scan_markers(FILE *f) {
    int b;
    if (fgetc(f) != 0xFF || fgetc(f) != JPEG_SOI) return -1; /* SOI? */

    while ((b = fgetc(f)) != EOF) {
        if (b != 0xFF) continue;          /* auf 0xFF synchronisieren */
        int code = fgetc(f);
        if (code == JPEG_EOI) break;      /* fertig */
        if (code == JPEG_SOI || code == 0xFF) continue;

        uint16_t len;
        fread(&len, 2, 1, f);
        len = ntohs(len) - 2;             /* Längenbytes herausrechnen */
        printf("Marker FF%02X, %u Byte Nutzdaten\n", code, len);
        fseek(f, len, SEEK_CUR);          /* Segment überspringen */
        if (code == JPEG_SOS) break;      /* danach kommen Bilddaten */
    }
    return 0;
}