PNG

#include <stdint.h>

/* Die ersten 8 Bytes jeder PNG-Datei */
static const uint8_t PNG_SIGNATURE[8] = {
    0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A
};

/* Jeder Chunk hat exakt diesen Rahmen (Big-Endian!) */
typedef struct {
    uint32_t length;     /* Länge des data-Feldes in Byte   */
    char     type[4];    /* z. B. "IHDR", "IDAT", "IEND"     */
    /* uint8_t data[length];  -- folgt direkt, variable Länge */
    uint32_t crc;        /* CRC-32 über type[] + data[]      */
} png_chunk_header_t;

#pragma pack(push, 1)         /* keine Ausrichtungs-Lücken */
typedef struct {
    uint32_t width;            /* Bildbreite  (Big-Endian)     */
    uint32_t height;           /* Bildhöhe    (Big-Endian)     */
    uint8_t  bit_depth;        /* 1,2,4,8 oder 16 Bit/Sample   */
    uint8_t  color_type;       /* 0,2,3,4,6 (siehe Tabelle)    */
    uint8_t  compression;      /* immer 0 = DEFLATE            */
    uint8_t  filter_method;    /* immer 0 = adaptive Filter    */
    uint8_t  interlace;        /* 0 = keins, 1 = Adam7         */
} png_ihdr_t;
#pragma pack(pop)

#include <stdio.h>
#include <string.h>
#include <arpa/inet.h>  /* ntohl: Big-Endian -> Host */

int read_png_header(FILE *f, png_ihdr_t *out) {
    uint8_t sig[8];
    if (fread(sig, 1, 8, f) != 8) return -1;
    if (memcmp(sig, PNG_SIGNATURE, 8) != 0) return -1; /* keine PNG */

    png_chunk_header_t ch;
    fread(&ch.length, 4, 1, f);
    fread(ch.type,    1, 4, f);
    if (memcmp(ch.type, "IHDR", 4) != 0) return -1;    /* IHDR fehlt */

    fread(out, 1, 13, f);
    out->width  = ntohl(out->width);   /* Byteorder korrigieren */
    out->height = ntohl(out->height);
    printf("PNG %ux%u, %d Bit, Farbtyp %d\n",
           out->width, out->height, out->bit_depth, out->color_type);
    return 0;
}