ZIP-Archiv

#include <stdint.h>

#pragma pack(push, 1)          /* exakt am Byte-Layout ausrichten */
typedef struct {
    uint32_t signature;        /* 0x04034B50  ("PK\3\4")          */
    uint16_t version_needed;
    uint16_t flags;
    uint16_t compression;      /* 0 = Stored, 8 = Deflate          */
    uint16_t mod_time;
    uint16_t mod_date;
    uint32_t crc32;            /* Prüfsumme der unkompr. Daten     */
    uint32_t comp_size;        /* komprimierte Größe               */
    uint32_t uncomp_size;      /* Originalgröße                    */
    uint16_t name_len;         /* Länge des Dateinamens            */
    uint16_t extra_len;        /* Länge des Extra-Feldes           */
    /* char filename[name_len]; */
    /* char extra[extra_len];   */
} zip_local_header_t;
#pragma pack(pop)

#pragma pack(push, 1)
typedef struct {
    uint32_t signature;        /* 0x02014B50  ("PK\1\2")          */
    uint16_t version_made_by;
    uint16_t version_needed;
    uint16_t flags;
    uint16_t compression;
    uint16_t mod_time, mod_date;
    uint32_t crc32;
    uint32_t comp_size, uncomp_size;
    uint16_t name_len, extra_len, comment_len;
    uint16_t disk_start;
    uint16_t internal_attr;
    uint32_t external_attr;
    uint32_t local_header_offset; /* Position des Local Headers   */
} zip_central_header_t;

typedef struct {
    uint32_t signature;        /* 0x06054B50  ("PK\5\6")          */
    uint16_t disk_number;
    uint16_t cd_start_disk;
    uint16_t cd_entries_disk;
    uint16_t cd_entries_total; /* Anzahl Einträge im Archiv        */
    uint32_t cd_size;          /* Größe des Central Directory      */
    uint32_t cd_offset;        /* Offset zum Central Directory     */
    uint16_t comment_len;
} zip_eocd_t;
#pragma pack(pop)

#include <stdio.h>

/* Sucht 0x06054B50 ("PK\5\6") rückwärts und liefert dessen Offset. */
long zip_find_eocd(FILE *f) {
    fseek(f, 0, SEEK_END);
    long size = ftell(f);
    long max_back = size < 65557 ? size : 65557;  /* 22 + max. Kommentar */

    for (long off = size - 22; off >= size - max_back; off--) {
        fseek(f, off, SEEK_SET);
        uint32_t sig;
        fread(&sig, 4, 1, f);
        if (sig == 0x06054B50) return off;  /* gefunden */
    }
    return -1;  /* keine gültige ZIP-Struktur */
}