c - CRC32C - appending 0s/CRC to message -

i trying better understanding of crc, stuck bit.

there few sample vectors here 1 can calculate correctly, stuck verifying calculated crc correct.

for example, given message of 32 bytes:

000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f

my understanding first append 32 bits of 0's payload:

000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f00000000

and calculate crc on message obtain 0x73c2a486

to verify crc correct should append crc value original value, in case:

000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f73c2a486

and should return 0, don't that.

would appreciate if point out going wrong.

edit:

sample code using:

static uint32_t crc32c_table_small[256] = {   0x00000000, 0xf26b8303, 0xe13b70f7, 0x1350f3f4, 0xc79a971f, 0x35f1141c, 0x26a1e7e8, 0xd4ca64eb,   0x8ad958cf, 0x78b2dbcc, 0x6be22838, 0x9989ab3b, 0x4d43cfd0, 0xbf284cd3, 0xac78bf27, 0x5e133c24,   0x105ec76f, 0xe235446c, 0xf165b798, 0x030e349b, 0xd7c45070, 0x25afd373, 0x36ff2087, 0xc494a384,   0x9a879fa0, 0x68ec1ca3, 0x7bbcef57, 0x89d76c54, 0x5d1d08bf, 0xaf768bbc, 0xbc267848, 0x4e4dfb4b,   0x20bd8ede, 0xd2d60ddd, 0xc186fe29, 0x33ed7d2a, 0xe72719c1, 0x154c9ac2, 0x061c6936, 0xf477ea35,   0xaa64d611, 0x580f5512, 0x4b5fa6e6, 0xb93425e5, 0x6dfe410e, 0x9f95c20d, 0x8cc531f9, 0x7eaeb2fa,   0x30e349b1, 0xc288cab2, 0xd1d83946, 0x23b3ba45, 0xf779deae, 0x05125dad, 0x1642ae59, 0xe4292d5a,   0xba3a117e, 0x4851927d, 0x5b016189, 0xa96ae28a, 0x7da08661, 0x8fcb0562, 0x9c9bf696, 0x6ef07595,   0x417b1dbc, 0xb3109ebf, 0xa0406d4b, 0x522bee48, 0x86e18aa3, 0x748a09a0, 0x67dafa54, 0x95b17957,   0xcba24573, 0x39c9c670, 0x2a993584, 0xd8f2b687, 0x0c38d26c, 0xfe53516f, 0xed03a29b, 0x1f682198,   0x5125dad3, 0xa34e59d0, 0xb01eaa24, 0x42752927, 0x96bf4dcc, 0x64d4cecf, 0x77843d3b, 0x85efbe38,   0xdbfc821c, 0x2997011f, 0x3ac7f2eb, 0xc8ac71e8, 0x1c661503, 0xee0d9600, 0xfd5d65f4, 0x0f36e6f7,   0x61c69362, 0x93ad1061, 0x80fde395, 0x72966096, 0xa65c047d, 0x5437877e, 0x4767748a, 0xb50cf789,   0xeb1fcbad, 0x197448ae, 0x0a24bb5a, 0xf84f3859, 0x2c855cb2, 0xdeeedfb1, 0xcdbe2c45, 0x3fd5af46,   0x7198540d, 0x83f3d70e, 0x90a324fa, 0x62c8a7f9, 0xb602c312, 0x44694011, 0x5739b3e5, 0xa55230e6,   0xfb410cc2, 0x092a8fc1, 0x1a7a7c35, 0xe811ff36, 0x3cdb9bdd, 0xceb018de, 0xdde0eb2a, 0x2f8b6829,   0x82f63b78, 0x709db87b, 0x63cd4b8f, 0x91a6c88c, 0x456cac67, 0xb7072f64, 0xa457dc90, 0x563c5f93,   0x082f63b7, 0xfa44e0b4, 0xe9141340, 0x1b7f9043, 0xcfb5f4a8, 0x3dde77ab, 0x2e8e845f, 0xdce5075c,   0x92a8fc17, 0x60c37f14, 0x73938ce0, 0x81f80fe3, 0x55326b08, 0xa759e80b, 0xb4091bff, 0x466298fc,   0x1871a4d8, 0xea1a27db, 0xf94ad42f, 0x0b21572c, 0xdfeb33c7, 0x2d80b0c4, 0x3ed04330, 0xccbbc033,   0xa24bb5a6, 0x502036a5, 0x4370c551, 0xb11b4652, 0x65d122b9, 0x97baa1ba, 0x84ea524e, 0x7681d14d,   0x2892ed69, 0xdaf96e6a, 0xc9a99d9e, 0x3bc21e9d, 0xef087a76, 0x1d63f975, 0x0e330a81, 0xfc588982,   0xb21572c9, 0x407ef1ca, 0x532e023e, 0xa145813d, 0x758fe5d6, 0x87e466d5, 0x94b49521, 0x66df1622,   0x38cc2a06, 0xcaa7a905, 0xd9f75af1, 0x2b9cd9f2, 0xff56bd19, 0x0d3d3e1a, 0x1e6dcdee, 0xec064eed,   0xc38d26c4, 0x31e6a5c7, 0x22b65633, 0xd0ddd530, 0x0417b1db, 0xf67c32d8, 0xe52cc12c, 0x1747422f,   0x49547e0b, 0xbb3ffd08, 0xa86f0efc, 0x5a048dff, 0x8ecee914, 0x7ca56a17, 0x6ff599e3, 0x9d9e1ae0,   0xd3d3e1ab, 0x21b862a8, 0x32e8915c, 0xc083125f, 0x144976b4, 0xe622f5b7, 0xf5720643, 0x07198540,   0x590ab964, 0xab613a67, 0xb831c993, 0x4a5a4a90, 0x9e902e7b, 0x6cfbad78, 0x7fab5e8c, 0x8dc0dd8f,   0xe330a81a, 0x115b2b19, 0x020bd8ed, 0xf0605bee, 0x24aa3f05, 0xd6c1bc06, 0xc5914ff2, 0x37faccf1,   0x69e9f0d5, 0x9b8273d6, 0x88d28022, 0x7ab90321, 0xae7367ca, 0x5c18e4c9, 0x4f48173d, 0xbd23943e,   0xf36e6f75, 0x0105ec76, 0x12551f82, 0xe03e9c81, 0x34f4f86a, 0xc69f7b69, 0xd5cf889d, 0x27a40b9e,   0x79b737ba, 0x8bdcb4b9, 0x988c474d, 0x6ae7c44e, 0xbe2da0a5, 0x4c4623a6, 0x5f16d052, 0xad7d5351 };    static inline uint32_t crc32c_software_simple(uint32_t crc, const uint8_t * data, size_t num_bytes)   {     while (num_bytes--)     {       crc = (crc >> 8) ^ crc32c_table_small[(crc & 0xff) ^ *data++];     }     return crc;   }      uint32_t num_bytes = 32;   uint32_t num_bytes_padded = num_bytes + sizeof(uint32_t);   uint8_t * test_data = (uint8_t*) malloc(num_bytes_padded);    for(uint32_t = num_bytes; < num_bytes_padded; i++) test_data[i] = 0;    for(uint32_t = 0; < num_bytes; i++)   {     test_data[i] = i;   }    binary(num_bytes_padded, test_data);   hex(num_bytes_padded, test_data);   uint32_t crc = 0xffffffff;   crc = ~crc32c_software_simple(crc, test_data, num_bytes_padded);    for(uint32_t = 0; < sizeof(uint32_t); i++) test_data[num_bytes + i] = ((uint8_t*)&crc)[i];    crc = 0xffffffff;   crc = ~crc32c_software_simple(crc, test_data, num_bytes_padded); 

the example code post complement of crc. cause verify crc constant non-zero value, in case verify crc == 0x48674bc7 if there no errors (regardless of message size). calling code needed fix on first call crc32c_software_simple, use num_bytes instead of num_bytes_padded, noted in comment below.

if there no post complement of crc, verify produce 0 crc.

the code pre-complement of crc, not affect verify.

int main() {       uint32_t num_bytes = 32;       uint32_t num_bytes_padded = num_bytes + sizeof(uint32_t);       uint8_t * test_data = (uint8_t*) malloc(num_bytes_padded);        for(uint32_t = num_bytes; < num_bytes_padded; i++) test_data[i] = 0;        for(uint32_t = 0; < num_bytes; i++)       {         test_data[i] = i;       }        uint32_t crc = 0xffffffff;       crc = ~crc32c_software_simple(crc, test_data, num_bytes);  // num_bytes fix        for(uint32_t = 0; < sizeof(uint32_t); i++) test_data[num_bytes + i] = ((uint8_t*)&crc)[i];        crc = 0xffffffff;       crc = ~crc32c_software_simple(crc, test_data, num_bytes_padded);       // if no errors, crc == 0x48674bc7     return 0; }