Erlang NIF function for WebSocket message masking

nif_mask.c

#include "erl_nif.h"

void mask(unsigned char *key, unsigned char *data, unsigned long len)
{
    unsigned int *p = (unsigned int *)data;
    unsigned int *k = (unsigned int *)key;
    for(int i=4; i<len; i+=4)
    {
        *(p+(i-4)/4) ^= *k;
    }

    int r = len % 4;
    if (r)
    {
        for(int i=0; i<r; i++)
        {
            data[len-r+i] ^= key[i];
        }
    }
}
/*
 * Erlang call would be like
 * nif_mask(Key, Data,Len)
 * where Key is 4 bytes binary
 *       Data is binary with length Len
 */
static ERL_NIF_TERM nif_mask(ErlNifEnv* env,int argc, const ERL_NIF_TERM argv[]) {
    ErlNifBinary data, key;
    enif_inspect_binary(env, argv[0], &key);
    enif_inspect_binary(env, argv[1], &data);
    enif_realloc_binary(&data, data.size);
    mask(key.data,data.data, data.size);
    return enif_make_binary(env, &data);
}

static ErlNifFunc nif_funcs[] = {
  {"nif_mask", 3, nif_mask}
  };

ERL_NIF_INIT(websocket_client,nif_funcs,NULL,NULL,NULL,NULL);