Question

How to get a 32 bit number in objective c when an byte array is passed to it, similarly as in java where,

ByteBuffer bb = ByteBuffer.wrap(truncation);   
return bb.getInt();

Where truncation is the byte array.
It returns 32 bit number.. Is this possible in objective c?

Answer 1

If the number is encoded in little-endian within the buffer, then use:

int32_t getInt32LE(const uint8_t *buffer)
{
    int32_t value = 0;
    unsigned length = 4;
    while (length > 0)
    {
        value <<= 8;
        value |= buffer[--length];
    }
    return value;
}

If the number is encoded in big-endian within the buffer, then use:

int32_t getInt32BE(const uint8_t *buffer)
{
    int32_t value = 0;
    for (unsigned i = 0; i < 4; i++)
    {
        value <<= 8;
        value |= *buffer++;
    }
    return value;
}

UPDATE If you are using data created on the same host then endianness is not an issue, in which case you can use a union as a bridge between the buffer and integers, which avoids some unpleasant casting:

union
{
    uint8_t b[sizeof(int32_t)];
    int32_t i;
} u;

memcpy(u.b, buffer, sizeof(u.b));
// value is u.i