Serialization of a long array (in C)

Question

in a C program I have an long* that I want to serialize (thus converting to chars). A long doesn't fit in a single char, and the size varies depending of the processor (can be 4 bytes or 8 bytes).

Theres a good way to make the serialization and de-serialization?

Answer 1

This is portable, but nowhere near as inefficient as using printf/scanf

void longtochar(char *buffer, unsigned long number) {
    int i;
    for (i=0; i<sizeof(long); i++) {
        buffer[i] = number & 0xFF; // place bottom 8 bits in char
        number = number >> 8; // shift down remaining bits
    }
    return; // the long is now stored in the first few (2,4,or 8) bytes of buffer
}

And to unpack it again (assuming long is the same size)

long chartolong(char *buffer) {
    long number = 0;
    int i;
    for (i=sizeof(long)-1; i>=0; i--) {
        number = number << 8; // left shift bits in long already
        number += buffer[i]; // add in bottom 8 bits
    }
    return number;
}

Do note the BIG assumption that long is the same length on both systems. Safe thing to do is #include <stdint.h> and use the types it provides (uint32_t or uint16_t).

Also, my code has it as an unsigned long. I don't have access to a C compiler right now, so I can't confirm if it would or not would not work with signed integers. If memory serves me, the behavior of it might be undefined (though it might not matter, how I handle it).