Sign extending to 32 bits, starting with n bits - C

Question

I am new to C and getting some practice with bit manipulation.

Suppose I have an n bit two's complement number such that n > 0 and n < 31. If I know the size of n in advance, how can I sign extend it to 32 bits?

If n was 16 bits,

int32_t extendMe(int16_t n) {
    return (int32_t) n;
}

assuming I have the data definitions.

Suppose I have an n bit value that I want to sign extend to 32, how can I accomplish this?

Thank you.

Answer 1

If this really is about interpreting arbitrary bit patterns as numbers represented in n bits using two's complement, here's some sloppy example code doing that:

#include <stdio.h>
#include <inttypes.h>

// this assumes the number is in the least significant `bits`, with
// the most significat of these being the sign bit.
int32_t fromTwosComplement(uint32_t pattern, unsigned int bits)
{
    // read sign bit
    int negative = !!(pattern & (1U << (bits-1)));

    // bit mask for all bits *except* the sign bit
    uint32_t mask = (1U << (bits-1)) - 1;

    // extract value without sign
    uint32_t val = pattern & mask;

    if (negative)
    {
        // if negative, apply two's complement
        val ^= mask;
        ++val;
        return -val;
    }
    else
    {
        return val;
    }
}

int main(void)
{
    printf("%" PRId32 "\n", fromTwosComplement(0x1f, 5)); // output -1
    printf("%" PRId32 "\n", fromTwosComplement(0x01, 5)); // output 1
}