Can I rely on sizeof(uint32_t) == 4?

Question

I know I can rely on sizeof(char) == 1, but what about sizeof(uint32_t) and sizeof(uint8_t)? Shouldn't it be 32bit (8bit) in size guessing from the name? Thanks!

Answer 1

Absolutely not, at least with regards to sizeof(uint32_t). sizeof returns the number of bytes, not the number of bits, and if a byte is 16 bits on the platform, sizeof(uint32_t) will be 2, not 4; if a byte is 32 bits (and such platforms actually exist), sizeof(uint32_t) will be 1 (and uint32_t could be a typedef to unsigned char).

Of course, in such cases, uint8_t won't be defined.

Answer 2

The fixed-sized types will always be exactly that size. If you're on some weird platform that doesn't have integer types of that size then they will be undefined.

Note that it doesn't necessarily follow that sizeof(uint32_t) == 4, if CHAR_BIT != 8; again this only occurs on weird platforms.