32 bit unsigned multiply on 64 bit causing undefined behavior?

Question

The simplest way to get the multiplication to happen in an unsigned type that is at least uint32_t, and also at least unsigned int, is to involve an expression of type unsigned int.

v = 1U * s1 * s2;

This either converts 1U to uint32_t, or s1 and s2 to unsigned int, depending on what's appropriate for your particular platform.

@Deduplicator comments that some compilers, where uint32_t is narrower than unsigned int, may warn about the implicit conversion in the assignment, and notes that such warnings are likely suppressable by making the conversion explicit:

v = (uint32_t) (1U * s1 * S2);

It looks a bit less elegant, in my opinion, though.

Congratulations on finding a friction point.

A possible way:

v = (uint32_t) (UINT_MAX<=0xffffffff
  ? s1 * s2
  : (unsigned)s1 * (unsigned)s2);

Anyway, looks like adding some typedefs to <stdint.h> for types guaranteed to be no smaller than int would be in order ;-).