Is using a union for type punning defined behavior in C?

Yes, in C99 and later. The C standard explicitly allows reading a union member that was not the last one written to. This is commonly used to interpret a float as its raw bit pattern, or to overlay a register value with a struct of named bitfields:

union float_bits {
    float f;
    uint32_t u;
};
union float_bits x;
x.f = 3.14f;
uint32_t raw = x.u; // Defined in C99+

However, this is undefined behavior in C++. If your codebase must compile as both C and C++, use memcpy() instead — the compiler will optimize it to a single register move.