reinterpret_cast是否安全 <bool*> 归零记忆？

Question

Context:

I have a chunk of memory that's shared between two processes via shm_open . I am guaranteed that after ftruncate ing and mmap ing it, the whole chunk has the bit pattern 00000... . I need to share a boolean value across the two processes.

A more concrete question:

Is the following guaranteed to be okay (the assertion doesn't fail and UB does not occur) on reasonable POSIX systems?

void *my_shared_memory_region = calloc(1024, 1);
bool *my_bool = reinterpret_cast<bool*>(my_shared_memory_region);
assert(*my_bool == false);

I believe that there are some restrictions on the actual values that can live inside of a bool , so I'm not sure. I think this question is distinct from this one because reinterpret_cast doesn't do the same kind of conversions that C-style casts do.

Answer 1

The core issue - even if the memory's all zeros, is it valid to read from it as if from a properly initialised bool - is the same as for this question .

Long story short: it's undefined behaviour that works on common systems but isn't guaranteed portable. Specific implementations are allowed to document behaviour for cases the Standard leaves undefined, so it's worth doing some research for the specific platforms/compilers you care about.