Hello, I'm wondering if G++ has special guarantees for accessing mmapped memory. My use-case: - I have some large data sets (>10GB) in a file, mostly float arrays, plus some POD management structs - I need to share this data read-only between unrelated processes To support this, I'd like to mmap the file containing the data into the processes that access it (properly aligned of course), and read it directly. I cannot memcpy the whole content, since then the data sharing between processes would be lost. However, my understanding is that C++ basically only allows memcpy from such data, but no direct access e.g. as float array (which would be UB in C++, because of the object lifetime rules). See https://stackoverflow.com/questions/55034863/dealing-with-undefined-behavior-when-using-reinterpret-cast-in-a-memory-mapping for a very similar question from someone else. My question: does g++ guarantee anything beyond ISO C++ in this regard? Using mmap to share data between processes sounds very useful, so it would be a pity if this was impossible. Alternatively, I could write the direct data access in C, and call that from C++. Basically, have a C function that mmaps the file, finds the start of the float array in the file, and returns a 'const float*' pointing to the start of the array. My understanding is that this is legal in C. Would the returned pointer also be legal to use in C++? (Assuming gcc and g++ as compiler; I'm asking about the guarantees GCC gives beyond ISO C and ISO C++). Thanks, Thomas
