> The second part is the misconception about the impact of UB. Making something UB does not dictate that its usage will initiate the rise of zombie velociraptors. It grants the implementation the power to decide the best course of action. That is, after all, what they’ve been doing all this time anyway.
Wrong. UB never happens. That is the promise the program writer makes to the compiler. UB never happens. A correct C program never executes UB. This allows the compiler to assume that anything that is UB never happens. Does some branch of your program unconditionally execute realloc(..., 0) after constant propagation? That branch never happens and can just be deleted.
Reading the defect report, they state "Classifying a call to realloc with a size of 0 as undefined behavior would allow POSIX to define the otherwise undefined behavior however they please." which is wrong. UB cannot be defined, if you define it, you are no longer writing standard C. It should instead have been classified as "implementation-defined behaviour".
In any case it's not that hard to just write a sane wrapper. This one is placed in the Public Domain:
void *sane_realloc(void *ptr, size_t sz)
{
if (sz == 0) {
free(ptr); /*free(NULL) is no-op*/
return NULL;
}
if (ptr == NULL) {
return malloc(sz);
}
return realloc(ptr, sz);
}
I am calling it sane and not safe, because it is not safe. You still have the confusion of what happens when the function returns NULL (was it allocation failure or did we free the object?) - check errno. However, it has the same fully defined semantics on most all implementations and acts like people would expect.
You may be tempted to make the function return the value of errno, mark it [[nodiscard]] and take a pointer-to-pointer-to-void, so that the value of the pointer will only be changed if the reallocation was successful. I am not sure if that is safer. You are trading one possible bug - null pointer on allocation failure, which then will cause a segmentation fault for another - stale pointer on allocation failure, but with updated size. The latter is more likely to be used in buffer overflow attacks than the former.