Since arrays are just contiguous data of the same type, and you don't need to explicitly put [] somewhere (e.g. you can int *p1 = malloc(sizeof(int) * 4);, how is it that when you realloc(p1, ...), it knows to move (if it has to move) exactly 4 ints worth of space, even if potentially there are other ints in memory?

To clarify the question: If you allocate an array in this way, and also just a single, seperate int - does that mean that these 4 1 total ints are never contiguous in memory, or is this "it's an array" information in the memory block somehow (e.g. they have some sort of delimiter?), or does the compiler infer and remember that from the malloc parameter? Or something else?

Basically, how does it ensure it moves only and exactly those 4, even when there are other blocks of memory of the same size that might be also contiguous?

CodePudding user response：

realloc is defined only when passed a pointer to memory allocated by a member of the malloc family of routines (or a null pointer). These routines keep records of the memory they have allocated. When you call realloc, it uses these records to know how long the allocated block is.

Often, the primary record for a block of memory is put into the bytes just before that block, so all realloc has to do is take the pointer you give it, subtract a known number of bytes from it, and look at the data at that new address, where it will find information about the size of the allocated block. However, other methods are possible too.