内存分配模拟问题

Question

I am simulating a memory block with a static array of structs that take the form of:

typedef struct memBlock
{
  unsigned int isAllocated = 0;
  unsigned int processID = 0;
}memBlock;

I have already got a malloc() -like function that will simulates malloc by taking in the requested memory size from a process and marking part of the array as occupied by setting the isAllocated equal to 1, and changing the PID to the process ID of the requesting process.

So now I want to write a function like free that will deallocate "memory" from this array by setting isAllocated to 0 and PID to 0, but how should I accomplish this? I know conceptually that I need to start at index 0 of this array and maybe check for the first index where isAllocated = 1 and a nonzero PID right? I already have this implemented so far:

void* custom_free(void* ptr, unsigned int size)
{
    unsigned int blockIndex = 0, sizeCount = 0, startIndex = 0;
    if(ptr){ free(ptr); }
}

Edit: Earlier in my code I have a malloc call for the memBlock structure like this:

memBlock *block = (memBlock *)malloc(BLOCK_SIZE);

where BLOCK_SIZE is a preprocessor defined constant

Answer 1

A common way to store information about allocated memory blocks by an allocator is to store the information structure in the memory block it allocates.

So instead of allocating size bytes, you allocate size + sizeof(memBlock) bytes, and use the pointer as a pointer to a memBlock structure and fill in the information you need. Then return a pointer to the memory after the memBlock structure (ie something like &((memBlock *) ptr)[1] ).

When freeing you subtract the size of a memBlock structure to get the original pointer, which points to the memBlock structure.

This of course makes a member such as isAllocated pretty much useless. If you have your own blocks, and only allocate in multiples of those blocks, you need to store information about which blocks was allocated, and the number of blocks as well.