How do malloc() and free() work?

Question

I want to know how malloc and free work.

int main() {
    unsigned char *p = (unsigned char*)malloc(4*sizeof(unsigned char));
    m         


        

Answer 1

malloc and free are implementation dependent. A typical implementation involves partitioning available memory into a "free list" - a linked list of available memory blocks. Many implementations artificially divide it into small vs large objects. Free blocks start with information about how big the memory block is and where the next one is, etc.

When you malloc, a block is pulled from the free list. When you free, the block is put back in the free list. Chances are, when you overwrite the end of your pointer, you are writing on the header of a block in the free list. When you free your memory, free() tries to look at the next block and probably ends up hitting a pointer that causes a bus error.