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

How malloc() and free() works depends on the runtime library used. Generally, malloc() allocates a heap (a block of memory) from the operating system. Each request to malloc() then allocates a small chunk of this memory be returning a pointer to the caller. The memory allocation routines will have to store some extra information about the block of memory allocated to be able to keep track of used and free memory on the heap. This information is often stored in a few bytes just before the pointer returned by malloc() and it can be a linked list of memory blocks.

By writing past the block of memory allocated by malloc() you will most likely destroy some of the book-keeping information of the next block which may be the remaining unused block of memory.

One place where you program may also crash is when copying too many characters into the buffer. If the extra characters are located outside the heap you may get an access violation as you are trying to write to non-existing memory.