What is behind the dynamic allocation function malloc ()?

Briefly:

Your program / process occupies a memory region that ranges from a x address to a y address called heap ). All its malloc s are allocated in this area between x and y . It maintains a data structure, let's say a list, which contains all the free spaces in the stack of this process.

When you call malloc , it looks through this list and looks for a space that is large enough for the size you want to allocate. If you have this size available, it returns a pointer to this space, registers that it will use it and from there your program will use that slice in memory.

When you deallocate by calling free() , it picks up that used space that will now be freed, unregisters it and now that freed space goes to the list and if there is any new allocation request, / p>

If you call malloc() and it can not find any large enough space on the stack, it uses syscall brk() to increase this range. That is, increase address y and reallocate all old addresses and new y to be a valid memory range.

brk() is a syscall , and therefore there is no way to do the same thing in the user layer.

Look for more information about syscall if you want to understand syscall itself.