how brk pointer grow after calling malloc

Question

I'm new to Linux and C, just want to ask how brk pointer grow after calling malloc. We know that the kernel maintains a brk pointer that points to the top of the heap. let's say there is no available free blocks when we call malloc to allocate the requested memory and we know malloc call mmap internally. Below is my questions:

Q1-when malloc call mmap internally, does malloc pass brk as the first argument to mmap as starting address like:

//not 100% correct onlyfor demo purpose
// void *mmap(void *start, size_t length, int prot, int flags, int fd, off_t offset);

void *malloc(size_t size){
  ...
  mmap(brk, size,...);  
  ...                    //line x
}

Q2- does other code section(after line x above) inside malloc make brk grow? or mmap internally make the brk grow?

Answer 1

The specifics depend on the malloc implementation, but at least in Doug Lea’s implementation (as used by default in the GNU C library), mmap and sbrk are used independently.

1. When malloc calls mmap, it uses mmap with no base address; effectively, it delegates the allocation to the kernel (after adjusting the requested size to take into account alignment constraints). There’s no expectation that the mmaped memory is contiguous with the existing heap, or that it can be re-used for any other allocation.

2. mmap doesn’t touch brk. If malloc doesn’t end up using mmap, it calls sbrk directly. The implementation is described in detail in comments at the top of the file; if you want to follow the malloc code, look for the sysmalloc function, and bear in mind that mmap is MMAP and sbrk is MORECORE in this code.