system calls - How are sbrk/brk implemented in Linux?

Question

I was thinking about how the Linux kernel implements system calls and I was wondering if someone could give me a high level view of how sbrk/brk work?

I've reviewed the kernel code, but there is just so much of it and I don't understand it. I was hoping for a summary from someone?

Answer 1

In a very high level view, the Linux kernel tracks the memory visible to a process as several "memory areas" (struct vm_area_struct). There is also a structure which represents (again in a very high level view) a process' whole address space (struct mm_struct). Each process (except some kernel threads) has exactly one struct mm_struct, which in turn points to all the struct vm_area_struct for the memory it can accesss.

The sys_brk system call (found in mm/mmap.c) simply adjusts some of these memory areas. (sbrk is a glibc wrapper around brk). It does so by comparing the old value of the brk address (found inside struct mm_struct) and the requested value.

It would be simpler to look at the mmap family of functions first, since brk is a special case of it.