如何在没有系统调用的情况下在堆上分配内存？

I was wondering that if the space required on heap is not large enough such that there is no need for a brk/sbrk system all (to shift the break pointer (brk) of data segment), how does a library function (such as malloc) allocates space on heap. I am not asking about the data-structures and algorithms for heap management. I am just asking how does malloc get the address of the first location of the heap if it doesn't invoke a system call. I am asking this because I have heard that it is not always necessary to invoke a system call (brk/sbrk) as these are only required to expand the space.Please correct me if I am wrong.I was wondering that if the space required on h