The malloc_trim() function attempts to release free memory at the top of the heap (by calling sbrk(2) with a suitable argument).
The pad argument specifies the amount of free space to leave untrimmed at the top of the heap. If this argument is 0, only the minimum amount of memory is maintained at the top of the heap (i.e., one page or less). A nonzero argument can be used to maintain some trailing space at the top of the heap in order to allow future allocations to be made without having to extend the heap with sbrk(2).
This function is automatically called by free(3) in certain circumstances; see the discussion of M_TOP_PAD and M_TRIM_THRESHOLD in mallopt(3).
This function cannot release free memory located at places other than the top of the heap.
This function releases only memory in the main arena.