When calling mallopt, the param argument specifies the parameter to be set, and value the new value to be set. Possible choices for param, as defined in malloc.h, are:

M_TRIM_THRESHOLD

This is the minimum size (in bytes) of the top-most, releasable chunk that will cause sbrk to be called with a negative argument in order to return memory to the system.

M_TOP_PAD

This parameter determines the amount of extra memory to obtain from the system when a call to sbrk is required. It also specifies the number of bytes to retain when shrinking the heap by calling sbrk with a negative argument. This provides the necessary hysteresis in heap size such that excessive amounts of system calls can be avoided.

M_MMAP_THRESHOLD

All chunks larger than this value are allocated outside the normal heap, using the mmap system call. This way it is guaranteed that the memory for these chunks can be returned to the system on free.

M_MMAP_MAX

The maximum number of chunks to allocate with mmap. Setting this to zero disables all use of mmap.