##### 3.2.2.8 Malloc Tunable Parameters

You can adjust some parameters for dynamic memory allocation with the `mallopt` function. This function is the general SVID/XPG interface, defined in `malloc.h`.

— Function: int mallopt (int param, int value)

> Preliminary: | MT-Unsafe init const:mallopt | AS-Unsafe init lock | AC-Unsafe init lock | See POSIX Safety Concepts.

> 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_MMAP_MAX`
>   The maximum number of chunks to allocate with `mmap`. Setting this to zero disables all use of `mmap`.
> `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`. Note that requests smaller than this threshold might still be allocated via `mmap`.
> `M_PERTURB`
>   If non-zero, memory blocks are filled with values depending on some low order bits of this parameter when they are allocated (except when allocated by `calloc`) and freed. This can be used to debug the use of uninitialized or freed heap memory. Note that this option does not guarantee that the freed block will have any specific values. It only guarantees that the content the block had before it was freed will be overwritten.
> `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_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.