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(libc.info)Malloc Tunable Parameters

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Malloc Tunable Parameters
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   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)
     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'.