mallinfo

       loc_get_state, malloc_set_state, mallinfo - dynamic allocation tuning

SYNOPSIS
       #include <malloc.h>

       int mallopt (int param, int value);
       int malloc_trim (size_t pad);
       size_t malloc_usable_size (void *ptr);
       void malloc_stats (void);
       void *malloc_get_state (void);
       int malloc_set_state (void *ptr);
       struct mallinfo mallinfo (void);

DESCRIPTION
       These functions allow glibc-specific interaction with the dynamic  mem-
       ory allocation routines.

       mallopt()  allows tuning of the parameters affecting allocation.  param
       is one of the constants listed below;  value  should  be  specified  in
       bytes.

       M_MXFAST
              Free  chunks  not  larger than the given value are not joined to
              adjacent free chunks; larger ones are joined.  This is  intended
              to optimize future requests for small chunks of the same size as
              previously freed  chunks.   Allowed  values  are  in  the  range
              [0-80];  a  value  of  0  causes  all  free chunks to be joined.
              Default: 64.

       M_TRIM_THRESHOLD
              Unused memory is returned to the OS when the size  available  to
              be returned exceeds the given value.

              Note  that  not  all unused memory is able to be returned to the
              OS; in particular, it is not possible to return an unused  block
              when  an  in-use block lies between it and the "top" of the data
              segment.  However, the free block may be used to satisfy  future
              allocation requests.

              Smaller values for this parameter cause sbrk() to be called more
              frequently with a negative argument, reducing memory usage,  but
              with  increased  overhead of extra syscalls.  A value of -1 dis-
              ables trimming.  Default: 128*1024.


       M_TOP_PAD
              When sbrk() is called with a positive argument to allocate addi-
              tional  address  space,  the given value specifies an additional
              amount to be allocated, beyond what is necessary to satisfy  the
              request.   This  value  also  defines an amount of address space
              which is not released to the OS when sbrk()  is  called  with  a
              negative  argument.  Again, the intent is to minimize the number
              of syscalls, without needlessly using  large  usage  of  memory.
              Default: 0 (allocation requests are internally rounded up to the

       M_MMAP_MAX
              The given value sets  the  maximum  number  of  mmap()-allocated
              chunks allowed to be in use at a given time (even if the size of
              the allocation request exceeds the value of the M_MMAP_THRESHOLD
              parameter).   This  is useful on systems where the mmap() imple-
              mentation scales poorly.  A value  of  0  disables  the  use  of
              mmap().  Default: 65536.

       M_CHECK_ACTION
              The 3 low bits of this value control the behavior when heap cor-
              ruption is detected.

              If set to 0, errors are silently ignored (but  the  behavior  of
              the program is undefined).

              If  the  low  bit  is  set,  an error message is printed; If the
              third-lowest bit is unset, the message is surrounded  by  aster-
              isks ("*") and includes the relevant pointer address.

              If  the second-lowest bit is set, the program is then terminated
              with abort().

              The default value for glibc 2.3.3 and  earlier  was  1,  causing
              only  an  informative message to be output.  glibc-2.3.4 changes
              the default to 3, which also causes the program to abort.

       malloc_trim() explicitly requests  that  any  unused  memory  space  be
       returned  to  the OS.  Note that this happens automatically when free()
       is called with a sufficiently large chunk; see the M_TRIM_THRESHOLD and
       M_TOP_PAD  parameters,  above.  pad specifies the number of bytes to be
       retained for use in future allocation requests; when called by  free(),
       this is the value of M_TOP_PAD.

       malloc_usable_size()  returns  the  number  of  bytes  available in the
       dynamically allocated  buffer  ptr,  which  may  be  greater  than  the
       requested  size  (but  is  guaranteed  to  be at least as large, if the
       request was successful).  Typically, you  should  store  the  requested
       allocation size rather than use this function.

       malloc_stats()  outputs  to stderr some information about the program's
       usage of dynamic memory.  Information for each arena is displayed.

              system bytes is the amount of address space obtained from the OS

              in use bytes is the amount of that space which the  program  has
              requested and are in use.

              max  mmap  regions is largest number of mmap() regions allocated
              at a given time.

              max mmap bytes is the largest total amount of address space ever
              allocated by mmap() at a given time.

                     not including that from mmap()

              int ordblks;
                     Count of independent, unused chunks

              int smblks;
                     Count of chunks small enough to qualify for fast  alloca-
                     tion  without  being joined to adjacent free chunks; this
                     field is not meaningful in the glibc implementation.

              int hblks;
                     Count of chunks allocated by mmap()

              int hblkhd;
                     Address space allocated by the  dynamic  allocation  rou-
                     tines using mmap()

              int usmblks;
                     Largest amount of address space ever used by the process;
                     this field is not meaningful in the glibc implementation.

              int fsmblks;
                     Total unused address space in small,  non-joined  chunks;
                     this field is not meaningful in the glibc implementation.

              int uordblks;
                     Dynamically  allocated address space used by the program,
                     including  book-keeping  overhead   per   allocation   of
                     sizeof(void *) bytes.

              int fordblks;
                     Unused address space, including that from mmap()

              int keepcost;
                     Upper  bound on the address space available for return to
                     the OS by malloc_trim().

RETURN VALUE
       mallopt() returns 1 on success, and 0 on failure.

       malloc_trim() returns 1 if any memory was returned to  the  OS,  and  0
       otherwise.

       malloc_usable_size()  returns  the  usable size of the allocated region
       beginning at ptr, or 0 if ptr is NULL.

       malloc_get_state() returns a pointer to a description of the  state  of
       the allocation routines, or NULL on error.

       malloc_set_state() returns 0 on success, and nonzero on error.

       mallinfo() returns a structure containing information about the dynamic
       memory use of the program.

       Since libc 5.4.23, the mallopt() tuning parameters  are  accessible  at
       runtime through the following environment variables:

              MALLOC_TRIM_THRESHOLD_
              MALLOC_TOP_PAD_
              MALLOC_MMAP_THRESHOLD_
              MALLOC_MMAP_MAX_
              MALLOC_CHECK_

       Only the first byte of MALLOC_CHECK_ is considered; "10" is interpreted
       as 1, and "64" is interpreted as 6.

CONFORMING TO
       mallopt() and mallinfo(), and M_MXFAST are defined by SVID/XPG.

       That standard also defines the mallopt() parameters M_NLBLKS,  M_GRAIN,
       and  M_KEEP,  but  these values have no effect in the glibc implementa-
       tion.

       The remainder of these functions and variables are GNU extensions,  and
       should not be used in programs intended to be portable.

AUTHOR
       glibc uses a dynamic allocation routines heavily based on the implemen-
       tation by Doug Lea <dl@cs.oswego.edu>.

SEE ALSO
       sbrk(2), mmap(2), stderr(2), malloc_hook(3), malloc(3), cfree(3), mema-
       lign(3), pvalloc(3).



GNU                               21 May 2006                       MALLOPT(3)
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