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       mremap - remap a virtual memory address


       #define _GNU_SOURCE         /* See feature_test_macros(7) */
       #include <sys/mman.h>

       void *mremap(void *old_address, size_t old_size,
                    size_t new_size, int flags, ... /* void *new_address */);


       mremap()  expands  (or  shrinks)  an existing memory mapping, potentially moving it at the
       same time (controlled by the flags argument and the available virtual address space).

       old_address is the old address of the virtual memory block that you  want  to  expand  (or
       shrink).   Note  that old_address has to be page aligned.  old_size is the old size of the
       virtual memory block.  new_size is the requested size of the virtual  memory  block  after
       the resize.  An optional fifth argument, new_address, may be provided; see the description
       of MREMAP_FIXED below.

       If the value of old_size is zero, and old_address  refers  to  a  shareable  mapping  (see
       mmap(2)  MAP_SHARED), then mremap() will create a new mapping of the same pages.  new_size
       will be the size of the new mapping and the location of the new mapping may  be  specified
       with  new_address;  see  the  description  of  MREMAP_FIXED  below.   If  a new mapping is
       requested via this method, then the MREMAP_MAYMOVE flag must also be specified.

       In Linux the memory is divided into pages.  A user process has  (one  or)  several  linear
       virtual  memory  segments.   Each  virtual memory segment has one or more mappings to real
       memory pages (in the page table).  Each virtual memory  segment  has  its  own  protection
       (access  rights),  which  may  cause  a  segmentation  violation if the memory is accessed
       incorrectly (e.g., writing to a read-only segment).  Accessing virtual memory  outside  of
       the segments will also cause a segmentation violation.

       mremap()  uses  the Linux page table scheme.  mremap() changes the mapping between virtual
       addresses and memory pages.  This can be used to implement a very efficient realloc(3).

       The flags bit-mask argument may be 0, or include the following flag:

              By default, if there is not sufficient space to expand a  mapping  at  its  current
              location,  then  mremap()  fails.   If  this  flag is specified, then the kernel is
              permitted to relocate the mapping to a new virtual address, if necessary.   If  the
              mapping  is  relocated, then absolute pointers into the old mapping location become
              invalid (offsets relative  to  the  starting  address  of  the  mapping  should  be

       MREMAP_FIXED (since Linux 2.3.31)
              This  flag serves a similar purpose to the MAP_FIXED flag of mmap(2).  If this flag
              is specified, then mremap() accepts  a  fifth  argument,  void *new_address,  which
              specifies  a page-aligned address to which the mapping must be moved.  Any previous
              mapping at the address range specified by new_address and new_size is unmapped.  If
              MREMAP_FIXED is specified, then MREMAP_MAYMOVE must also be specified.

       If  the  memory segment specified by old_address and old_size is locked (using mlock(2) or
       similar), then this lock is maintained when the segment is resized and/or relocated.  As a
       consequence, the amount of memory locked by the process may change.


       On success mremap() returns a pointer to the new virtual memory area.  On error, the value
       MAP_FAILED (that is, (void *) -1) is returned, and errno is set appropriately.


       EAGAIN The caller tried to expand a memory segment  that  is  locked,  but  this  was  not
              possible without exceeding the RLIMIT_MEMLOCK resource limit.

       EFAULT "Segmentation fault." Some address in the range old_address to old_address+old_size
              is an invalid virtual memory address for this process.  You  can  also  get  EFAULT
              even  if  there  exist  mappings  that cover the whole address space requested, but
              those mappings are of different types.

       EINVAL An invalid argument was given.  Possible causes are:

              *  old_address was not page aligned;

              *  a value other than MREMAP_MAYMOVE or MREMAP_FIXED was specified in flags;

              *  new_size was zero;

              *  new_size or new_address was invalid;

              *  the new address range specified by new_address and new_size overlapped  the  old
                 address range specified by old_address and old_size;

              *  MREMAP_FIXED was specified without also specifying MREMAP_MAYMOVE;

              *  old_size was zero and old_address does not refer to a shareable mapping (but see

              *  old_size was zero and the MREMAP_MAYMOVE flag was not specified.

       ENOMEM The memory area cannot  be  expanded  at  the  current  virtual  address,  and  the
              MREMAP_MAYMOVE  flag is not set in flags.  Or, there is not enough (virtual) memory


       This call is Linux-specific, and should not be used in programs intended to be portable.


       Prior to version 2.4, glibc did  not  expose  the  definition  of  MREMAP_FIXED,  and  the
       prototype for mremap() did not allow for the new_address argument.

       If  mremap()  is  used  to  move or expand an area locked with mlock(2) or equivalent, the
       mremap() call will make a best effort to populate the new area  but  will  not  fail  with
       ENOMEM if the area cannot be populated.


       Before  Linux  4.14, if old_size was zero and the mapping referred to by old_address was a
       private mapping (mmap(2) MAP_PRIVATE), mremap() created a new private mapping unrelated to
       the  original mapping.  This behavior was unintended and probably unexpected in user-space
       applications (since the intention of mremap() is to create a  new  mapping  based  on  the
       original  mapping).   Since  Linux  4.14,  mremap()  fails  with  the error EINVAL in this


       brk(2), getpagesize(2), getrlimit(2), mlock(2), mmap(2), sbrk(2), malloc(3), realloc(3)

       Your favorite text book on operating systems for more information on paged  memory  (e.g.,
       Modern  Operating  Systems  by  Andrew  S. Tanenbaum, Inside Linux by Randolf Bentson, The
       Design of the UNIX Operating System by Maurice J. Bach)


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