mremap(2) [redhat man page]

MREMAP(2)						     Linux Programmer's Manual							 MREMAP(2)

NAME

       mremap - re-map a virtual memory address

SYNOPSIS

       #include <unistd.h>
       #include <sys/mman.h>

       void * mremap(void *old_address, size_t old_size , size_t new_size, unsigned long flags);

DESCRIPTION

       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.

       The flags argument is a bitmap of flags.

       In Linux the memory is divided into pages.  A user process has (one or) several linear virtual memory segments.	Each virtual  memory  seg-
       ment  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.

FLAGS

       MREMAP_MAYMOVE
	      indicates if the operation should fail, or change the virtual address if the resize cannot be done at the current virtual address.

RETURN VALUE

       On success mremap returns a pointer to the new virtual memory area.  On error, -1 is returned, and errno is set appropriately.

ERRORS

       EINVAL An invalid argument was given. Most likely old_address was not page aligned.

       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.

       EAGAIN The memory segment is locked and cannot be re-mapped.

       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 available.

NOTES

       With  current  glibc  includes,	in  order  to  get  the  definition  of  MREMAP_MAYMOVE,  you  need to define _GNU_SOURCE before including
       <sys/mman.h>.

CONFORMING TO

       This call is Linux-specific, and should not be used in programs intended to  be	portable.   4.2BSD  had  a  (never  actually  implemented)
       mremap(2) call with completely different semantics.

SEE ALSO

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

       Your  favorite  OS text book for more information on paged memory.  (Modern Operating Systems by Andrew S. Tannenbaum, Inside Linux by Ran-
       dolf Bentson, The Design of the UNIX Operating System by Maurice J. Bach.)

Linux 1.3.87							    1996-04-12								 MREMAP(2)

MREMAP(2)						     Linux Programmer's Manual							 MREMAP(2)

NAME

       mremap - remap a virtual memory address

SYNOPSIS

       #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 */);

DESCRIPTION

       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 seg-
       ment 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:

       MREMAP_MAYMOVE
	      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  relo-
	      cated,  then absolute pointers into the old mapping location become invalid (offsets relative to the starting address of the mapping
	      should be employed).

       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 argu-
	      ment,  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.

RETURN VALUE

       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.

ERRORS

       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 BUGS);

	      *  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 available.

CONFORMING TO

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

NOTES

       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.

BUGS

       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 appli-
       cations (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 scenario.

SEE ALSO

       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)

COLOPHON

       This page is part of release 4.15 of the Linux man-pages project.  A description of the project, information about reporting bugs, and  the
       latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.

Linux								    2017-09-25								 MREMAP(2)