mremap(2) — Linux manual page


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

NAME         top

       mremap - remap a virtual memory address

SYNOPSIS         top

       #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         top

       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.

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

              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 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 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

       MREMAP_DONTUNMAP (since Linux 5.7)
              This flag, which must be used in conjunction with
              MREMAP_MAYMOVE, remaps a mapping to a new address but does not
              unmap the mapping at old_address.

              The MREMAP_DONTUNMAP flag can be used only with private
              anonymous mappings (see the description of MAP_PRIVATE and
              MAP_ANONYMOUS in mmap(2)).

              After completion, any access to the range specified by
              old_address and old_size will result in a page fault.  The
              page fault will be handled by a userfaultfd(2) handler if the
              address is in a range previously registered with
              userfaultfd(2).  Otherwise, the kernel allocates a zero-filled
              page to handle the fault.

              The MREMAP_DONTUNMAP flag may be used to atomically move a
              mapping while leaving the source mapped.  See NOTES for some
              possible applications of MREMAP_DONTUNMAP.

       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         top

       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         top

       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 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 or
                 MREMAP_DONTUNMAP 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 or MREMAP_DONTUNMAP was specified without also
                 specifying MREMAP_MAYMOVE;

              *  MREMAP_DONTUNMAP was specified, but one or more pages in
                 the range specified by old_address and old_size were not
                 private anonymous;

              *  MREMAP_DONTUNMAP was specified and old_size was not equal
                 to new_size;

              *  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

       ENOMEM Not enough memory was available to complete the operation.
              Possible causes are:

              *  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.

              *  MREMAP_DONTUNMAP was used causing a new mapping to be
                 created that would exceed the (virtual) memory available.
                 Or, it would exceed the maximum number of allowed mappings.

CONFORMING TO         top

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

NOTES         top

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

       In Linux, memory is divided into pages.  A 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 (SIGSEGV) 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.

       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

       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.

       Possible applications for MREMAP_DONTUNMAP include:

       *  Non-cooperative userfaultfd(2): an application can yank out a
          virtual address range using MREMAP_DONTUNMAP and then employ a
          userfaultfd(2) handler to handle the page faults that subsequently
          occur as other threads in the process touch pages in the yanked

       *  Garbage collection: MREMAP_DONTUNMAP can be used in conjunction
          with userfaultfd(2) to implement garbage collection algorithms
          (e.g., in a Java virtual machine).  Such an implementation can be
          cheaper (and simpler) than conventional garbage collection
          techniques that involve marking pages with protection PROT_NONE in
          conjunction with the of a SIGSEGV handler to catch accesses to
          those pages.

BUGS         top

       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 scenario.

SEE ALSO         top

       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 Randolph Bentson, The Design of the UNIX Operating
       System by Maurice J. Bach)

COLOPHON         top

       This page is part of release 5.09 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

Linux                            2020-06-09                        MREMAP(2)

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