mremap(2) — Linux manual page


mremap(2)                  System Calls Manual                 mremap(2)

NAME         top

       mremap - remap a virtual memory address

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

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

       void *mremap(void old_address[.old_size], 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 the description of MAP_SHARED in mmap(2)),
       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

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

              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 specified.

       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 to indicate the error.

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

              •  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.

STANDARDS         top


HISTORY         top

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

NOTES         top

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

       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 populated.

       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 range.

       •  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 use 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 (see the description of
       MAP_PRIVATE in mmap(2)), 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

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Linux man-pages 6.9.1          2024-05-02                      mremap(2)

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