mremap

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 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:

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 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.

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 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

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 5.04 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/.

Referenced By

getrlimit(2), ioctl_userfaultfd(2), memusage(1), mmap(2), mmap2(2), remap_file_pages(2), stress-ng(1), syscalls(2), userfaultfd(2).