.\" Copyright (c) 1996 Tom Bjorkholm .\" .\" %%%LICENSE_START(GPLv2+_DOC_FULL) .\" This is free documentation; you can redistribute it and/or .\" modify it under the terms of the GNU General Public License as .\" published by the Free Software Foundation; either version 2 of .\" the License, or (at your option) any later version. .\" .\" The GNU General Public License's references to "object code" .\" and "executables" are to be interpreted as the output of any .\" document formatting or typesetting system, including .\" intermediate and printed output. .\" .\" This manual is distributed in the hope that it will be useful, .\" but WITHOUT ANY WARRANTY; without even the implied warranty of .\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the .\" GNU General Public License for more details. .\" .\" You should have received a copy of the GNU General Public .\" License along with this manual; if not, see .\" . .\" %%%LICENSE_END .\" .\" 1996-04-11 Tom Bjorkholm .\" First version written (1.3.86) .\" 1996-04-12 Tom Bjorkholm .\" Update for Linux 1.3.87 and later .\" 2005-10-11 mtk: Added NOTES for MREMAP_FIXED; revised EINVAL text. .\" .TH MREMAP 2 2017-09-25 "Linux" "Linux Programmer's Manual" .SH NAME mremap \- remap a virtual memory address .SH SYNOPSIS .nf .BR "#define _GNU_SOURCE" " /* See feature_test_macros(7) */" .B #include .PP .BI "void *mremap(void *" old_address ", size_t " old_size , .BI " size_t " new_size ", int " flags ", ... /* void *" new_address " */);" .fi .SH DESCRIPTION .BR mremap () expands (or shrinks) an existing memory mapping, potentially moving it at the same time (controlled by the \fIflags\fP argument and the available virtual address space). .PP \fIold_address\fP is the old address of the virtual memory block that you want to expand (or shrink). Note that \fIold_address\fP has to be page aligned. \fIold_size\fP is the old size of the virtual memory block. \fInew_size\fP is the requested size of the virtual memory block after the resize. An optional fifth argument, .IR new_address , may be provided; see the description of .B MREMAP_FIXED below. .PP If the value of \fIold_size\fP is zero, and \fIold_address\fP refers to a shareable mapping (see .BR mmap (2) .BR MAP_SHARED ), then .BR mremap () will create a new mapping of the same pages. \fInew_size\fP will be the size of the new mapping and the location of the new mapping may be specified with \fInew_address\fP; see the description of .B MREMAP_FIXED below. If a new mapping is requested via this method, then the .B MREMAP_MAYMOVE flag must also be specified. .PP 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. .PP .BR mremap () uses the Linux page table scheme. .BR mremap () changes the mapping between virtual addresses and memory pages. This can be used to implement a very efficient .BR realloc (3). .PP The \fIflags\fP bit-mask argument may be 0, or include the following flag: .TP .B MREMAP_MAYMOVE By default, if there is not sufficient space to expand a mapping at its current location, then .BR 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). .TP .BR MREMAP_FIXED " (since Linux 2.3.31)" This flag serves a similar purpose to the .B MAP_FIXED flag of .BR mmap (2). If this flag is specified, then .BR mremap () accepts a fifth argument, .IR "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 .I new_address and .I new_size is unmapped. If .B MREMAP_FIXED is specified, then .B MREMAP_MAYMOVE must also be specified. .PP If the memory segment specified by .I old_address and .I old_size is locked (using .BR 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. .SH RETURN VALUE On success .BR mremap () returns a pointer to the new virtual memory area. On error, the value .B MAP_FAILED (that is, \fI(void\ *)\ \-1\fP) is returned, and \fIerrno\fP is set appropriately. .SH ERRORS .TP .B EAGAIN The caller tried to expand a memory segment that is locked, but this was not possible without exceeding the .B RLIMIT_MEMLOCK resource limit. .TP .B EFAULT "Segmentation fault." Some address in the range \fIold_address\fP to \fIold_address\fP+\fIold_size\fP is an invalid virtual memory address for this process. You can also get .B EFAULT even if there exist mappings that cover the whole address space requested, but those mappings are of different types. .TP .B EINVAL An invalid argument was given. Possible causes are: .RS .IP * 3 \fIold_address\fP was not page aligned; .IP * a value other than .B MREMAP_MAYMOVE or .B MREMAP_FIXED was specified in .IR flags ; .IP * .I new_size was zero; .IP * .I new_size or .I new_address was invalid; .IP * the new address range specified by .I new_address and .I new_size overlapped the old address range specified by .I old_address and .IR old_size ; .IP * .B MREMAP_FIXED was specified without also specifying .BR MREMAP_MAYMOVE ; .IP * \fIold_size\fP was zero and \fIold_address\fP does not refer to a shareable mapping (but see BUGS); .IP * \fIold_size\fP was zero and the .BR MREMAP_MAYMOVE flag was not specified. .RE .TP .B ENOMEM The memory area cannot be expanded at the current virtual address, and the .B MREMAP_MAYMOVE flag is not set in \fIflags\fP. Or, there is not enough (virtual) memory available. .SH 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) .\" .BR mremap (2) .\" call with completely different semantics. .SH NOTES Prior to version 2.4, glibc did not expose the definition of .BR MREMAP_FIXED , and the prototype for .BR mremap () did not allow for the .I new_address argument. .PP If .BR mremap () is used to move or expand an area locked with .BR mlock (2) or equivalent, the .BR mremap () call will make a best effort to populate the new area but will not fail with .B ENOMEM if the area cannot be populated. .SH BUGS Before Linux 4.14, if .I old_size was zero and the mapping referred to by .I old_address was a private mapping .RB ( mmap "(2) " MAP_PRIVATE ), .BR 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 .BR mremap () is to create a new mapping based on the original mapping). Since Linux 4.14, .\" commit dba58d3b8c5045ad89c1c95d33d01451e3964db7 .BR mremap () fails with the error .B EINVAL in this scenario. .SH SEE ALSO .BR brk (2), .BR getpagesize (2), .BR getrlimit (2), .BR mlock (2), .BR mmap (2), .BR sbrk (2), .BR malloc (3), .BR realloc (3) .PP Your favorite text book on operating systems for more information on paged memory (e.g., \fIModern Operating Systems\fP by Andrew S. Tanenbaum, \fIInside Linux\fP by Randolf Bentson, \fIThe Design of the UNIX Operating System\fP by Maurice J. Bach) .SH COLOPHON This page is part of release 4.16 of the Linux .I 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/.