mirror of https://github.com/mkerrisk/man-pages
214 lines
6.2 KiB
Groff
214 lines
6.2 KiB
Groff
.\" Copyright (c) 1996 Tom Bjorkholm <tomb@mydata.se>
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.\"
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.\" %%%LICENSE_START(GPLv2+_doc_full)
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.\" This is free documentation; you can redistribute it and/or
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.\" modify it under the terms of the GNU General Public License as
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.\" published by the Free Software Foundation; either version 2 of
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.\" the License, or (at your option) any later version.
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.\"
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.\" The GNU General Public License's references to "object code"
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.\" and "executables" are to be interpreted as the output of any
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.\" document formatting or typesetting system, including
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.\" intermediate and printed output.
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.\"
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.\" This manual is distributed in the hope that it will be useful,
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.\" but WITHOUT ANY WARRANTY; without even the implied warranty of
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.\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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.\" GNU General Public License for more details.
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.\"
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.\" You should have received a copy of the GNU General Public
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.\" License along with this manual; if not, see
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.\" <http://www.gnu.org/licenses/>.
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.\" %%%LICENSE_END
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.\"
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.\" 1996-04-11 Tom Bjorkholm <tomb@mydata.se>
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.\" First version written (1.3.86)
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.\" 1996-04-12 Tom Bjorkholm <tomb@mydata.se>
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.\" Update for Linux 1.3.87 and later
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.\" 2005-10-11 mtk: Added NOTES for MREMAP_FIXED; revised EINVAL text.
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.\"
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.TH MREMAP 2 2010-06-10 "Linux" "Linux Programmer's Manual"
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.SH NAME
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mremap \- remap a virtual memory address
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.SH SYNOPSIS
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.nf
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.BR "#define _GNU_SOURCE" " /* See feature_test_macros(7) */"
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.br
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.B #include <sys/mman.h>
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.sp
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.BI "void *mremap(void *" old_address ", size_t " old_size ,
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.BI " size_t " new_size ", int " flags ", ... /* void *" new_address " */);"
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.fi
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.SH DESCRIPTION
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.BR mremap ()
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expands (or shrinks) an existing memory mapping, potentially
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moving it at the same time (controlled by the \fIflags\fP argument and
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the available virtual address space).
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\fIold_address\fP is the old address of the virtual memory block that you
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want to expand (or shrink).
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Note that \fIold_address\fP has to be page
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aligned.
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\fIold_size\fP is the old size of the
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virtual memory block.
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\fInew_size\fP is the requested size of the
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virtual memory block after the resize.
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An optional fifth argument,
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.IR new_address ,
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may be provided; see the description of
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.B MREMAP_FIXED
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below.
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In Linux the memory is divided into pages.
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A user process has (one or)
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several linear virtual memory segments.
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Each virtual memory segment has one
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or more mappings to real memory pages (in the page table).
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Each virtual memory segment has its own
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protection (access rights), which may cause
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a segmentation violation if the memory is accessed incorrectly (e.g.,
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writing to a read-only segment).
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Accessing virtual memory outside of the
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segments will also cause a segmentation violation.
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.BR mremap ()
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uses the Linux page table scheme.
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.BR mremap ()
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changes the
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mapping between virtual addresses and memory pages.
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This can be used to implement a very efficient
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.BR realloc (3).
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The \fIflags\fP bit-mask argument may be 0, or include the following flag:
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.TP
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.B MREMAP_MAYMOVE
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By default, if there is not sufficient space to expand a mapping
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at its current location, then
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.BR mremap ()
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fails.
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If this flag is specified, then the kernel is permitted to
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relocate the mapping to a new virtual address, if necessary.
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If the mapping is relocated,
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then absolute pointers into the old mapping location
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become invalid (offsets relative to the starting address of
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the mapping should be employed).
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.TP
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.BR MREMAP_FIXED " (since Linux 2.3.31)"
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This flag serves a similar purpose to the
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.B MAP_FIXED
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flag of
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.BR mmap (2).
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If this flag is specified, then
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.BR mremap ()
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accepts a fifth argument,
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.IR "void *new_address" ,
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which specifies a page-aligned address to which the mapping must
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be moved.
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Any previous mapping at the address range specified by
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.I new_address
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and
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.I new_size
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is unmapped.
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If
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.B MREMAP_FIXED
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is specified, then
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.B MREMAP_MAYMOVE
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must also be specified.
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.PP
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If the memory segment specified by
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.I old_address
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and
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.I old_size
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is locked (using
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.BR mlock (2)
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or similar), then this lock is maintained when the segment is
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resized and/or relocated.
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As a consequence, the amount of memory locked by the process may change.
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.SH RETURN VALUE
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On success
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.BR mremap ()
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returns a pointer to the new virtual memory area.
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On error, the value
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.B MAP_FAILED
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(that is, \fI(void\ *)\ \-1\fP) is returned,
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and \fIerrno\fP is set appropriately.
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.SH ERRORS
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.TP
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.B EAGAIN
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The caller tried to expand a memory segment that is locked,
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but this was not possible without exceeding the
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.B RLIMIT_MEMLOCK
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resource limit.
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.TP
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.B EFAULT
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"Segmentation fault." Some address in the range
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\fIold_address\fP to \fIold_address\fP+\fIold_size\fP is an invalid
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virtual memory address for this process.
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You can also get
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.B EFAULT
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even if there exist mappings that cover the
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whole address space requested, but those mappings are of different types.
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.TP
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.B EINVAL
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An invalid argument was given.
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Possible causes are: \fIold_address\fP was not
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page aligned; a value other than
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.B MREMAP_MAYMOVE
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or
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.B MREMAP_FIXED
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was specified in
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.IR flags ;
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.I new_size
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was zero;
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.I new_size
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or
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.I new_address
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was invalid;
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or the new address range specified by
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.I new_address
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and
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.I new_size
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overlapped the old address range specified by
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.I old_address
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and
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.IR old_size ;
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or
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.B MREMAP_FIXED
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was specified without also specifying
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.BR MREMAP_MAYMOVE .
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.TP
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.B ENOMEM
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The memory area cannot be expanded at the current virtual address, and the
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.B MREMAP_MAYMOVE
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flag is not set in \fIflags\fP.
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Or, there is not enough (virtual) memory available.
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.SH CONFORMING TO
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This call is Linux-specific, and should not be used in programs
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intended to be portable.
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.\" 4.2BSD had a (never actually implemented)
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.\" .BR mremap (2)
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.\" call with completely different semantics.
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.SH NOTES
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Prior to version 2.4, glibc did not expose the definition of
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.BR MREMAP_FIXED ,
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and the prototype for
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.BR mremap ()
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did not allow for the
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.I new_address
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argument.
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.SH SEE ALSO
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.BR brk (2),
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.BR getpagesize (2),
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.BR getrlimit (2),
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.BR mlock (2),
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.BR mmap (2),
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.BR sbrk (2),
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.BR malloc (3),
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.BR realloc (3)
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Your favorite text book on operating systems
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for more information on paged memory
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(e.g., \fIModern Operating Systems\fP by Andrew S. Tanenbaum,
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\fIInside Linux\fP by Randolf Bentson,
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\fIThe Design of the UNIX Operating System\fP by Maurice J. Bach)
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