man2/mremap.2

.\" Copyright (c) 1996 Tom Bjorkholm <tomb@mydata.se>
.\"
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.\" 1996-04-11 Tom Bjorkholm <tomb@mydata.se>
.\"            First version written (1.3.86)
.\" 1996-04-12 Tom Bjorkholm <tomb@mydata.se>
.\"            Update for Linux 1.3.87 and later
.\" 2005-10-11 mtk: Added NOTES for MREMAP_FIXED; revised EINVAL text.
.\"
.TH MREMAP 2 2019-03-06 "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 <sys/mman.h>
.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)