man-pages/man2/mremap.2

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.\" 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
.\"
.TH MREMAP 2 2005-09-13 "Linux 2.6.13" "Linux Programmer's Manual"
.SH NAME
mremap \- re-map a virtual memory address 
.SH SYNOPSIS
.B #define _GNU_SOURCE
.br
.B #include <unistd.h>
.br
.B #include <sys/mman.h>
.sp
.BI "void * mremap(void *" old_address ", size_t " old_size
.BI ", size_t " new_size ", unsigned long " flags );
.fi
.SH DESCRIPTION
\fBmremap\fR() expands (or shrinks) an existing memory mapping, potentially 
moving it at the same time (controlled by the \fIflags\fR argument and 
the available virtual address space).

\fIold_address\fR is the old address of the virtual memory block that you
want to expand (or shrink).  Note that \fIold_address\fR has to be page 
aligned. \fIold_size\fR is the old size of the 
virtual memory block.  \fInew_size\fR is the requested size of the
virtual memory block after the resize. 

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.

\fBmremap\fR() uses the Linux page table scheme.
\fBmremap\fR() changes the 
mapping between virtual addresses and memory pages.  This can be used to
implement a very efficient \fBrealloc\fR().

The \fIflags\fR 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).
.PP
If the memory segment specified by 
.I old_address
and 
.I old_size
is locked (using
.BR mlock ()
or similar), then this lock is maintained when the segment is
resized and/or relocated.
As a consequence, the amount memory locked by the process may change.
.SH "RETURN VALUE"
On success \fBmremap\fR() returns a pointer to the new virtual memory area.
On error, the value
.B MAP_FAILED
(that is, (void *) \-1) is returned, and \fIerrno\fR 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
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 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. Most likely \fIold_address\fR was not 
page aligned.
.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 "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)
.P
Your favorite OS text book for more information on paged memory.
(\fIModern Operating Systems\fR by Andrew S. Tannenbaum, 
\fIInside Linux\fR by Randolf Bentson,
\fIThe Design of the UNIX Operating System\fR by Maurice J. Bach.)