MREMAP(2)

NAME
mremap - re-map a virtual memory address

SYNOPSIS
#define _GNU_SOURCE
#include <unistd.h>
#include <sys/mman.h>

void * mremap(void *old_address, size_t old_size , size_t new_size, unsigned long flags);

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. 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(). 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).

If the memory segment specified by old_address and old_size is locked (using 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.

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; or the new address range specified by new_address and new_size overlapped the old address range specified by old_address and old_size; or MREMAP_FIXED was specified without also specifying MREMAP_MAYMOVE.

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.

NOTES
Linux 2.3.31 added an additional flags value: MREMAP_FIXED, with a similar purpose to the MAP_FIXED flag for mmap(2). If this flag is specified, then mremap() accepts a fifth argument, new_address, which specifies a fixed address to which the mapping should be moved. This address must be page aligned. If MREMAP_FIXED is specified, then MREMAP_MAYMOVE must also be specified. Currently, neither the definition of MREMAP_FIXED nor a prototype for the new_address argument are exposed by glibc.

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) mremap(2) call with completely different semantics.

SEE ALSO
brk(2), getpagesize(2), getrlimit(2), mlock(2), mmap(2), sbrk(2), malloc(3), realloc(3)

Your favorite OS text book for more information on paged memory. (Modern Operating Systems by Andrew S. Tannenbaum, Inside Linux by Randolf Bentson, The Design of the UNIX Operating System by Maurice J. Bach.)