MREMAP(2) Linux Programmer's Manual MREMAP(2)
mremap - re-map a virtual memory address
void * mremap(void *old_address, size_t old_size , size_t new_size,
unsigned long flags);
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
The flags argument is a bitmap of flags.
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 viola-
mremap uses the Linux page table scheme. mremap changes the mapping
between virtual addresses and memory pages. This can be used to imple-
ment a very efficient realloc.
indicates if the operation should fail, or change the virtual
address if the resize cannot be done at the current virtual
On success mremap returns a pointer to the new virtual memory area. On
error, -1 is returned, and errno is set appropriately.
EINVAL An invalid argument was given. Most likely old_address was not
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 map-
pings that cover the whole address space requested, but those
mappings are of different types.
EAGAIN The memory segment is locked and cannot be re-mapped.
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.
With current glibc includes, in order to get the definition of
MREMAP_MAYMOVE, you need to define _GNU_SOURCE before including
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.
getpagesize(2), realloc(3), malloc(3), brk(2), sbrk(2), mmap(2)
Your favorite OS text book for more information on paged memory. (Mod-
ern Operating Systems by Andrew S. Tannenbaum, Inside Linux by Randolf
Bentson, The Design of the UNIX Operating System by Maurice J. Bach.)
Linux 1.3.87 1996-04-12 MREMAP(2)