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 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.
indicates if the operation should fail, or change
the virtual address if the resize cannot be done at
the current virtual address.
On success mremap returns a pointer to the new virtual
memory area. On error, -1 is returned, and errno is set
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.
EAGAIN The memory segment is locked and cannot be re-
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)
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 dif
getpagesize(2), realloc(3), malloc(3), brk(2), sbrk(2),
Your favorite OS text book for more information on paged
memory. (Modern Operating Systems by Andrew S. Tannen
baum, Inside Linux by Randolf Bentson, The Design of the
UNIX Operating System by Maurice J. Bach.)