MREMAP(2)

NAME
mremap - re-map a virtual memory address

SYNOPSIS
#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.

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.

FLAGS

MREMAP_MAYMOVE
    indicates if the operation should fail, or change the virtual address if the resize cannot be done at the current virtual address.

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 memory segment is locked and cannot be re-mapped.

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. Most likely old_address was not page aligned.

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
With current glibc includes, in order to get the definition of MREMAP_MAYMOVE, you need to define _GNU_SOURCE before including <sys/mman.h>.

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), 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.)