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. An optional fifth argument, new_address, may be provided; see the description of MREMAP_FIXED below.

If the value of old_size is zero, and old_address refers to a shareable mapping (see mmap(2) MAP_SHARED), then mremap() will create a new mapping of the same pages. new_size will be the size of the new mapping and the location of the new mapping may be specified with new_address; see the description of MREMAP_FIXED below. If a new mapping is requested via this method, then the MREMAP_MAYMOVE flag must also be specified.

The flags bit-mask argument may be 0, or include the following flags:

If the memory segment specified by old_address and old_size is locked (using mlock(2) or similar), then this lock is maintained when the segment is resized and/or relocated. As a consequence, the amount of 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 to indicate the error.

ERRORS

CONFORMING TO

This call is Linux-specific, and should not be used in programs intended to be portable.

NOTES

mremap() changes the mapping between virtual addresses and memory pages. This can be used to implement a very efficient realloc(3).

In Linux, memory is divided into pages. A 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 (SIGSEGV) 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.

If mremap() is used to move or expand an area locked with mlock(2) or equivalent, the mremap() call will make a best effort to populate the new area but will not fail with ENOMEM if the area cannot be populated.

Prior to version 2.4, glibc did not expose the definition of MREMAP_FIXED, and the prototype for mremap() did not allow for the new_address argument.

BUGS

Before Linux 4.14, if old_size was zero and the mapping referred to by old_address was a private mapping (mmap(2) MAP_PRIVATE), mremap() created a new private mapping unrelated to the original mapping. This behavior was unintended and probably unexpected in user-space applications (since the intention of mremap() is to create a new mapping based on the original mapping). Since Linux 4.14, mremap() fails with the error EINVAL in this scenario.

SEE ALSO

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

Your favorite text book on operating systems for more information on paged memory (e.g., Modern Operating Systems by Andrew S. Tanenbaum, Inside Linux by Randolph Bentson, The Design of the UNIX Operating System by Maurice J. Bach)