MMAP(2) BSD System Calls Manual MMAP(2)NAMEmmap — allocate memory, or map files or devices into memory
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/mman.h>
void *
mmap(void *addr, size_t len, int prot, int flags, int fd, off_t offset);
DESCRIPTION
The mmap() system call causes the pages starting at addr and continuing
for at most len bytes to be mapped from the object described by fd,
starting at byte offset offset. If offset or len is not a multiple of
the pagesize, the mapped region may extend past the specified range. Any
extension beyond the end of the mapped object will be zero-filled.
The addr argument is used by the system to determine the starting address
of the mapping, and its interpretation is dependent on the setting of the
MAP_FIXED flag. If MAP_FIXED is specified in flags, the system will try
to place the mapping at the specified address, possibly removing a map‐
ping that already exists at that location. If MAP_FIXED is not speci‐
fied, then the system will attempt to use the range of addresses starting
at addr if they do not overlap any existing mappings, including memory
allocated by malloc(3) and other such allocators. Otherwise, the system
will choose an alternate address for the mapping (using an implementation
dependent algorithm) that does not overlap any existing mappings. In
other words, without MAP_FIXED the system will attempt to find an empty
location in the address space if the specified address range has already
been mapped by something else. If addr is zero and MAP_FIXED is not
specified, then an address will be selected by the system so as not to
overlap any existing mappings in the address space. In all cases, the
actual starting address of the region is returned. If MAP_FIXED is spec‐
ified, a successful mmap deletes any previous mapping in the allocated
address range. Previous mappings are never deleted if MAP_FIXED is not
specified.
The protections (region accessibility) are specified in the prot argument
by or'ing the following values:
PROT_NONE Pages may not be accessed.
PROT_READ Pages may be read.
PROT_WRITE Pages may be written.
PROT_EXEC Pages may be executed.
Note that, due to hardware limitations, on some platforms PROT_WRITE may
imply PROT_READ, and PROT_READ may imply PROT_EXEC. Portable programs
should not rely on these flags being separately enforcable.
The flags argument specifies the type of the mapped object, mapping
options and whether modifications made to the mapped copy of the page are
private to the process (copy-on-write) or are to be shared with other
references. Sharing, mapping type and options are specified in the flags
argument by or'ing the following values:
MAP_ANON Map anonymous memory not associated with any specific
file. The offset argument is ignored. Mac OS X spe‐
cific: the file descriptor used for creating MAP_ANON
regions can be used to pass some Mach VM flags, and can
be specified as -1 if no such flags are associated with
the region. Mach VM flags are defined in <mach/vm_sta‐
tistics.h> and the ones that currently apply to mmap
are:
VM_FLAGS_PURGABLE to create Mach purgable (i.e.
volatile) memory
VM_MAKE_TAG(tag) to associate an 8-bit tag with the
region
<mach/vm_statistics.h> defines some preset tags (with a
VM_MEMORY_ prefix). Users are encouraged to use tags
between 240 and 255. Tags are used by tools such as
vmmap(1) to help identify specific memory regions.
VM_FLAGS_SUPERPAGE_SIZE_* to use superpages for the
allocation. See <mach/vm_statistics.h> for supported
architectures and sizes (or use VM_FLAGS_SUPER‐
PAGE_SIZE_ANY to have the kernel choose a size). The
specified size must be divisible by the superpage size
(except for VM_FLAGS_SUPERPAGE_SIZE_ANY), and if you
use MAP_FIXED, the specified address must be properly
aligned. If the system cannot satisfy the request with
superpages, the call will fail. Note that currently,
superpages are always wired and not inherited by chil‐
dren of the process.
MAP_FILE Mapped from a regular file. (This is the default map‐
ping type, and need not be specified.)
MAP_FIXED Do not permit the system to select a different address
than the one specified. If the specified address can‐
not be used, mmap() will fail. If MAP_FIXED is speci‐
fied, addr must be a multiple of the pagesize. If a
MAP_FIXED request is successful, the mapping estab‐
lished by mmap() replaces any previous mappings for the
process' pages in the range from addr to addr + len.
Use of this option is discouraged.
MAP_HASSEMAPHORE Notify the kernel that the region may contain sema‐
phores and that special handling may be necessary.
MAP_PRIVATE Modifications are private (copy-on-write).
MAP_SHARED Modifications are shared.
MAP_NOCACHE Pages in this mapping are not retained in the kernel's
memory cache. If the system runs low on memory, pages
in MAP_NOCACHE mappings will be among the first to be
reclaimed. This flag is intended for mappings that
have little locality and provides a hint to the kernel
that pages in this mapping are unlikely to be needed
again in the near future.
Conforming applications must specify either MAP_PRIVATE or MAP_SHARED.
The close(2) system call does not unmap pages, see munmap(2) for further
information.
The current design does not allow a process to specify the location of
swap space. In the future we may define an additional mapping type,
MAP_SWAP, in which the file descriptor argument specifies a file or
device to which swapping should be done.
RETURN VALUES
Upon successful completion, mmap() returns a pointer to the mapped
region. Otherwise, a value of MAP_FAILED is returned and errno is set to
indicate the error.
ERRORS
The mmap() system call will fail if:
[EACCES] The flag PROT_READ was specified as part of the prot
argument and fd was not open for reading. The flags
MAP_SHARED and PROT_WRITE were specified as part of
the flags and prot argument and fd was not open for
writing.
[EBADF] The fd argument is not a valid open file descriptor.
[EINVAL] MAP_FIXED was specified and the addr argument was not
page aligned, or part of the desired address space
resides out of the valid address space for a user
process.
[EINVAL] flags does not include either MAP_PRIVATE or
MAP_SHARED.
[EINVAL] The len argument was negative.
[EINVAL] The offset argument was not page-aligned based on the
page size as returned by getpagesize(3).
[ENODEV] MAP_ANON has not been specified and the file fd refers
to does not support mapping.
[ENOMEM] MAP_FIXED was specified and the addr argument was not
available. MAP_FIXED was specified and the address
range specified exceeds the address space limit for
the process. MAP_ANON was specified and insufficient
memory was available.
[ENXIO] Addresses in the specified range are invalid for fd.
[EOVERFLOW] Addresses in the specified range exceed the maximum
offset set for fd.
LEGACY SYNOPSIS
#include <sys/types.h>
#include <sys/mman.h>
The include file <sys/types.h> is necessary.
COMPATIBILITYmmap() now returns with errno set to EINVAL in places that historically
succeeded. The rules have changed as follows:
· The flags parameter must specify either MAP_PRIVATE or MAP_SHARED.
· The size parameter must not be 0.
· The off parameter must be a multiple of pagesize, as returned by
sysconf().
SEE ALSOmadvise(2), mincore(2), minherit(2), mlock(2), mprotect(2), msync(2),
munlock(2), munmap(2), shmat(2), getpagesize(3)BSD April 21, 2006 BSD
