mmap(2)
NAME
mmap - map pages of memory
SYNOPSIS
#include <sys/mman.h>
void *mmap(void *addr, size_t len, int prot, int flags, int
fildes, off_t off);
DESCRIPTION
The mmap() function establishes a mapping between a
process's address space and a file or shared memory object.
The format of the call is as follows:
pa = mmap(addr, len, prot, flags, fildes, off);
The mmap() function establishes a mapping between the
address space of the process at an address pa for len bytes
to the memory object represented by the file descriptor
fildes at offset off for len bytes. The value of pa is a
function of the addr argument and values of flags, further
described below. A successful mmap() call returns pa as its
result. The address range starting at pa and continuing for
len bytes will be legitimate for the possible (not neces-
sarily current) address space of the process. The range of
bytes starting at off and continuing for len bytes will be
legitimate for the possible (not necessarily current)
offsets in the file or shared memory object represented by
fildes.
The mmap() function allows [pa, pa + len) to extend beyond
the end of the object both at the time of the mmap() and
while the mapping persists, such as when the file is created
prior to the mmap() call and has no contents, or when the
file is truncated. Any reference to addresses beyond the end
of the object, however, will result in the delivery of a
SIGBUS or SIGSEGV signal. The mmap() function cannot be used
to implicitly extend the length of files.
The mapping established by mmap() replaces any previous map-
pings for those whole pages containing any part of the
address space of the process starting at pa and continuing
for len bytes.
If the size of the mapped file changes after the call to
mmap() as a result of some other operation on the mapped
file, the effect of references to portions of the mapped
region that correspond to added or removed portions of the
file is unspecified.
The mmap() function is supported for regular files and
shared memory objects. Support for any other type of file is
unspecified.
The prot argument determines whether read, write, execute,
or some combination of accesses are permitted to the data
being mapped. The prot argument should be either PROT_NONE
or the bitwise inclusive OR of one or more of the other
flags in the following table, defined in the header
<sys/mman.h>.
PROT_READ
Data can be read.
PROT_WRITE
Data can be written.
PROT_EXEC
Data can be executed.
PROT_NONE
Data cannot be accessed.
If an implementation of mmap() for a specific platform can-
not support the combination of access types specified by
prot, the call to mmap() fails. An implementation may permit
accesses other than those specified by prot; however, the
implementation will not permit a write to succeed where
PROT_WRITE has not been set or permit any access where
PROT_NONE alone has been set. Each platform-specific imple-
mentation of mmap() supports the following values of prot:
PROT_NONE, PROT_READ, PROT_WRITE, and the inclusive OR of
PROT_READ and PROT_WRITE. On some platforms, the PROT_WRITE
protection option is implemented as PROT_READ|PROT_WRITE and
PROT_EXEC as PROT_READ|PROT_EXEC. The file descriptor fildes
is opened with read permission, regardless of the protection
options specified. If PROT_WRITE is specified, the applica-
tion must have opened the file descriptor fildes with write
permission unless MAP_PRIVATE is specified in the flags
argument as described below.
The flags argument provides other information about the
handling of the mapped data. The value of flags is the bit-
wise inclusive OR of these options, defined in <sys/mman.h>:
MAP_SHARED
Changes are shared.
MAP_PRIVATE
Changes are private.
MAP_FIXED
Interpret addr exactly.
MAP_NORESERVE
Do not reserve swap space.
MAP_ANON
Map anonymous memory.
MAP_ALIGN
Interpret addr as required aligment.
The MAP_SHARED and MAP_PRIVATE options describe the disposi-
tion of write references to the underlying object. If
MAP_SHARED is specified, write references will change the
memory object. If MAP_PRIVATE is specified, the initial
write reference will create a private copy of the memory
object page and redirect the mapping to the copy. The
private copy is not created until the first write; until
then, other users who have the object mapped MAP_SHARED can
change the object. Either MAP_SHARED or MAP_PRIVATE must be
specified, but not both. The mapping type is retained across
fork(2).
When MAP_FIXED is set in the flags argument, the system is
informed that the value of pa must be addr, exactly. If
MAP_FIXED is set, mmap() may return (void *)-1 and set errno
to EINVAL. If a MAP_FIXED request is successful, the map-
ping established by mmap() replaces any previous mappings
for the process's pages in the range [pa, pa + len). The use
of MAP_FIXED is discouraged, since it may prevent a system
from making the most effective use of its resources.
When MAP_FIXED is set and the requested address is the same
as previous mapping, the previous address is unmapped and
the new mapping is created on top of the old one.
When MAP_FIXED is not set, the system uses addr to arrive at
pa. The pa so chosen will be an area of the address space
that the system deems suitable for a mapping of len bytes to
the file. The mmap() function interprets an addr value of 0
as granting the system complete freedom in selecting pa,
subject to constraints described below. A non-zero value of
addr is taken to be a suggestion of a process address near
which the mapping should be placed. When the system selects
a value for pa, it will never place a mapping at address 0,
nor will it replace any extant mapping, nor map into areas
considered part of the potential data or stack "segments".
When MAP_ALIGN is set, the system is informed that the
alignment of pa must be the same as addr. The alignment
value in addr must be 0 or some power of two multiple of
page size as returned by sysconf(3C). If addr is 0, the
system will choose a suitable alignment.
The MAP_NORESERVE option specifies that no swap space be
reserved for a mapping. Without this flag, the creation of a
writable MAP_PRIVATE mapping reserves swap space equal to
the size of the mapping; when the mapping is written into,
the reserved space is employed to hold private copies of
the data. A write into a MAP_NORESERVE mapping produces
results which depend on the current availability of swap
space in the system. If space is available, the write
succeeds and a private copy of the written page is created;
if space is not available, the write fails and a SIGBUS or
SIGSEGV signal is delivered to the writing process.
MAP_NORESERVE mappings are inherited across fork(); at the
time of the fork(), swap space is reserved in the child for
all private pages that currently exist in the parent;
thereafter the child's mapping behaves as described above.
When MAP_ANON is set in flags, and fildes is set to -1,
mmap() provides a direct path to return anonymous pages to
the caller. This operation is equivalent to passing mmap()
an open file descriptor on /dev/zero with MAP_ANON elided
from the flags argument.
The off argument is constrained to be aligned and sized
according to the value returned by sysconf(3C) when passed
_SC_PAGESIZE or _SC_PAGE_SIZE. When MAP_FIXED is specified,
the addr argument must also meet these constraints. The
system performs mapping operations over whole pages. Thus,
while the len argument need not meet a size or alignment
constraint, the system will include, in any mapping opera-
tion, any partial page specified by the range [pa, pa +
len).
The system will always zero-fill any partial page at the end
of an object. Further, the system will never write out any
modified portions of the last page of an object which are
beyond its end. References to whole pages following the end
of an object will result in the delivery of a SIGBUS or SIG-
SEGV signal. SIGBUS signals may also be delivered on various
file system conditions, including quota exceeded errors.
The mmap() function adds an extra reference to the file
associated with the file descriptor fildes which is not
removed by a subsequent close(2) on that file descriptor.
This reference is removed when there are no more mappings to
the file by a call to the munmap(2) function.
The st_atime field of the mapped file may be marked for
update at any time between the mmap() call and the
corresponding munmap(2) call. The initial read or write
reference to a mapped region will cause the file's st_atime
field to be marked for update if it has not already been
marked for update.
The st_ctime and st_mtime fields of a file that is mapped
with MAP_SHARED and PROT_WRITE, will be marked for update at
some point in the interval between a write reference to the
mapped region and the next call to msync(3C) with MS_ASYNC
or MS_SYNC for that portion of the file by any process. If
there is no such call, these fields may be marked for update
at any time after a write reference if the underlying file
is modified as a result.
If the process calls mlockall(3C) with the MCL_FUTURE flag,
the pages mapped by all future calls to mmap() will be
locked in memory. In this case, if not enough memory could
be locked, mmap() fails and sets errno to EAGAIN.
RETURN VALUES
Upon successful completion, the mmap() function returns the
address at which the mapping was placed (pa); otherwise, it
returns a value of MAP_FAILED and sets errno to indicate the
error. The symbol MAP_FAILED is defined in the header
<sys/mman.h>. No successful return from mmap() will return
the value MAP_FAILED.
If mmap() fails for reasons other than EBADF, EINVAL or
ENOTSUP, some of the mappings in the address range starting
at addr and continuing for len bytes may have been unmapped.
ERRORS
The mmap() function will fail if:
EACCES
The fildes file descriptor is not open for read,
regardless of the protection specified; or fildes is
not open for write and PROT_WRITE was specified for a
MAP_SHARED type mapping.
EAGAIN
The mapping could not be locked in memory.
There was insufficient room to reserve swap space for
the mapping.
EBADF The fildes file descriptor is not open (and MAP_ANON
was not specified).
EINVAL
The arguments addr (if MAP_FIXED was specified) or off
are not multiples of the page size as returned by sys-
conf().
The argument addr (if MAP_ALIGN was specified) is not
0 or some power of two multiple of page size as
returned by sysconf(3C).
MAP_FIXED and MAP_ALIGN are both specified.
The field in flags is invalid (neither MAP_PRIVATE or
MAP_SHARED is set).
The argument len has a value equal to 0.
MAP_ANON was specified, but the file descriptor was
not -1.
EMFILE
The number of mapped regions would exceed an
implementation-dependent limit (per process or per
system).
ENODEV
The fildes argument refers to an object for which
mmap() is meaningless, such as a terminal.
ENOMEM
The MAP_FIXED option was specified and the range
[addr, addr + len) exceeds that allowed for the
address space of a process.
The MAP_FIXED option was not specified and there is
insufficient room in the address space to effect the
mapping.
The mapping could not be locked in memory, if required
by mlockall(3C), because it would require more space
than the system is able to supply.
The composite size of len plus the lengths obtained
from all previous calls to mmap() exceeds RLIMIT_VMEM
(see getrlimit(2)).
ENOTSUP
The system does not support the combination of
accesses requested in the prot argument.
ENXIO Addresses in the range [off, off + len) are invalid
for the object specified by fildes.
The MAP_FIXED option was specified in flags and the
combination of addr, len and off is invalid for the
object specified by fildes.
EOVERFLOW
The file is a regular file and the value of off plus
len exceeds the offset maximum establish in the open
file description associated with fildes.
The mmap() function may fail if:
EAGAIN
The file to be mapped is already locked using advisory
or mandatory record locking. See fcntl(2).
USAGE
Use of mmap() may reduce the amount of memory available to
other memory allocation functions.
MAP_ALIGN is useful to assure a properly aligned value of pa
for subsequent use with memcntl(2) and the MC_HAT_ADVISE
command.
This is best used for large, long-lived, and heavily refer-
enced regions. MAP_FIXED and MAP_ALIGN are always
mutually-exclusive.
Use of MAP_FIXED may result in unspecified behavior in
further use of brk(2), sbrk(2), malloc(3C), and shmat(2).
The use of MAP_FIXED is discouraged, as it may prevent an
implementation from making the most effective use of
resources.
The application must ensure correct synchronization when
using mmap() in conjunction with any other file access
method, such as read(2) and write(2), standard input/output,
and shmat(2).
The mmap() function has a transitional interface for 64-bit
file offsets. See lf64(5).
The mmap() function allows access to resources using address
space manipulations instead of the read()/write() interface.
Once a file is mapped, all a process has to do to access it
is use the data at the address to which the object was
mapped.
Consider the following pseudo-code:
fildes = open(...)
lseek(fildes, offset, whence)
read(fildes, buf, len)
/* use data in buf */
The following is a rewrite using mmap():
fildes = open(...)
address = mmap((caddr_t) 0, len, (PROT_READ | PROT_WRITE),
MAP_PRIVATE, fildes, offset)
/* use data at address */
ATTRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
____________________________________________________________
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
|_____________________________|_____________________________|
| Interface Stability | Standard |
|_____________________________|_____________________________|
| MT-Level | Async-Signal-Safe |
|_____________________________|_____________________________|
SEE ALSO
close(2), exec(2), fcntl(2), fork(2), getrlimit(2),
memcntl(2), mprotect(2), munmap(2), shmat(2), lockf(3C),
mlockall(3C), msync(3C), plock(3C), sysconf(3C), attri-
butes(5), lf64(5), standards(5), null(7D), zero(7D)
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