MMAP(2) System Calls 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 necessarily 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 mappings 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 cannot 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 implementation 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 application 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 bitwise 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.


MAP_TEXT
Map text.


MAP_INITDATA
Map initialized data segment.


MAP_32BIT
Map to the lower 32 bits of address space.


The MAP_SHARED and MAP_PRIVATE options describe the disposition 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 mapping 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 MAP_TEXT option informs the system that the mapped region will be
used primarily for executing instructions. This information can help the
system better utilize MMU resources on some platforms. This flag is
always passed by the dynamic linker when it maps text segments of shared
objects. When the MAP_TEXT option is used for regular file mappings on
some platforms, the system can choose a mapping size larger than the page
size returned by sysconf(3C). The specific page sizes that are used
depend on the platform and the alignment of the addr and len arguments.
Several diffrent mapping sizes can be used to map the region with larger
page sizes used in the parts of the region that meet alignment and size
requirements for those page sizes.


The MAP_INITDATA option informs the system that the mapped region is an
initialized data segment of an executable or shared object. When the
MAP_INITDATA option is used for regular file mappings on some platforms,
the system can choose a mapping size larger than the page size returned
by sysconf(). The MAP_INITDATA option should be used only by the dynamic
linker for mapping initialized data of shared objects.


The MAP_32BIT option informs the system that the search space for mapping
assignment should be limited to the first 32 bits (4 Gbytes) of the
caller's address space. This flag is accepted in both 32-bit and 64-bit
process models, but does not alter the mapping strategy when used in a
32-bit process model.


The off argument is constrained to be aligned and sized according to the
value returned by sysconf() 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 operation, 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 SIGSEGV 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.


The mmap() function aligns based on the length of the mapping. When
determining the amount of space to add to the address space, mmap()
includes two 8-Kbyte pages, one at each end of the mapping that are not
mapped and are therefore used as "red-zone" pages. Attempts to reference
these pages result in access violations.


The size requested is incremented by the 16 Kbytes for these pages and is
then subject to rounding constraints. The constraints are:

o For 32-bit processes:

If length > 4 Mbytes
round to 4-Mbyte multiple
elseif length > 512 Kbytes
round to 512-Kbyte multiple
else
round to 64-Kbyte multiple


o For 64-bit processes:

If length > 4 Mbytes
round to 4-Mbyte multiple
else
round to 1-Mbyte multiple


The net result is that for a 32-bit process:

o If an mmap() request is made for 4 Mbytes, it results in 4
Mbytes + 16 Kbytes and is rounded up to 8 Mbytes.

o If an mmap() request is made for 512 Kbytes, it results in 512
Kbytes + 16 Kbytes and is rounded up to 1 Mbyte.

o If an mmap() request is made for 1 Mbyte, it results in 1
Mbyte + 16 Kbytes and is rounded up to 1.5 Mbytes.

o Each 8-Kbyte mmap request "consumes" 64 Kbytes of virtual
address space.


To obtain maximal address space usage for a 32-bit process:

o Combine 8-Kbyte requests up to a limit of 48 Kbytes.

o Combine amounts over 48 Kbytes into 496-Kbyte chunks.

o Combine amounts over 496 Kbytes into 4080-Kbyte chunks.


To obtain maximal address space usage for a 64-bit process:

o Combine amounts < 1008 Kbytes into chunks <= 1008 Kbytes.

o Combine amounts over 1008 Kbytes into 4080-Kbyte chunks.


The following is the output from a 32-bit program demonstrating this:

map 8192 bytes: 0xff390000
map 8192 bytes: 0xff380000
64-Kbyte delta between starting addresses.


map 512 Kbytes: 0xff180000
map 512 Kbytes: 0xff080000
1-Mbyte delta between starting addresses.


map 496 Kbytes: 0xff000000
map 496 Kbytes: 0xfef80000
512-Kbyte delta between starting addresses


map 1 Mbyte: 0xfee00000
map 1 Mbyte: 0xfec80000
1536-Kbyte delta between starting addresses


map 1008 Kbytes: 0xfeb80000
map 1008 Kbytes: 0xfea80000
1-Mbyte delta between starting addresses


map 4 Mbytes: 0xfe400000
map 4 Mbytes: 0xfdc00000
8-Mbyte delta between starting addresses


map 4080 Kbytes: 0xfd800000
map 4080 Kbytes: 0xfd400000
4-Mbyte delta between starting addresses


The following is the output of the same program compiled as a 64-bit
application:

map 8192 bytes: 0xffffffff7f000000
map 8192 bytes: 0xffffffff7ef00000
1-Mbyte delta between starting addresses


map 512 Kbytes: 0xffffffff7ee00000
map 512 Kbytes: 0xffffffff7ed00000
1-Mbyte delta between starting addresses


map 496 Kbytes: 0xffffffff7ec00000
map 496 Kbytes: 0xffffffff7eb00000
1-Mbyte delta between starting addresses


map 1 Mbyte: 0xffffffff7e900000
map 1 Mbyte: 0xffffffff7e700000
2-Mbyte delta between starting addresses


map 1008 Kbytes: 0xffffffff7e600000
map 1008 Kbytes: 0xffffffff7e500000
1-Mbyte delta between starting addresses


map 4 Mbytes: 0xffffffff7e000000
map 4 Mbytes: 0xffffffff7d800000
8-Mbyte delta between starting addresses


map 4080 Kbytes: 0xffffffff7d400000
map 4080 Kbytes: 0xffffffff7d000000
4-Mbyte delta between starting addresses


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 sysconf().

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.

MAP_TEXT was specified but PROT_EXEC was not.

MAP_TEXT and MAP_INITDATA were both specified.


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 referenced 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 attributes:


+--------------------+-------------------+
| 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),
mmapobj(2), mprotect(2), munmap(2), shmat(2), lockf(3C), mlockall(3C),
msync(3C), plock(3C), sysconf(3C), attributes(5), lf64(5), standards(5),
null(7D), zero(7D)


February 4, 2009 MMAP(2)