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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 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.
SunOS 5.10 Last change: 7 Apr 2005 1
System Calls mmap(2)
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.
SunOS 5.10 Last change: 7 Apr 2005 2
System Calls mmap(2)
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.
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.
SunOS 5.10 Last change: 7 Apr 2005 3
System Calls mmap(2)
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 sys-
tem 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 dif-
frent mapping sizes can be used to map the region with
larger page sizes used in the parts of the region that meet
SunOS 5.10 Last change: 7 Apr 2005 4
System Calls mmap(2)
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 reg-
ular file mappings on some platforms, the system can choose
a mapping size larger than the page size returned by sys-
conf(). The MAP_INITDATA option should be used only by the
dynamic linker for mapping initialized data of shared
objects.
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 sys-
tem 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.
SunOS 5.10 Last change: 7 Apr 2005 5
System Calls mmap(2)
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 map-
ping. 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 there-
fore 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 con-
straints 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 vir-
tual address space.
To obtain maximal address space usage for a 32-bit process:
SunOS 5.10 Last change: 7 Apr 2005 6
System Calls mmap(2)
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 demon-
strating 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
SunOS 5.10 Last change: 7 Apr 2005 7
System Calls mmap(2)
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
SunOS 5.10 Last change: 7 Apr 2005 8
System Calls mmap(2)
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 speci-
fied; or fildes is not open for write and
PROT_WRITE was specified for a MAP_SHARED
type mapping.
SunOS 5.10 Last change: 7 Apr 2005 9
System Calls mmap(2)
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 speci-
fied) or off are not multiples of the page
size as returned by sysconf().
The argument addr (if MAP_ALIGN was speci-
fied) 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 speci-
fied.
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 ter-
minal.
ENOMEM The MAP_FIXED option was specified and the
range [addr, addr + len) exceeds that
SunOS 5.10 Last change: 7 Apr 2005 10
System Calls mmap(2)
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 asso-
ciated 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
SunOS 5.10 Last change: 7 Apr 2005 11
System Calls mmap(2)
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 attri-
butes:
____________________________________________________________
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
|_____________________________|_____________________________|
| Interface Stability | Standard |
|_____________________________|_____________________________|
| MT-Level | Async-Signal-Safe |
|_____________________________|_____________________________|
SunOS 5.10 Last change: 7 Apr 2005 12
System Calls mmap(2)
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)
SunOS 5.10 Last change: 7 Apr 2005 13
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