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mmap(2)
NAME
mmap - Map a file system object into virtual memory
SYNOPSIS
#include <sys/mman.h>
void *mmap(
void *addr,
size_t len,
int prot,
int flags,
int filedes,
off_t off );
[Tru64 UNIX] The following definition of the mmap() function does not
conform to current standards and is supported only for backward
compatibility:
caddr_t mmap(
caddr_t addr,
size_t len,
int prot,
int flags,
int filedes,
off_t off );
STANDARDS
Interfaces documented on this reference page conform to industry standards
as follows:
mmap(): XSH4.2, XSH5.0
Refer to the standards(5) reference page for more information about
industry standards and associated tags.
Standards: standards(5)
PARAMETERS
addr
Specifies the starting address of the new region (truncated to a page
boundary).
len Specifies the length in bytes of the new region (rounded up to a page
boundary).
prot
Specifies access permissions as either PROT_NONE or the result of a
logical OR operation on any combination of PROT_READ, PROT_WRITE, and
PROT_EXEC.
flags
Specifies attributes of the mapped region as the results of a bitwise-
inclusive OR operation on any combination of MAP_FILE, MAP_ANONYMOUS,
MAP_VARIABLE, MAP_FIXED, MAP_SHARED, MAP_PRIVATE, MAP_INHERIT, or
MAP_UNALIGNED.
filedes
Specifies the file to be mapped to the new mapped file region returned
by open().
off Specifies the offset into the file that gets mapped at address addr.
DESCRIPTION
The mmap() function creates a new mapped file region, a new private region,
or a new shared memory region.
The addr and len parameters specify the requested starting address and
length in bytes for the new region. The address is a multiple of the page
size returned by the sysconf(_SC_PAGE_SIZE) call.
If the len parameter is not a multiple of the page size returned by the
sysconf(_SC_PAGE_SIZE) call, then the result of any reference to an address
between the end of the region and the end of the page containing the end of
the region is undefined.
The flags parameter specifies attributes of the mapped region. Values of
the flags parameter are constructed by a bitwise-inclusive OR operation on
the flags from the following list of symbolic names, which are defined in
the <sys/mman.h> file:
MAP_FILE
Create a mapped file region.
MAP_ANONYMOUS
Create an unnamed memory region.
MAP_VARIABLE
Place region at the computed address.
MAP_FIXED
Place region at a fixed address.
MAP_SHARED
Share changes.
MAP_PRIVATE
Changes are private.
MAP_INHERIT
Region is not unmapped by an exec call.
MAP_UNALIGNED
Do not verify that the file offset is page aligned.
The MAP_FILE and MAP_ANONYMOUS flags control whether the region to be
mapped is a mapped file region or an anonymous shared memory region. One of
these flags must be selected.
If MAP_FILE is set in the flags parameter:
· A new mapped file region is created, mapping the file associated with
the filedes parameter.
· The off parameter specifies the file byte offset at which the mapping
starts. This offset must be a multiple of the page size returned by
the sysconf(_SC_PAGE_SIZE) call.
· If the end of the mapped file region is beyond the end of the file,
the result of any reference to an address in the mapped file region
corresponding to an offset beyond the end of the file is unspecified.
If MAP_ANONYMOUS is set in the flags parameter:
· A new memory region is created and initialized to all zeros. This
memory region can be shared only with descendents of the current
process.
· If the filedes parameter is not -1, the mmap() function fails.
The new region is placed at the requested address if the requested address
is not null and it is possible to place the region at this address. When
the requested address is null or the region cannot be placed at the
requested address, the MAP_VARIABLE and MAP_FIXED flags control the
placement of the region. One of these flags must be selected.
If MAP_VARIABLE is set in the flags parameter:
· If the requested address is null or if it is not possible for the
system to place the region at the requested address, the region is
placed at an address selected by the system.
If MAP_FIXED is set in the flags parameter:
· If the requested address is not null, the mmap() function succeeds
even if the requested address is already part of another region. (If
the address is within an existing region, the effect on the pages
within that region and within the area of the overlap produced by the
two regions is the same as if they were unmapped. In other words,
whatever is mapped between addr and addr + len will be unmapped.)
· If the requested address is null and MAP_FIXED is specified, the
region is placed at the default exact mapping address for the region.
The call places the region at this value exactly, replacing previous
mappings if necessary. The exact mapping address is determined from a
combination of the flag and protection parameters passed to the mmap()
function.
The MAP_PRIVATE and MAP_SHARED flags control the visibility of
modifications to the mapped file or shared memory region. One of these
flags must be selected.
If MAP_SHARED is set in the flags parameter:
· If the region is a mapped region, modifications to the region are
visible to other processes that have mapped the same region using
MAP_SHARED.
· If the region is a mapped file region, modifications to the region are
written to the file. (Note that the modifications are not immediately
written to the file because of buffer cache delay; that is, the write
to the file does not occur until there is a need to reuse the buffer
cache. If the modifications must be written to the file immediately,
the msync() function can be used to ensure that this is done.)
If MAP_PRIVATE is set in the flags parameter:
· Modifications to the mapped region by the calling process are not
visible to other processes that have mapped the same region using
either MAP_PRIVATE or MAP_SHARED.
· Modifications to the mapped region by the calling process are not
written to the file.
It is unspecified whether modifications by processes that have mapped the
region using MAP_SHARED are visible to other processes that have mapped the
same region using MAP_PRIVATE.
If MAP_INHERIT is set in the flags parameter:
· The mapped region cannot be removed from the address space of the
calling process by an exec system call.
If MAP_UNALIGNED is set in the flags parameter:
· The mmap() system call is prevented from verifying that the file
offset is page aligned.
The prot parameter specifies the mapped region's access permissions. The
<sys/mman.h> header file defines the following access options:
PROT_READ
The mapped region can be read.
PROT_WRITE
The mapped region can be written.
PROT_EXEC
The mapped region can be executed.
PROT_NONE
The mapped region cannot be accessed.
The prot parameter can be either PROT_NONE or the results of a logical OR
operation on any combination of PROT_READ, PROT_WRITE, and PROT_EXEC. If
PROT_NONE is not specified, access permissions may be granted to the region
in addition to those explicitly requested, except that write access is not
granted unless PROT_WRITE is specified.
If the region is a mapped file that was mapped with MAP_SHARED, the mmap()
function grants read or execute access permission only if the file
descriptor used to map the file is open for reading, and grants write
access permission only if the file descriptor used to map the file is open
for writing. If the region is a mapped file that was mapped with
MAP_PRIVATE, the mmap() function grants read, write, or execute access
permission only if the file descriptor used to map the file is open for
reading. If the region is a shared memory region that was mapped with
MAP_ANONYMOUS, the mmap() function grants all requested access permissions.
After the successful completion of the mmap() function, the filedes
parameter can be closed without effect on the mapped region or on the
contents of the mapped file. Each mapped region creates a file reference,
similar to an open file descriptor, which prevents the file data from being
deallocated.
Modifications made to the file by using the write() function are visible to
mapped regions, and modifications to a mapped region are visible with the
read() function.
After a call to the fork() function, the child process inherits all mapped
regions with the same sharing and protection attributes as in the parent
process. Each mapped file and shared memory region created with the mmap()
function is unmapped by a successful call to any of the exec functions,
unless that region is made inheritable across the exec call.
NOTES
The memory acquired with the mmap() function is not locked, regardless of
the previous use of the plock() function.
An mmap() call will fail on a file that has direct I/O enabled.
For AdvFS files, use the msync() function to write out modified file data
in a memory region established by mmap(). This is necessary to ensure the
integrity of the changes in the data being written.
RETURN VALUES
Upon successful completion, the mmap() function returns the address at
which the mapping was placed. Otherwise, mmap() returns the value of
MAP_FAILED. The MAP_FAILED symbol is defined in the <sys/mman.h> header
file.
ERRORS
If the mmap() function fails, errno is set to one of the following values:
[EACCES]
The file referred to by filedes is not open for read access, or the
file is not open for write access and PROT_WRITE was set for a
MAP_SHARED mapping operation.
[EBADF]
The filedes parameter is not a valid file descriptor.
[EFAULT]
The addr parameter is an invalid address.
[EINVAL]
The flags or prot parameter is invalid, or the addr parameter or off
parameter is not a multiple of the page size returned by the
sysconf(_SC_PAGE_SIZE) call. This error is also returned if the file is
already open for atomic write data logging or direct I/O.
[ENODEV]
The file descriptor filedes refers to an object that cannot be mapped,
such as a terminal.
[ENOMEM]
There is not enough address space to map len bytes.
[ENXIO]
The addresses specified by the range [off, off + len] are invalid for
filedes.
SEE ALSO
Functions: exec(2), fcntl(2), fork(2), madvise(2), mprotect(2), msync(2),
munmap(2), plock(2), sysconf(3)
Standards: standards(5)
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Index for
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Alphabetical
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