man3/shm_open.3

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.TH SHM_OPEN 3 2009-02-25 "Linux" "Linux Programmer's Manual"
.SH NAME
shm_open, shm_unlink \- create/open or unlink POSIX shared memory objects
.SH SYNOPSIS
.B #include <sys/mman.h>
.br
.BR "#include <sys/stat.h>" "        /* For mode constants */"
.br
.BR "#include <fcntl.h>" "           /* For O_* constants */"
.sp
.BI "int shm_open(const char *" name ", int " oflag ", mode_t " mode );
.sp
.BI "int shm_unlink(const char *" name );
.sp
Link with \fI\-lrt\fP.
.SH DESCRIPTION
.BR shm_open ()
creates and opens a new, or opens an existing, POSIX shared memory object.
A POSIX shared memory object is in effect a handle which can
be used by unrelated processes to
.BR mmap (2)
the same region of shared memory.
The
.BR shm_unlink ()
function performs the converse operation,
removing an object previously created by
.BR shm_open ().
.LP
The operation of
.BR shm_open ()
is analogous to that of
.BR open (2).
.I name
specifies the shared memory object to be created or opened.
For portable use,
a shared memory object should be identified by a name of the form
.IR /somename ;
that is, a null-terminated string of up to
.BI NAME_MAX
(i.e., 255) characters consisting of an initial slash,
.\" glibc allows the initial slash to be omitted, and makes
.\" multiple initial slashes equivalent to a single slash.
.\" This differs from the implementation of POSIX message queues.
followed by one or more characters, none of which are slashes.
.\" glibc allows subdirectory components in the name, in which
.\" case the subdirectory must exist under /dev/shm, and allow the
.\" required permissions if a user wants to create a shared memory
.\" object in that subdirectory.
.LP
.I oflag
is a bit mask created by ORing together exactly one of
.B O_RDONLY
or
.B O_RDWR
and any of the other flags listed here:
.TP 1.1i
.B O_RDONLY
Open the object for read access.
A shared memory object opened in this way can only be
.BR mmap (2)ed
for read (\fBPROT_READ\fP) access.
.TP
.B O_RDWR
Open the object for read-write access.
.TP
.B O_CREAT
Create the shared memory object if it does not exist.
The user and group ownership of the object are taken
from the corresponding effective IDs of the calling process,
.\" In truth it is actually the file system IDs on Linux, but these
.\" are nearly always the same as the effective IDs.  (MTK, Jul 05)
and the object's
permission bits are set according to the low-order 9 bits of
.IR mode ,
except that those bits set in the process file mode
creation mask (see
.BR umask (2))
are cleared for the new object.
A set of macro constants which can be used to define
.I mode
is listed in
.BR open (2).
(Symbolic definitions of these constants can be obtained by including
.IR <sys/stat.h> .)
.sp
A new shared memory object initially has zero length\(emthe size of the
object can be set using
.BR ftruncate (2).
The newly allocated bytes of a shared memory
object are automatically initialized to 0.
.TP
.B O_EXCL
If
.B O_CREAT
was also specified, and a shared memory object with the given
.I name
already exists, return an error.
The check for the existence of the object, and its creation if it
does not exist, are performed atomically.
.TP
.B O_TRUNC
If the shared memory object already exists, truncate it to zero bytes.
.LP
Definitions of these flag values can be obtained by including
.IR <fcntl.h> .
.LP
On successful completion
.BR shm_open ()
returns a new file descriptor referring to the shared memory object.
This file descriptor is guaranteed to be the lowest-numbered file descriptor
not previously opened within the process.
The
.B FD_CLOEXEC
flag (see
.BR fcntl (2))
is set for the file descriptor.

The file descriptor is normally used in subsequent calls
to
.BR ftruncate (2)
(for a newly created object) and
.BR mmap (2).
After a call to
.BR mmap (2)
the file descriptor may be closed without affecting the memory mapping.

The operation
of
.BR shm_unlink ()
is analogous to
.BR unlink (2):
it removes a shared memory object name, and, once all processes
have unmapped the object, de-allocates and
destroys the contents of the associated memory region.
After a successful
.BR shm_unlink (),
attempts to
.BR shm_open ()
an object with the same
.I name
will fail (unless
.B O_CREAT
was specified, in which case a new, distinct object is created).
.SH "RETURN VALUE"
On success,
.BR shm_open ()
returns a nonnegative file descriptor.
On failure,
.BR shm_open ()
returns \-1.
.BR shm_unlink ()
returns 0 on success, or \-1 on error.
.SH ERRORS
On failure,
.I errno
is set to indicate the cause of the error.
Values which may appear in
.I errno
include the following:
.TP
.B EACCES
Permission to
.BR shm_unlink ()
the shared memory object was denied.
.TP
.B EACCES
Permission was denied to
.BR shm_open ()
.I name
in the specified
.IR mode ,
or
.B O_TRUNC
was specified and the caller does not have write permission on the object.
.TP
.B EEXIST
Both
.B O_CREAT
and
.B O_EXCL
were specified to
.BR shm_open ()
and the shared memory object specified by
.I name
already exists.
.TP
.B EINVAL
The
.I name
argument to
.BR shm_open ()
was invalid.
.TP
.B EMFILE
The process already has the maximum number of files open.
.TP
.B ENAMETOOLONG
The length of
.I name
exceeds
.BR PATH_MAX .
.TP
.B ENFILE
The limit on the total number of files open on the system has been
reached.
.TP
.B ENOENT
An attempt was made to
.BR shm_open ()
a
.I name
that did not exist, and
.B O_CREAT
was not specified.
.TP
.B ENOENT
An attempt was to made to
.BR shm_unlink ()
a
.I name
that does not exist.
.SH VERSIONS
These functions are provided in glibc 2.2 and later.
.SH "CONFORMING TO"
POSIX.1-2001.
.LP
POSIX.1-2001 says that the group ownership of a newly created shared
memory object is set to either the calling process's effective group ID
or "a system default group ID".
.SH "NOTES"
.LP
POSIX leaves the behavior of the combination of
.B O_RDONLY
and
.B O_TRUNC
unspecified.
On Linux, this will successfully truncate an existing
shared memory object\(emthis may not be so on other UNIX systems.
.LP
The POSIX shared memory object implementation on Linux 2.4 makes use
of a dedicated file system, which is normally
mounted under
.IR /dev/shm .
.SH "SEE ALSO"
.BR close (2),
.BR fchmod (2),
.BR fchown (2),
.BR fcntl (2),
.BR fstat (2),
.BR ftruncate (2),
.BR mmap (2),
.BR open (2),
.BR umask (2),
.BR shm_overview (7)