mirror of https://github.com/mkerrisk/man-pages
301 lines
7.8 KiB
Groff
301 lines
7.8 KiB
Groff
.\" Copyright (C) 2002 Michael Kerrisk <mtk.manpages@gmail.com>
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\" %%%LICENSE_END
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.\"
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.\" FIXME . Add an example to this page
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.TH SHM_OPEN 3 2017-05-03 "Linux" "Linux Programmer's Manual"
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.SH NAME
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shm_open, shm_unlink \- create/open or unlink POSIX shared memory objects
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.SH SYNOPSIS
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.B #include <sys/mman.h>
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.br
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.BR "#include <sys/stat.h>" " /* For mode constants */"
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.br
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.BR "#include <fcntl.h>" " /* For O_* constants */"
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.PP
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.BI "int shm_open(const char *" name ", int " oflag ", mode_t " mode );
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.PP
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.BI "int shm_unlink(const char *" name );
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.PP
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Link with \fI\-lrt\fP.
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.SH DESCRIPTION
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.BR shm_open ()
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creates and opens a new, or opens an existing, POSIX shared memory object.
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A POSIX shared memory object is in effect a handle which can
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be used by unrelated processes to
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.BR mmap (2)
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the same region of shared memory.
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The
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.BR shm_unlink ()
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function performs the converse operation,
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removing an object previously created by
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.BR shm_open ().
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.PP
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The operation of
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.BR shm_open ()
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is analogous to that of
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.BR open (2).
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.I name
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specifies the shared memory object to be created or opened.
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For portable use,
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a shared memory object should be identified by a name of the form
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.IR /somename ;
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that is, a null-terminated string of up to
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.BI NAME_MAX
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(i.e., 255) characters consisting of an initial slash,
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.\" glibc allows the initial slash to be omitted, and makes
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.\" multiple initial slashes equivalent to a single slash.
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.\" This differs from the implementation of POSIX message queues.
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followed by one or more characters, none of which are slashes.
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.\" glibc allows subdirectory components in the name, in which
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.\" case the subdirectory must exist under /dev/shm, and allow the
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.\" required permissions if a user wants to create a shared memory
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.\" object in that subdirectory.
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.PP
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.I oflag
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is a bit mask created by ORing together exactly one of
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.B O_RDONLY
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or
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.B O_RDWR
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and any of the other flags listed here:
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.TP 1.1i
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.B O_RDONLY
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Open the object for read access.
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A shared memory object opened in this way can be
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.BR mmap (2)ed
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only for read
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.RB ( PROT_READ )
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access.
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.TP
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.B O_RDWR
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Open the object for read-write access.
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.TP
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.B O_CREAT
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Create the shared memory object if it does not exist.
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The user and group ownership of the object are taken
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from the corresponding effective IDs of the calling process,
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.\" In truth it is actually the filesystem IDs on Linux, but these
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.\" are nearly always the same as the effective IDs. (MTK, Jul 05)
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and the object's
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permission bits are set according to the low-order 9 bits of
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.IR mode ,
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except that those bits set in the process file mode
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creation mask (see
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.BR umask (2))
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are cleared for the new object.
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A set of macro constants which can be used to define
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.I mode
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is listed in
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.BR open (2).
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(Symbolic definitions of these constants can be obtained by including
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.IR <sys/stat.h> .)
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.IP
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A new shared memory object initially has zero length\(emthe size of the
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object can be set using
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.BR ftruncate (2).
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The newly allocated bytes of a shared memory
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object are automatically initialized to 0.
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.TP
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.B O_EXCL
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If
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.B O_CREAT
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was also specified, and a shared memory object with the given
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.I name
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already exists, return an error.
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The check for the existence of the object, and its creation if it
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does not exist, are performed atomically.
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.TP
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.B O_TRUNC
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If the shared memory object already exists, truncate it to zero bytes.
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.PP
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Definitions of these flag values can be obtained by including
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.IR <fcntl.h> .
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.PP
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On successful completion
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.BR shm_open ()
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returns a new file descriptor referring to the shared memory object.
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This file descriptor is guaranteed to be the lowest-numbered file descriptor
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not previously opened within the process.
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The
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.B FD_CLOEXEC
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flag (see
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.BR fcntl (2))
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is set for the file descriptor.
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.PP
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The file descriptor is normally used in subsequent calls
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to
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.BR ftruncate (2)
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(for a newly created object) and
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.BR mmap (2).
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After a call to
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.BR mmap (2)
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the file descriptor may be closed without affecting the memory mapping.
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.PP
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The operation
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of
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.BR shm_unlink ()
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is analogous to
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.BR unlink (2):
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it removes a shared memory object name, and, once all processes
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have unmapped the object, de-allocates and
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destroys the contents of the associated memory region.
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After a successful
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.BR shm_unlink (),
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attempts to
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.BR shm_open ()
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an object with the same
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.I name
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will fail (unless
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.B O_CREAT
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was specified, in which case a new, distinct object is created).
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.SH RETURN VALUE
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On success,
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.BR shm_open ()
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returns a nonnegative file descriptor.
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On failure,
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.BR shm_open ()
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returns \-1.
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.BR shm_unlink ()
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returns 0 on success, or \-1 on error.
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.SH ERRORS
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On failure,
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.I errno
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is set to indicate the cause of the error.
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Values which may appear in
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.I errno
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include the following:
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.TP
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.B EACCES
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Permission to
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.BR shm_unlink ()
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the shared memory object was denied.
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.TP
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.B EACCES
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Permission was denied to
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.BR shm_open ()
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.I name
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in the specified
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.IR mode ,
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or
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.B O_TRUNC
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was specified and the caller does not have write permission on the object.
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.TP
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.B EEXIST
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Both
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.B O_CREAT
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and
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.B O_EXCL
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were specified to
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.BR shm_open ()
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and the shared memory object specified by
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.I name
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already exists.
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.TP
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.B EINVAL
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The
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.I name
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argument to
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.BR shm_open ()
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was invalid.
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.TP
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.B EMFILE
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The per-process limit on the number of open file descriptors has been reached.
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.TP
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.B ENAMETOOLONG
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The length of
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.I name
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exceeds
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.BR PATH_MAX .
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.TP
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.B ENFILE
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The system-wide limit on the total number of open files has been reached.
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.TP
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.B ENOENT
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An attempt was made to
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.BR shm_open ()
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a
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.I name
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that did not exist, and
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.B O_CREAT
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was not specified.
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.TP
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.B ENOENT
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An attempt was to made to
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.BR shm_unlink ()
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a
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.I name
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that does not exist.
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.SH VERSIONS
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These functions are provided in glibc 2.2 and later.
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.SH ATTRIBUTES
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For an explanation of the terms used in this section, see
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.BR attributes (7).
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.TS
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allbox;
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lbw24 lb lb
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l l l.
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Interface Attribute Value
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T{
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.BR shm_open (),
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.BR shm_unlink ()
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T} Thread safety MT-Safe locale
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.TE
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.sp 1
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.SH CONFORMING TO
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POSIX.1-2001, POSIX.1-2008.
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.PP
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POSIX.1-2001 says that the group ownership of a newly created shared
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memory object is set to either the calling process's effective group ID
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or "a system default group ID".
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POSIX.1-2008 says that the group ownership
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may be set to either the calling process's effective group ID
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or, if the object is visible in the filesystem,
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the group ID of the parent directory.
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.SH NOTES
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.PP
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POSIX leaves the behavior of the combination of
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.B O_RDONLY
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and
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.B O_TRUNC
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unspecified.
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On Linux, this will successfully truncate an existing
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shared memory object\(emthis may not be so on other UNIX systems.
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.PP
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The POSIX shared memory object implementation on Linux makes use
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of a dedicated
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.BR tmpfs (5)
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filesystem that is normally mounted under
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.IR /dev/shm .
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.SH SEE ALSO
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.BR close (2),
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.BR fchmod (2),
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.BR fchown (2),
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.BR fcntl (2),
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.BR fstat (2),
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.BR ftruncate (2),
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.BR memfd_create (2),
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.BR mmap (2),
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.BR open (2),
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.BR umask (2),
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.BR shm_overview (7)
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