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.TH SHM_OPEN 3 2017-09-15 "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 */"

.PP

.BI "int shm_open(const char *" name ", int " oflag ", mode_t " mode );

.PP

.BI "int shm_unlink(const char *" name );

.PP

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 ().

.PP

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.

.PP

.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 be

.BR mmap (2)ed

only for read

.RB ( PROT_READ )

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 filesystem 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> .)

.IP

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.

.PP

Definitions of these flag values can be obtained by including

.IR <fcntl.h> .

.PP

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.

.PP

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.

.PP

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

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 per-process limit on the number of open file descriptors has been reached.

.TP

.B ENAMETOOLONG

The length of

.I name

exceeds

.BR PATH_MAX .

.TP

.B ENFILE

The system-wide limit on the total number of open files 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 ATTRIBUTES

For an explanation of the terms used in this section, see

.BR attributes (7).

.TS

allbox;

lbw24 lb lb

l l l.

Interface	Attribute	Value

T{

.BR shm_open (),

.BR shm_unlink ()

T}	Thread safety	MT-Safe locale

.TE

.sp 1

.SH CONFORMING TO

POSIX.1-2001, POSIX.1-2008.

.PP

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".

POSIX.1-2008 says that the group ownership

may be set to either the calling process's effective group ID

or, if the object is visible in the filesystem,

the group ID of the parent directory.

.SH NOTES

.PP

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.

.PP

The POSIX shared memory object implementation on Linux makes use

of a dedicated

.BR tmpfs (5)

filesystem that 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 memfd_create (2),

.BR mmap (2),

.BR open (2),

.BR umask (2),

.BR shm_overview (7)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I man-pages

project.

A description of the project,

information about reporting bugs,

and the latest version of this page,

can be found at

\%https://www.kernel.org/doc/man\-pages/.