path: root/man3/shm_open.3

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.TH SHM_OPEN 3 2002-02-22 "Linux 2.4" "Linux Programmer's Manual"
.SH NAME
shm_open, shm_unlink \- Create/open or unlink POSIX shared memory objects
.SH SYNOPSIS
.B #include <sys/types.h>
.br
.B #include <sys/mman.h>
.sp
.BI "int shm_open(const char *" name ", int " oflag ", mode_t " mode );
.sp
.BI "int shm_unlink(const char *" name );
.SH DESCRIPTION
.B 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 
.B shm_unlink
function performs the converse operation, 
removing an object previously created by
.BR shm_open .
.LP
The operation of
.B 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, 
.I name
should have an initial slash (/) and contain no embedded slashes.
.\" The names used may or may not live in a file system, and may or may not
.\" survive a reboot. Names starting with a slash are also visible to other
.\" processes. Other names have implementation-defined effect.
.LP
.I oflag
is a bit mask created by ORing together exactly one of
.B O_RDONLY 
or
.B O_RWDR
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 set as for
.BR open (2),
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).)
.sp
A new shared memory object initially has zero length \- the size of the
object can be set using
.BR ftruncate (2).
(The newly-allocated bytes of a shared memory
object are automatically initialised to 0.)
.TP 
.B O_EXCL
If 
.B O_CREAT
was also specified, and a share 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
On successful completion
.B 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 
.B 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 
.B 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,
.B shm_open
returns a non-negative file descriptor.  On failure,
.B shm_open
returns \-1.
.B 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
.B shm_unlink
the shared memory object was denied.
.TP
.B EACCES
Permission was denied to 
.B 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
.B shm_open
and the shared memory object specified by
.I name
already exists.
.TP
.B EINVAL
The
.I name
argument to 
.B 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
.B 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
.B shm_unlink
a 
.I name 
that does not exist.
.SH "NOTES"
These functions are provided in glibc 2.2 and later.  Programs using these 
functions must specify the 
.B \-lrt
flag to
.B cc
in order to link against the required ("realtime") library.
.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 \- this may not be so on other Unices.
.LP
The POSIX shared memory object implementation on Linux 2.4 makes use
of a dedicated file system, which is normally
mounted under 
.BR /dev/shm .
.SH "CONFORMING TO"
POSIX 1003.1 (2001).
.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)