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SHM_OPEN(2) | System Calls Manual | SHM_OPEN(2) |
NAME¶
shm_open, shm_unlink — shared memory object operationsLIBRARY¶
Standard C Library (libc, -lc)SYNOPSIS¶
#include <sys/types.h>#include <sys/mman.h> int
shm_open(const char *path, int flags, mode_t mode); int
shm_unlink(const char *path);
DESCRIPTION¶
The shm_open() system call opens (or optionally creates) a POSIX shared memory object named path. The flags argument contains a subset of the flags used by open(2). An access mode of eitherO_RDONLY
or O_RDWR
must be
included in flags. The optional flags
O_CREAT
, O_EXCL
, and
O_TRUNC
may also be specified.
If O_CREAT
is specified, then a new shared memory object
named path will be created if it does not exist. In this
case, the shared memory object is created with mode mode
subject to the process' umask value. If both the
O_CREAT
and O_EXCL
flags are
specified and a shared memory object named path already
exists, then shm_open() will fail with
EEXIST.
Newly created objects start off with a size of zero. If an existing shared
memory object is opened with O_RDWR
and the
O_TRUNC
flag is specified, then the shared memory
object will be truncated to a size of zero. The size of the object can be
adjusted via ftruncate(2) and queried via
fstat(2).
The new descriptor is set to close during execve(2) system
calls; see close(2) and fcntl(2).
As a FreeBSD extension, the constant SHM_ANON
may be
used for the path argument to
shm_open(). In this case, an anonymous, unnamed shared
memory object is created. Since the object has no name, it cannot be removed
via a subsequent call to shm_unlink(). Instead, the shared
memory object will be garbage collected when the last reference to the shared
memory object is removed. The shared memory object may be shared with other
processes by sharing the file descriptor via fork(2) or
sendmsg(2). Attempting to open an anonymous shared memory
object with O_RDONLY
will fail with
EINVAL
. All other flags are ignored.
The shm_unlink() system call removes a shared memory object
named path.
RETURN VALUES¶
If successful, shm_open() returns a non-negative integer, and shm_unlink() returns zero. Both functions return -1 on failure, and set errno to indicate the error.COMPATIBILITY¶
The path argument does not necessarily represent a pathname (although it does in most other implementations). Two processes opening the same path are guaranteed to access the same shared memory object if and only if path begins with a slash (‘/
’) character.
Only the O_RDONLY
, O_RDWR
,
O_CREAT
, O_EXCL
, and
O_TRUNC
flags may be used in portable programs.
The result of using open(2), read(2), or
write(2) on a shared memory object, or on the descriptor
returned by shm_open(), is undefined. It is also undefined
whether the shared memory object itself, or its contents, persist across
reboots.
In FreeBSD, read(2) and write(2) on a shared
memory object will fail with EOPNOTSUPP
and neither
shared memory objects nor their contents persist across reboots.
ERRORS¶
The following errors are defined for shm_open():- [
EINVAL
] - A flag other than
O_RDONLY
,O_RDWR
,O_CREAT
,O_EXCL
, orO_TRUNC
was included in flags. - [
EMFILE
] - The process has already reached its limit for open file descriptors.
- [
ENFILE
] - The system file table is full.
- [
EINVAL
] O_RDONLY
was specified while creating an anonymous shared memory object viaSHM_ANON
.- [
EFAULT
] - The path argument points outside the process' allocated address space.
- [
ENAMETOOLONG
] - The entire pathname exceeded 1023 characters.
- [
EINVAL
] - The path does not begin with a slash
(‘
/
’) character. - [
ENOENT
] O_CREAT
is specified and the named shared memory object does not exist.- [
EEXIST
] O_CREAT
andO_EXCL
are specified and the named shared memory object dies exist.- [
EACCES
] - The required permissions (for reading or reading and writing) are denied.
- [
EFAULT
] - The path argument points outside the process' allocated address space.
- [
ENAMETOOLONG
] - The entire pathname exceeded 1023 characters.
- [
ENOENT
] - The named shared memory object does not exist.
- [
EACCES
] - The required permissions are denied. shm_unlink() requires write permission to the shared memory object.
SEE ALSO¶
close(2), ftruncate(2), fstat(2), mmap(2), munmap(2)STANDARDS¶
The shm_open() and shm_unlink() functions are believed to conform to IEEE Std 1003.1b-1993 (“POSIX.1b”).HISTORY¶
The shm_open() and shm_unlink() functions first appeared in FreeBSD 4.3. The functions were reimplemented as system calls using shared memory objects directly rather than files in FreeBSD 7.0.AUTHORS¶
Garrett A. Wollman ⟨wollman@FreeBSD.org⟩ (C library support and this manual page)Matthew Dillon ⟨dillon@FreeBSD.org⟩ (
MAP_NOSYNC
)March 20, 2007 | Debian |