SHM_OPEN(2) BSD System Calls Manual SHM_OPEN(2)NAME
shm_open, shm_unlink — shared memory object operations
LIBRARY
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 either O_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 speci‐
fied, 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 mem‐
ory object is created. Since the object has no name, it cannot be
removed via a subsequent call to shm_unlink(). Instead, the shared mem‐
ory 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,
or O_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 via SHM_ANON.
[EFAULT] The path argument points outside the process' allo‐
cated 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 and O_EXCL are specified and the named shared
memory object dies exist.
[EACCES] The required permissions (for reading or reading and
writing) are denied.
The following errors are defined for shm_unlink():
[EFAULT] The path argument points outside the process' allo‐
cated 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 ALSOclose(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.1”).
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 man‐
ual page)
Matthew Dillon ⟨dillon@FreeBSD.org⟩ (MAP_NOSYNC)
BSD March 20, 2007 BSD