shm_open(2) [osx man page]
SHM_OPEN(2) BSD System Calls Manual SHM_OPEN(2) NAME
shm_open -- open a shared memory object SYNOPSIS
#include <sys/mman.h> #include <fcntl.h> int shm_open(const char *name, int oflag, ...); The parameter "mode_t mode" is optional. DESCRIPTION
The shared memory object referenced by name is opened for reading and/or writing as specified by the argument oflag and the file descriptor returned to the calling process. The returned file descriptor will be the lowest non-open file descriptor for the calling process, and is not shared with any other processes, as it is a new file descriptor. The new file descriptor will have the FD_CLOEXEC flag set. Repeated calls to shm_open with the same string value for name() will return a file descriptor referring to the same shared memory object, provided that the object has not been unlinked by a call to shm_unlink(). The oflag argument may indicate the file is to be created if it does not exist (by specifying the O_CREAT flag), in which case the file is created with mode mode as described in chmod(2) and modified by the process' umask value (see umask(2)). The value of oflag is formed by or'ing the following values: O_RDONLY open for reading only O_RDWR open for reading and writing O_CREAT create object if it does not exist O_EXCL error if create and object exists O_TRUNC truncate size to 0 Exactly one of O_RDONLY or O_RDWR must be specified. If O_TRUNC is specified and the file exists, the file is truncated to zero length. If O_EXCL is set with O_CREAT and the file already exists, shm_open() returns an error. This may be used to implement a simple exclusive access locking mechanism. If successful, shm_open() returns a non-negative integer, termed a file descriptor. It returns -1 and sets errno on failure. The file pointer used to mark the current position within the memory object is set to the beginning of the object. When a new shared memory object is created it is given the owner and group corresponding to the effective user and group of the calling process. There is no visible entry in the file system for the created object in this implementation. When a shared memory object is created, it persists until it it unlinked and all other references are gone. Objects do not persist across a system reboot. The system imposes a limit on the number of file descriptors open simultaneously by one process. getdtablesize(2) returns the current system limit. ERRORS
The named object is opened unless: [EACCES] The required permissions (for reading and/or writing) are denied for the given flags. [EACCES] O_CREAT is specified, the object does not exist, and permission to create the object is denied. [EEXIST] O_CREAT and O_EXCL were specified and the object exists. [EINTR] The shm_open() operation was interrupted by a signal. [EINVAL] The shm_open() operation is not supported. [EMFILE] The process has already reached its limit for open file descriptors. [ENAMETOOLONG] name exceeded the name size limit. This is currently PSHMNAMLEN characters (defined in <sys/posix_shm.h>), but this may change in the future. [ENFILE] The system file table is full. [ENOENT] O_CREAT is not set and the named object does not exist. [ENOSPC] O_CREAT is specified, the file does not exist, and there is insufficient space available to create the object. SEE ALSO
chmod(2), close(2), getdtablesize(2), mmap(2), shm_unlink(2), umask(2) HISTORY
shm_open() is specified in the POSIX Realtime Extension (1003.1b-1993/1003.1i-1995). Darwin August 29, 2008 Darwin
Check Out this Related Man Page
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 spec- ified, 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, 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' 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 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' 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.1''). HISTORY
The shm_open() and shm_unlink() functions first appeared in FreeBSD 4.3. The functions were reimplemented as system calls using shared mem- ory 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) BSD
March 20, 2007 BSD