shm_open(2) [mojave 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
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shm_open(3) Library Functions Manual shm_open(3) NAME
shm_open - Opens a shared memory object, creating the object if necessary (P1003.1b) LIBRARY
Realtime Library (librt.so, librt.a) SYNOPSIS
#include <sys/mman.h> int shm_open ( const char *name, int oflag, mode_t mode); PARAMETERS
*name Points to a string naming the shared memory object. oflag Specifies flags that define file status and file access modes. This argument is constructed from the bitwise inclusive OR of zero or more of the flags defined in the <fcntl.h> header file. mode Specifies the shared memory object's permission bits. This argument is used only when the shared memory object is being created. DESCRIPTION
The shm_open function establishes a connection between a shared memory object and a file descriptor. It creates an open file descriptor that refers to the shared memory object and a file descriptor that refers to that open file descriptor. This file descriptor is used by other functions to refer to the shared memory object. The name points to a string naming a shared memory object. The name can be a path- name, in which case other processes referring to the same pathname refer to the same shared memory object. When a shared memory object is created, its state and all data associated with it persist until the shared memory is unlinked. The shm_open function returns a file descriptor that is the lowest numbered file descriptor not currently open for that process. File sta- tus flags and access modes are set according to the oflag argument. These flags are defined in the <fcntl.h> header file and can have zero or more of the following values: o O_RDONLY - Open for read access only. o O_RDWR - Open for read or write access. o O_CREAT - Create the shared memory if the memory object does not exist already. The user ID and group ID of the shared memory object are identical to those of the calling process. The shared memory object's permission bits are set to the value of mode, except those set in the file mode creation mask of the process. o O_EXCL - Prevent the opening of a shared memory object if O_CREAT is set and the shared memory object already exists. Use this flag only in combination with O_CREAT. o O_TRUNC - Truncate the shared memory object to zero length if it is successfully opened for read or write access (O_RDWR). The initial contents of the shared memory object are binary zeros. RETURN VALUES
On a successful call to shm_open, a nonnegative integer is returned that represents the lowest-numbered unused file descriptor. The file descriptor points to the shared memory object. Otherwise, -1 is returned and errno is set to indicate the error. ERRORS
The shm_open function fails under the following conditions: [EACCES] Permission to create the shared memory object is denied. The shared memory object exists and the permissions specified by oflag are denied. O_TRUNC is specified and write permission is denied. [EEXIST] O_CREAT and O_EXCL are set, but the named shared memory object already exists. [EINTR] A signal has interrupted the shm_open operation. [EINVAL] The shm_open operation is not supported for the given name. [EMFILE] Too many file descriptors are currently in use by this process. [ENAMETOOLONG] The length of the path argument exceeds PATH_MAX, or a pathname component is longer than NAME_MAX while _POSIX_NO_TRUNC is in effect. [ENFILE] Too many shared memory objects are currently open in the system. [ENOENT] O_CREAT is not set and the named shared memory object does not exist. [ENOSPC] Memory space for creation of the new shared memory object is insufficient. RELATED INFORMATION
Functions: close(2), dup(2), exec(2), fcntl(2), fstat(2), mmap(2), umask(2), shm_unlink(3) Guide to Realtime Programming delim off shm_open(3)