SHM_OPEN(2) BSD System Calls Manual SHM_OPEN(2)
shm_open -- open a shared memory object
shm_open(const char *name, int flags, mode_t mode);
The shared memory object referenced by name is opened for reading and/or
writing as specified by the argument flags 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 flags argu-
ment 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
The flags specified are 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
The new descriptor is set to remain open across execve system calls; see
close(2) and fcntl(2).
The system imposes a limit on the number of file descriptors open simul-
taneously by one process. Getdtablesize(2) returns the current system
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
[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
[ENAMETOOLONG] name exceeded SHM_NAME_MAX characters.
[ENFILE] The system file table is full.
[ENOENT] O_CREAT is not set and the named object does not
[ENOSPC] O_CREAT is specified, the file does not exist, and
there is insufficient space available to create the
chmod(2), close(2), getdtablesize(2), mmap(2), shm_unlink(2), umask(2)
shm_open() is specified in the POSIX Realtime Extension
Darwin September 20, 1999 Darwin