|SHM_OPEN(2)||System Calls Manual||SHM_OPEN(2)|
shm_open — open a
shared memory object
shm_open(const char *name,
int oflag, ...);
The parameter "mode_t mode" is optional.
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
FD_CLOEXEC flag set. Repeated calls to
shm_open with the same string value for
will return a file descriptor referring to the same shared memory object,
provided that the object has not been unlinked by a call to
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_RDWR must be specified.
O_TRUNC is specified and
the file exists, the file is truncated to zero length. If
O_EXCL is set with
and the file already exists,
returns an error. This may be used to implement a simple exclusive access
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
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.
The named object is opened unless:
O_CREATis specified, the object does not exist, and permission to create the object is denied.
O_EXCLwere specified and the object exists.
shm_open() operation was interrupted by a signal.
shm_open() operation is not supported.
PSHMNAMLENcharacters (defined in
<sys/posix_shm.h>), but this may change in the future.
O_CREATis not set and the named object does not exist.
O_CREATis specified, the file does not exist, and there is insufficient space available to create the object.
shm_open() is specified in the POSIX
Realtime Extension (1003.1b-1993/1003.1i-1995).
|August 29, 2008||Darwin|