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- Shared Memory Interfaces
- ************************
- Shared memory interfaces are only available with the NuttX kernel build
- (``CONFIG_BUILD_KERNEL=y``). These interfaces support user memory
- regions that can be shared between multiple user processes. Shared
- memory interfaces:
- - :c:func:`shmget`
- - :c:func:`shmat`
- - :c:func:`shmctl`
- - :c:func:`shmdt`
- Functions
- ---------
- .. c:function:: int shmget(key_t key, size_t size, int shmflg)
- Returns the shared memory identifier associated with ``key``.
- A shared memory identifier, associated data structure, and shared memory
- segment of at least size bytes is created for ``key`` if one of the
- following is true:
- - The argument ``key`` is equal to ``IPC_PRIVATE``.
- - The argument ``key`` does not already have a shared memory identifier
- associated with it and ``(shmflg & IPC_CREAT)`` is non-zero.
- Upon creation, the data structure associated with the new shared memory
- identifier will be initialized as follows:
- - The low-order nine bits of ``shm_perm.mode`` are set equal to the
- low-order nine bits of ``shmflg``.
- - The value of ``shm_segsz`` is set equal to the value of size.
- - The values of ``shm_lpid``, ``shm_nattch``, ``shm_atime``, and
- ``shm_dtime`` are set equal to 0.
- - The value of ``shm_ctime`` is set equal to the current time.
- When the shared memory segment is created, it will be initialized with
- all zero values.
- :param key: The key that is used to access the unique shared memory
- identifier.
- :param size: The shared memory region that is created will be at least
- this size in bytes.
- :param shmflg: See ``IPC_*`` definitions in ``sys/ipc.h``. Only the
- values ``IPC_PRIVATE`` or ``IPC_CREAT`` are supported.
- :return: Upon successful completion, ``shmget()`` will return
- a non-negative integer, namely a shared memory identifier; otherwise, it
- will return -1 and set ``errno`` to indicate the error.
- - ``EACCES``. A shared memory identifier exists for key but operation
- permission as specified by the low-order nine bits of ``shmflg``
- would not be granted.
- - ``EEXIST``. A shared memory identifier exists for the argument key
- but ``(shmflg & IPC_CREAT) && (shmflg & IPC_EXCL)`` are non-zero.
- - ``EINVAL``. A shared memory segment is to be created and the value of
- size is less than the system-imposed minimum or greater than the
- system-imposed maximum.
- - ``EINVAL``. No shared memory segment is to be created and a shared
- memory segment exists for key but the size of the segment associated
- with it is less than size and size is not 0.
- - ``ENOENT``. A shared memory identifier does not exist for the
- argument key and ``(shmflg & IPC_CREAT)`` is 0.
- - ``ENOMEM``. A shared memory identifier and associated shared memory
- segment will be created, but the amount of available physical memory
- is not sufficient to fill the request.
- - ``ENOSPC``. A shared memory identifier is to be created, but the
- system-imposed limit on the maximum number of allowed shared memory
- identifiers system-wide would be exceeded.
- **POSIX Deviations**
- - The values of ``shm_perm.cuid``, ``shm_perm.uid``, ``shm_perm.cgid``,
- and ``shm_perm.gid`` should be set equal to the effective user ID and
- effective group ID, respectively, of the calling process. The NuttX
- ``ipc_perm`` structure, however, does not support these fields
- because user and group IDs are not yet supported by NuttX.
- .. c:function:: void *shmat(int shmid, FAR const void *shmaddr, int shmflg)
- Attaches the shared memory
- segment associated with the shared memory identifier specified by
- ``shmid`` to the address space of the calling process. The segment is
- attached at the address specified by one of the following criteria:
- - If ``shmaddr`` is a null pointer, the segment is attached at the
- first available address as selected by the system.
- - If ``shmaddr`` is not a null pointer and ``(shmflg & SHM_RND)`` is
- non-zero, the segment is attached at the address given by
- ``(shmaddr - ((uintptr_t)shmaddr % SHMLBA))``.
- - If ``shmaddr`` is not a null pointer and ``(shmflg & SHM_RND)`` is 0,
- the segment is attached at the address given by ``shmaddr``.
- - The segment is attached for reading if ``(shmflg & SHM_RDONLY)`` is
- non-zero and the calling process has read permission; otherwise, if
- it is 0 and the calling process has read and write permission, the
- segment is attached for reading and writing.
- :param shmid: Shared memory identifier
- :param smaddr: Determines mapping of the shared memory region
- :param shmflg: See ``SHM_*`` definitions in ``include/sys/shm.h``. Only
- ``SHM_RDONLY`` and ``SHM_RND`` are supported.
- :return: Upon successful completion, ``shmat()`` will
- increment the value of ``shm_nattch`` in the data structure associated
- with the shared memory ID of the attached shared memory segment and
- return the segment's start address. Otherwise, the shared memory segment
- will not be attached, ``shmat()`` will return -1, and ``errno`` will be
- set to indicate the error.
- - ``EACCES``. Operation permission is denied to the calling process
- - ``EINVAL``. The value of ``shmid`` is not a valid shared memory
- identifier, the ``shmaddr`` is not a null pointer, and the value of
- ``(shmaddr -((uintptr_t)shmaddr % SHMLBA))`` is an illegal address
- for attaching shared memory; or the ``shmaddr`` is not a null
- pointer, ``(shmflg & SHM_RND)`` is 0, and the value of ``shmaddr`` is
- an illegal address for attaching shared memory.
- - ``EMFILE``. The number of shared memory segments attached to the
- calling process would exceed the system-imposed limit.
- - ``ENOMEM``. The available data space is not large enough to
- accommodate the shared memory segment.
- .. c:function:: int shmctl(int shmid, int cmd, FAR struct shmid_ds *buf)
- Provides a variety of shared
- memory control operations as specified by ``cmd``. The following values
- for ``cmd`` are available:
- - ``IPC_STAT``. Place the current value of each member of the
- ``shmid_ds`` data structure associated with ``shmid`` into the
- structure pointed to by ``buf``.
- - ``IPC_SET``. Set the value of the ``shm_perm.mode`` member of the
- ``shmid_ds`` data structure associated with ``shmid`` to the
- corresponding value found in the structure pointed to by ``buf``.
- - ``IPC_RMID``. Remove the shared memory identifier specified by
- ``shmid`` from the system and destroy the shared memory segment and
- ``shmid_ds`` data structure associated with it.
- :param shmid: Shared memory identifier
- :param cmd: ``shmctl()`` command
- :param buf: Data associated with the ``shmctl()`` command
- :return: Upon successful completion, ``shmctl()`` will return
- 0; otherwise, it will return -1 and set ``errno`` to indicate the error.
- - ``EACCES``. The argument ``cmd`` is equal to ``IPC_STAT`` and the
- calling process does not have read permission.
- - ``EINVAL``. The value of ``shmid`` is not a valid shared memory
- identifier, or the value of ``cmd``\ is not a valid command.
- - ``EPERM``. The argument ``cmd`` is equal to ``IPC_RMID`` or
- ``IPC_SET`` and the effective user ID of the calling process is not
- equal to that of a process with appropriate privileges and it is not
- equal to the value of ``shm_perm.cuid`` or ``shm_perm.uid`` in the
- data structure associated with ``shmid``.
- - ``EOVERFLOW``. The ``cmd`` argument is ``IPC_STAT`` and the ``gid``
- or ``uid`` value is too large to be stored in the structure pointed
- to by the ``buf`` argument.
- **POSIX Deviations**
- - ``IPC_SET``. Does not set the ``shm_perm.uid`` or
- ``shm_perm.gid``\ members of the ``shmid_ds`` data structure
- associated with ``shmid`` because user and group IDs are not yet
- supported by NuttX
- - ``IPC_SET``. Does not restrict the operation to processes with
- appropriate privileges or matching user IDs in ``shmid_ds`` data
- structure associated with ``shmid``. Again because user IDs and
- user/group privileges are are not yet supported by NuttX
- - ``IPC_RMID``. Does not restrict the operation to processes with
- appropriate privileges or matching user IDs in ``shmid_ds`` data
- structure associated with ``shmid``. Again because user IDs and
- user/group privileges are are not yet supported by NuttX
- .. c:function:: int shmdt(FAR const void *shmaddr)
- Detaches the shared memory
- segment located at the address specified by ``shmaddr`` from the address
- space of the calling process.
- :param shmid: Shared memory identifier
- :return: Upon successful completion, ``shmdt()`` will
- decrement the value of ``shm_nattch`` in the data structure associated
- with the shared memory ID of the attached shared memory segment and
- return 0.
- Otherwise, the shared memory segment will not be detached, ``shmdt()``
- will return -1, and ``errno`` will be set to indicate the error.
- - ``EINVAL``. The value of ``shmaddr`` is not the data segment start
- address of a shared memory segment.
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