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