rtapi_shmem — Functions for managing shared memory blocks


int rtapi_shmem_new ( int key, int module_id, unsigned long int size )

int rtapi_shmem_delete ( int shmem_id, int module_id )

int rtapi_shmem_getptr(int shmem_id, void ptr, unsigned long int * sizeptr )

int rtapi_shmem_exists ( int key )


Identifies the memory block. Key must be nonzero.
All modules wishing to use the same memory must use the same key.

Module identifier returned by a prior call to rtapi_init.

The desired size of the shared memory block, in bytes

The pointer to the shared memory block.
Note that the block may be mapped at a different address for different modules.

Pointer to an unsigned long, or 0.
If non-zero, set to the size of the shared memory block.


rtapi_shmem_new allocates a block of shared memory.

key identifies the memory block, and must be non-zero.
All modules wishing to access the same memory must use the same key.

module_id is the ID of the module that is making the call (see rtapi_init).
The block will be at least size bytes, and may be rounded up.
Allocating many small blocks may be very wasteful. When a particular block is allocated for the first time, the first 4 bytes are zeroed.
Subsequent allocations of the same block by other modules or processes will not touch the contents of the block.
Applications can use those bytes to see if they need to initialize the block, or if another module already did so.

On success, it returns a positive integer ID, which is used for all subsequent calls dealing with the block.
On failure it returns a negative error code.

Calling rtapi_shmem_new with a zero size argument will attach to an existing shared memory segment; if this segment does not exist a negative error code is returned.

rtapi_shmem_delete frees the shared memory block associated with shmem_id. module_id is the ID of the calling module.
Returns a status code.

rtapi_shmem_getptr sets *ptr to point to shared memory block associated with shmem_id.

rtapi_shmem_exists may be used to test if a shared memory segment exists.
A value of 1 means the segment exists, a value of 0 means it doesnt.


rtapi_shmem_getptr may be called from user code, init/cleanup code, or realtime tasks.

rtapi_shmem_new and rtapi_shmem_dete may not be called from realtime tasks.