13.8 Stream Pairs

Since there are many possible processes/event loop combinations, the event code is responsible for creating an appropriate lstream type. The process implementation does not care about that implementation.

The Create stream pair function is passed two void* values, which identify process-dependent ’handles’. The process implementation uses these handles to communicate with child processes. The function must be prepared to receive handle types of any process implementation. Since only one process implementation exists in a particular XEmacs configuration, preprocessing is a means of compiling in the support for the code which deals with particular handle types.

For example, a unixoid type loop, which relies on file descriptors, may be asked to create a pair of streams by a unix-style process implementation. In this case, the handles passed are unix file descriptors, and the code may deal with these directly. Although, the same code may be used on Win32 system with X-Windows. In this case, Win32 process implementation passes handles of type HANDLE, and the create_io_streams function must call appropriate function to get file descriptors given HANDLEs, so that these descriptors may be passed to XtAddInput.

The handle given may have special denying value, in which case the corresponding lstream should not be created.

The return value of the function is a unique stream identifier. It is used by processes implementation, in its platform-independent part. There is the get_process_from_usid function, which returns process object given its USID. The event stream is responsible for converting its internal handle type into USID.

Example is the TTY event stream. When a file descriptor signals input, the event loop must determine process to which the input is destined. Thus, the implementation uses process input stream file descriptor as USID, by simply casting the fd value to USID type.

There are two special USID values. One, USID_ERROR, indicates that the stream pair cannot be created. The second, USID_DONTHASH, indicates that streams are created, but the event stream does not wish to be able to find the process by its USID. Specifically, if an event stream implementation never calls get_process_from_usid, this value should always be returned, to prevent accumulating useless information on USID to process relationship.