The Lua for LabVIEW calling mechanism

The problem with integrating scripting languages into LabVIEW such that LabVIEW can be called, and thus scripted, is that there is no API for calling back into LabVIEW from C code called by LabVIEW. Lua for LabVIEW solves this by calling LabVIEW from LabVIEW via a specially constructed C function that interrupts the virtual machine so that a LabVIEW to LabVIEW call can take place instead. This is evident when an error occurs inside a LabVIEW-side function that is being called from Lua: the error message will contain both a Lua stack trace that claims the error happened inside a C function, which is true from Lua's perspective, and a call chain that claims that LabVIEW failed while calling LabVIEW, which is true from LabVIEW's perspective. This mechanism is made possible by the versatile virtual machine control and customisation features offered by Lua.

One alternative means of scripting LabVIEW is to implement a scripting language in LabVIEW (G). This has in fact been done for several specialized scripting systems, but creating a general purpose language in LabVIEW that approaches the quality of existing C-based scripting languages is a nearly impossible task. Another alternative is to run scripts as part of  an external process, and implement a calling mechanism based on inter-process communication, e.g. over TCP/IP. This is cumbersome in terms of deployment, initialisation, and binding. It also carries a large amount of overhead since two process switches are required per call and arguments and results will have to be passed across process boundaries. Advantages of the Lua for LabVIEW approach are:

The lack of provisions for calling LabVIEW from C code running under the LabVIEW runtime has to do with the special nature of scheduling in LabVIEW. Instead of an execution context being defined by a thread with an associated stack, LabVIEW is able to multiplex the available threads in a cooperative and pre-emptive manner over the various bits of LabVIEW code that require execution. Execution in a dataflow system is required when all input data is available, and this has allowed LabVIEW to be designed to exploit parallel processing for run-of-the-mill diagram code that has no special provisions. This is an amazing and excellent feature. It makes that LabVIEW code will happily scale along in these days of simultaneous multi threading and cheap simultaneous multi processing without requiring any further investment on the part of the programmer.

The above does suggest, with a high degree of confidence, that LabVIEW does not store and restore C stack frames when it reschedules threads as this would be both costly and a misfit of the data-flow paradigm. Instead LabVIEW looks at the data structures associated with the compiled diagram to determine what has to happen next. One consequence is that LabVIEW does not support recursive VI calls. Another consequence is that when LabVIEW calls into C code (which does require the C stack), the calling thread is blocked for the duration of the execution of the C code. Furthermore, that C code will have difficulty with calling back into LabVIEW. The reason is that, once called back to the LabVIEW side, the thread stack holds the calling frames of all underlying C calls. Thus, LabVIEW would either need to support a mechanism that stores and restores this stack so that the thread can participate in normal scheduling (which is technically difficult), or the thread must remain reserved while the LabVIEW-side call is ongoing. The latter would limit the number of LabVIEW-side calls to one less than the number of available threads, which makes it incompatible with single threading mode and, in case of multi-threading, would bring the runtime to a grinding halt when having more than a couple of active scripts.

A disadvantage of the Lua for LabVIEW calling mechanism is that LabVIEW cannot be called via an additional C function: while a normal C function is being called, the Lua virtual machine cannot be interrupted because the C stack cannot be unwound. There are in fact a couple of C functions in the Lua C libraries that take a function as a parameter and make calls on the passed function. When passing a LabVIEW-side function to such a C function, a runtime error is thrown. The same occurs when trying to call LabVIEW from a meta method: these are implemented as internal C functions. You are unlikely to encounter this issue unless you use meta tables, coroutines, or elaborate functional programming techniques. The most useful of the offending C library functions is pcall which allows function calls to be protected by catching their errors. To compensate, Lua for LabVIEW provides an alternate means of protecting LabVIEW calls (the lv.pcall function) that works without making use of the C stack. Note that it is often possible to implement equivalents of C functions in Lua, and Lua functions have no trouble with calling into LabVIEW.

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