Call Data Type

MDSplus provides a call data type used for describing references to external functions. The MDSplus expression evaluator has the ability to call external functions in shared libararies or DLL's. The call datatype describes the type of the return value of the function, the name of the library or DLL where the function can be found, the name of the function and the list of arguments to present to the function when calling it. The expression evaluator in MDSplus has syntax for easily building instances of this data type as will as builtin functions for constructing them. The call to the external function occurs when the expression containing a call item is evaluated. The following example shows an example of calling an external function:

mysharedlib->myfunction(42)

The code above is the most common form of the function reference. When evaluated, the function called "myfunction" found in the library "mysharedlib" will be called with one argument, 42. The return value of the function is assumed to be a 32-bit signed integer.

build_call(kind(1),"mysharedlib","myfunction",42)

The above code creates the same function as the previous example but uses the build_call function to construct the call item. The behavior would be identical to the previous example.

_x=zero(100)
build_call(kind(1e6),"/home/twf/mylibraries/libmysharedlib","myfunction2",val(42),ref(_x))

This example builds a similar call but in this case the function return is handled as a floating point value and the "myfunction2" function is called with two arguments, 42 passed by value and a TDI variable passed by reference. This example uses the full file specification (without the file type).

MDSplus uses operating system specific rules to locate shared libraries when the full file specification of the library is not given. On OpenVMS, the system would first look for a logical name matching the name of the library and if one is found it would activate the image defined by the logical name. If their is no such logical name, the system would then look in the directory search path defined by SYS$SHARE for the library. The file type for shared libraries on OpenVMS is ".EXE". On Unix systems the system looks for a library path environment variable such as LD_LIBRARY_PATH or SHLIB_PATH and looks in these directories. If there is no such environment variable, the system looks in system library directories (usually /usr/lib or /usr/local/lib) for the shared library. The file type for shared libraries on Unix is generally ".so",".sl" or sometimes ".a". On Windows systems, the system looks for shared libraries in the directory where the application was run or in the directory list specified by the "PATH" environment variable. The file type of shared libraries on Windows is ".dll".

The following table lists some of the functions available for creating and accessing the call function: