External C-functions

In the previous section we have shown, that external function can be specified in XASM using the asm construct. Alternatively, external functions can be implemented in C. The corresponding XASM-declaration is given as follows:

The c_name specifies the name of the C function; it can be omitted, if it is equal to the XASM-name of the function. Depending on the access mode of the XASM-function, the corresponding C-function prototypes differ slightly:

• If the access mode is ``monitored'', the C-functions are defined as follows:

  ASMOBJ c_name(ASM a, int argc, ASMOBJ* argv);

  where ``ASMOBJ'' is the C-type representing elements of the superuniverse; ``ASM'' is a C-struct containing information related to parent asm. These types are specified in the header file ``xasm.h'' which must be included in those files containing external function implementations. The arguments of the function call can be accessed via the ``argv'' field using ``argc'' as argument count. The first argument, argv[0], always contains the name of the corresponding XASM-function as string element. The result of the external function that is accessible in the calling asm is returned by the C-function as ``ASMOBJ''.
• If the access mode is ``output'', the C-functions are defined as follows:

  void cname(ASM a, int argc, ASMOBJ* argv, ASMOBJ val);

  In this case, additionally the value that is used in the update representing the call of the external function can be accessed using the ``val'' parameter.

As an example, the following C-code contains the implementation of the ``stderr'' function previously used in one of the examples:

The XASM-library function ``str_obj'' returns the string representation of an ASMOBJ. The corresponding declaration of the external C-function in an asm has the following format: