Interfacing C Programs to PL/M-51

You can easily interface C51 to routines written in PL/M-51. Intel’s PL/M-51 is a popular programming language that is similar to C in many ways. The PL/M-51 compiler generates object files in the OMF-51 format. You can access PL/M-51 functions from C by declaring them with the alien function type specifier. Public variables declared in the PL/M-51 module are available to your C programs.

C51 can optionally operate with PL/M-51 parameter passing conventions. The alien function type specifier is used to declare public or external functions that are compatible with PL/M-51 in any memory model. For example:

extern alien char plm_func (int, char);

alien unsigned int c_func (
  unsigned char x,
  unsigned char y)
{
return (x * y);
}

Parameters and return values of PL/M-51 functions may be any of the following types: bit, char, unsigned char, int, and unsigned int. Other types, including long, float, and all types of pointers, can be declared in C functions with the alien type specifier.  However, use these types with care because PL/M-51 does not directly support 32-bit binary integers or floating-point numbers.

PL/M-51 does not support variable-length argument lists. Therefore, functions declared using the alien type specifier must have a fixed number of arguments. The ellipsis notation used for variable-length argument lists is not allowed for alien functions and will cause C51 to generate an error message. For example:

extern alien unsigned int  plm_i (char, int, ...);

*** ERROR IN LINE 1 OF A.C: 'plm_i': Var_parms on alien function