The C interface file

To make your C functions visible to the runtime system, you must define all the functions in the C interface file.

The C interface file is a C source file that defines the usrFunctions array. This array defines C functions that can be called from programs.

The last record of the usrFunctions array must be a line with all the elements set to NULL/0, to define the end of the list.

Each element of the usrFunctions array must include the following members:
  1. The first member is the name of the function, provided as a (const char *) character string.
  2. The second member is the C function symbol, provided as an (int (*function) (int)) C function pointer.
  3. The third member is the number of parameters passed to the function through the runtime stack, provided as an (int); use a negative value like -1 to specify a variable number of arguments.
  4. The fourth member is the number of values returned by the function, provided as an (int); use a negative value like -1 to specify a variable number of return values.
Note: The third and fourth member of a UsrFunction element can be defined as a negative value (-1), to indicate a variable number of arguments and/or return values.

You typically do a forward declaration of your C functions, before the usrFunctions array initializer:

#include "f2c/fglExt.h"

int c_init(int);
int c_set_trace(int);
int c_get_message(int);
int c_compare(int);
int c_generate(int);

UsrFunction usrFunctions[]={
  { "init",         c_init,         0, 0 },
  { "set_trace",    c_set_trace,    1, 0 },
  { "get_message",  c_get_message,  1, 1 },
  { "compare",      c_compare,     -1, 1 }, /* var. numb. params */
  { "generate",     c_generate,     1,-1 }, /* var. numb. returns */
  { NULL,           NULL,           0, 0 }
};
Note that the UsrFunction structure contains an additional member, dedicated for internal use. If you experience compiler warnings because of uninitialized structure members, simply complete the C function definitions with a fifth zero value: 
/* Avoids C compiler warnings because of un-initialized structure members */

UsrFunction usrFunctions[]={
  { "init",         c_init,         0, 0, 0 },
            /* member for internal use ---^ */
  ...