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5.25.2 Declaring C Functions

Before you can call lseek or dlseek, you have to declare it. The declaration consists of two parts:

The C part
is the C declaration of the function, or more typically and portably, a C-style #include of a file that contains the declaration of the C function.
The Forth part
declares the Forth types of the parameters and the Forth word name corresponding to the C function.

For the words lseek and dlseek mentioned earlier, the declarations are:

     \c #define _FILE_OFFSET_BITS 64
     \c #include <sys/types.h>
     \c #include <unistd.h>
     c-function lseek lseek n n n -- n
     c-function dlseek lseek n d n -- d

The C part of the declarations is prefixed by \c, and the rest of the line is ordinary C code. You can use as many lines of C declarations as you like, and they are visible for all further function declarations.

The Forth part declares each interface word with c-function, followed by the Forth name of the word, the C name of the called function, and the stack effect of the word. The stack effect contains an arbitrary number of types of parameters, then --, and then exactly one type for the return value. The possible types are:

single-cell integer
address (single-cell)
double-cell integer
floating-point value
C function pointer
no value (used as return type for void functions)

To deal with variadic C functions, you can declare one Forth word for every pattern you want to use, e.g.:

     \c #include <stdio.h>
     c-function printf-nr printf a n r -- n
     c-function printf-rn printf a r n -- n

Note that with C functions declared as variadic (or if you don't provide a prototype), the C interface has no C type to convert to, so no automatic conversion happens, which may lead to portability problems in some cases. In such cases you can perform the conversion explicitly on the C level, e.g., as follows:

     \c #define printfll(s,ll) printf(s,(long long)ll)
     c-function printfll printfll a n -- n

Here, instead of calling printf() directly, we define a macro that casts (converts) the Forth single-cell integer into a C long long before calling printf().

\c       "rest-of-line" –         gforth       “backslash-c”

One line of C declarations for the C interface

c-function       "forth-name" "c-name" "{type}" "–" "type" –         gforth       “c-function”

Define a Forth word forth-name. Forth-name has the specified stack effect and calls the C function c-name.

In order to work, this C interface invokes GCC at run-time and uses dynamic linking. If these features are not available, there are other, less convenient and less portable C interfaces in lib.fs and oldlib.fs. These interfaces are mostly undocumented and mostly incompatible with each other and with the documented C interface; you can find some examples for the lib.fs interface in lib.fs.