ANSI C

In computer programming, ANSI C (or ISO C, or C89) is a specification for the C language and an update to the original K&R version of C. Programs written the ANSI C language have access to a few extra features inspired by C++; the main difference between old C and ANSI C seems to be in declarations of function parameters. These days, ANSI C is routine and C programmers almost always use it.

• The Portland Pattern Repository wiki has a page: Wiki:AnsiCee.
• Wikipedia has one: Wikipedia:ANSI C.

However, NetHack was a very old program dating from before ANSI C's first spec in 1989. Today's version can take advantage of certain ANSI C features, and code for this is in tradstdc.h. Is NetHack written in ANSI C? Yes and no, depending on what tradstdc.h decides to do.

The "void" type

In C, the "void" type indicates a function that does not return a value. The original C did not have a "void" type; programmers often declared functions to return "int" and ignore the value. (This is why compiling doesn't fail if you forget to return a value from a non-void function.)

It became common to #define void int to cosmetically declare a void function. (The preprocessor would change every "void" to "int" and the C compiler would have no concept of void.) Later, many C compiler vendors started including the void keyword. C++ had a void keyword. So ANSI decided to put "void" in ANSI C.

If you find a void-free compiler to build NetHack with, then the procedure is to uncomment the #define NOVOID line at config.h#line239 so that tradstdc.h#line23 defines void.

void pointers, null pointers

In ANSI C, the data type pointer-to-void (void *) can be used to hold the value of any other type of pointer, without requiring an explicit cast (or type conversion). NetHack defines genericptr_t for this purpose.

NetHack also does without the keyword NULL, which is defined as a pointer value that cannot be dereferenced and compares equal to a value of zero in integer or boolean types. The pre-ANSI equivalent is (char *)0, (genericptr_t)0 etc.

Function declarations in ANSI C

Prototypes

An old style declaration gives only the return type:

char *xname();

while a prototype gives the types of the parameters:

char *xname(struct obj *optr);

The name of the parameter, "optr" in the example above, is optional; and NetHack usually omits it.

The NDECL, FDECL and VDECL macros create either prototypes or old-style declarations. Separate macros are used to support an overlaid build on MS-DOS (which is not officially supported, and barely practical today). NDECL is used if there are no parameters, FDECL for a fixed parameter list of at least one parameter, and VDECL for a variable parameter list.

Default promotions

With the functions defined in the old style, integer types smaller than int are promoted to int or unsigned, preserving their signedness. If a prototype is present, on most compilers the parameters must match the promoted types. A few older compilers will match a prototype to an unpromoted parameter type.

global.h defines seven types to be used in prototypes where the function has a type with a default promotion. The types are CHAR_P, SCHAR_P, UCHAR_P, XCHAR_P, SHORT_P, BOOLEAN_P and ALIGNTYP_P, and they correspond to char, schar, uchar, xchar, short, boolean and aligntyp. The X11 interface defines a DIMENSION_P type, corresponding to Dimension.

Variable parameter lists

The first C compilers used barely-portable hacks to support variadic functions such as printf. The symbol USE_OLDARGS enables these in NetHack.

Later pre-ANSI compilers provided a header, varargs.h, to support variadic functions. USE_VARARGS enables these in NetHack, and they look like this:

#include <varargs.h>
void
function(format, va_alist)
char *format;
va_dcl /* no semicolon */
{
    va_list the_args;

    va_start(the_args);
    vprintf(format, the_args);
    va_end(the_args);
}

(There is another macro, va_arg to extract an argument. No part of NetHack uses it.)

This usage could not be made compatible with prototypes, and so ANSI C uses a different system; USE_STDARG enables it in NetHack:

#include <stdarg.h>
void
function(const char *format, ...)
{
    va_list the_args;

    va_start(the_args, format); /* use the last parameter before the ... */
    vprintf(format, the_args);
    va_end(the_args);
}

The source code for the variadic functions in NetHack is ugly, to say the least.

Note for the ambitious newbie

In case you consider creating a project of your own, be it a Rogue-like game or some other application, consider using a more modern language than C. Nowadays there are lots of platform-independent, high-level alternatives like Java, Python or Perl just to mention a few. Such an application would be far easier to debug and maintain than its counterpart written in C. If you feel an urge to squeeze a bit more power out of the machine for your advanced ANSI-graphics and its pixelshading algorithms, at least consider using C++. If you, despite every sane thought, decide that C is the language you want to use, use the latest version of C, C99. The latest version contains many corrections and improvements and will cause you less trouble.

If you choose to develop your code in C or C++, and are using something like gcc to compile, use the options "-Wall -ansi -pedantic". You can also use "-std=c99" for the C99 version of C.

Vanilla NetHack continues to use C because of inertia (a 150,000+ line program is non-trivial to translate) and because of its stated goal: to get the game working on as many different types of hardware and under as many different operating systems as is practical.