How to process custom arguments
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Scconfig first processes the known arguments (defined in the table in
default/arg.c). Unknown (custom) arguments are assumed to be project-specific
and are passed to hook_custom_arg(), defined in your hooks.c:

int hook_custom_arg(const char *key, const char *value) {}

If the user argument was --foo=bar, this function gets "foo" as key and
"bar" as value. There are multiple options on parsing the arguments.

1. Classic C

Just write a bunch of if (strcmp(key, "foo") == 0) { } statements, e.g.:

	if (strcmp(key, "prefix") == 0) {
		report("Setting prefix to '%s'\n", value);
		put("/local/prefix", strclone(value));
		return 1;
	}
	if (strcmp(key, "debug") == 0) {
		put("/local/pcb/debug", strue);
		pup_set_debug(strue);
		return 1;
	}

The code is free to do anything upon detecting arguments; the typical
behavior is to set tree nodes to affect the generated files later.

Return 1 means the argument has been accepted. Return 0 will tell scconfig
to report an error and exit.

2. Semi-automatic, using arg_auto_set

#include "util/arg_auto_set.h"

const arg_auto_set_t disable_libs[] = {
	{"enable-bison",      "/local/pcb/want_bison",        arg_true,      "$enable generating language files"},
	{"disable-bison",     "/local/pcb/want_bison",        arg_false,     "$disable generating language files"},
	{"enable-dmalloc",    "/local/pcb/want_dmalloc",      arg_true,      "$compile with lib dmalloc"},
	{"disable-dmalloc",   "/local/pcb/want_dmalloc",      arg_false,     "$compile without lib dmalloc"},
	{"disable-gd",        "libs/gui/gd",                  arg_lib_nodes, "$do not use gd (many exporters need it)"},
	{NULL, NULL, NULL, NULL}
};

int hook_custom_arg(const char *key, const char *value)
{
	return arg_auto_set(key, value, disable_libs);
}


Function arg_auto_set() searches the table for a matching key in the first
column. When found, it executes the instructions in the 3rd column on the
subtree specified in the 2nd column. Instruction arg_true and arg_false
will assume the subtree path points to a leaf and sets it to hardwired
strue or sfalse. arg_lib_nodes disables a library, setting the present-string
to sfalse and making ldflags and cflags empty under the subtree - this
will inhibit detection of the library as scconfig will think the library
already got detected and was not found.

(The user can specify application-local instructions; see also:
src/util/arg_auto_set.[ch]; a "$" in the second column of an
arg_auto_set_node_t table will set the given node to the value supplied
by the user in the CLI argument).

An user readable help can be generated from the dable using
arg_auto_print_options(). The 4th column is the help text. If it starts
with '$', the '$' is replaced by the first column and padding spaces as
needed to have a tabular output:

 --enable-bison             enable generating language files
 --disable-bison            disable generating language files
 --enable-dmalloc           compile with lib dmalloc
 --disable-dmalloc          compile without lib dmalloc
 --disable-gd               do not use gd (many exporters need it)

3. The two methods can be used in parallel

In such setup hook_custom_arg() starts with a bunch of if (strcmp()) statements
and the final statement is a

	return arg_auto_set(key, value, table_name);