ecdcdea1af
static since otherwise gcc4 optimises them away completely. Also, it really *is* an error if the table start and end are unused, so they certainly shouldn't be marked with attribute unused.
230 lines
6.3 KiB
C
230 lines
6.3 KiB
C
#ifndef _GPXE_TABLES_H
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#define _GPXE_TABLES_H
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/** @page ifdef_harmful #ifdef considered harmful
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*
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* Overuse of @c #ifdef has long been a problem in Etherboot.
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* Etherboot provides a rich array of features, but all these features
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* take up valuable space in a ROM image. The traditional solution to
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* this problem has been for each feature to have its own @c #ifdef
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* option, allowing the feature to be compiled in only if desired.
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*
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* The problem with this is that it becomes impossible to compile, let
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* alone test, all possible versions of Etherboot. Code that is not
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* typically used tends to suffer from bit-rot over time. It becomes
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* extremely difficult to predict which combinations of compile-time
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* options will result in code that can even compile and link
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* correctly.
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*
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* To solve this problem, we have adopted a new approach from
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* Etherboot 5.5 onwards. @c #ifdef is now "considered harmful", and
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* its use should be minimised. Separate features should be
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* implemented in separate @c .c files, and should \b always be
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* compiled (i.e. they should \b not be guarded with a @c #ifdef @c
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* MY_PET_FEATURE statement). By making (almost) all code always
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* compile, we avoid the problem of bit-rot in rarely-used code.
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*
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* The file config.h, in combination with the @c make command line,
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* specifies the objects that will be included in any particular build
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* of Etherboot. For example, suppose that config.h includes the line
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*
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* @code
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*
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* #define CONSOLE_SERIAL
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* #define DOWNLOAD_PROTO_TFTP
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*
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* @endcode
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*
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* When a particular Etherboot image (e.g. @c bin/rtl8139.zdsk) is
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* built, the options specified in config.h are used to drag in the
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* relevant objects at link-time. For the above example, serial.o and
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* tftp.o would be linked in.
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*
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* There remains one problem to solve: how do these objects get used?
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* Traditionally, we had code such as
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*
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* @code
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*
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* #ifdef CONSOLE_SERIAL
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* serial_init();
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* #endif
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*
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* @endcode
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*
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* in main.c, but this reintroduces @c #ifdef and so is a Bad Idea.
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* We cannot simply remove the @c #ifdef and make it
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*
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* @code
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*
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* serial_init();
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*
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* @endcode
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*
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* because then serial.o would end up always being linked in.
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*
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* The solution is to use @link tables.h linker tables @endlink.
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*
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*/
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/** @file
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*
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* Linker tables
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*
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* Read @ref ifdef_harmful first for some background on the motivation
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* for using linker tables.
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*
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* This file provides macros for dealing with linker-generated tables
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* of fixed-size symbols. We make fairly extensive use of these in
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* order to avoid @c #ifdef spaghetti and/or linker symbol pollution.
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* For example, instead of having code such as
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*
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* @code
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*
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* #ifdef CONSOLE_SERIAL
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* serial_init();
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* #endif
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*
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* @endcode
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*
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* we make serial.c generate an entry in the initialisation function
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* table, and then have a function call_init_fns() that simply calls
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* all functions present in this table. If and only if serial.o gets
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* linked in, then its initialisation function will be called. We
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* avoid linker symbol pollution (i.e. always dragging in serial.o
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* just because of a call to serial_init()) and we also avoid @c
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* #ifdef spaghetti (having to conditionalise every reference to
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* functions in serial.c).
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*
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* The linker script takes care of assembling the tables for us. All
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* our table sections have names of the format @c .tbl.NAME.NN where
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* @c NAME designates the data structure stored in the table (e.g. @c
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* init_fn) and @c NN is a two-digit decimal number used to impose an
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* ordering upon the tables if required. @c NN=00 is reserved for the
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* symbol indicating "table start", and @c NN=99 is reserved for the
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* symbol indicating "table end".
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*
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* As an example, suppose that we want to create a "frobnicator"
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* feature framework, and allow for several independent modules to
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* provide frobnicating services. Then we would create a frob.h
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* header file containing e.g.
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*
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* @code
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*
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* struct frobnicator {
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* const char *name; // Name of the frobnicator
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* void ( *frob ) ( void ); // The frobnicating function itself
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* };
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*
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* #define __frobnicator __table ( frobnicators, 01 )
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*
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* @endcode
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*
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* Any module providing frobnicating services would look something
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* like
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*
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* @code
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*
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* #include "frob.h"
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*
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* static void my_frob ( void ) {
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* // Do my frobnicating
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* ...
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* }
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*
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* struct frob my_frobnicator __frobnicator = {
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* .name = "my_frob",
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* .frob = my_frob,
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* };
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*
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* @endcode
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*
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* The central frobnicator code (frob.c) would use the frobnicating
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* modules as follows
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*
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* @code
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*
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* #include "frob.h"
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*
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* static struct frob frob_start[0] __table_start ( frobnicators );
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* static struct frob frob_end[0] __table_end ( frobnicators );
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*
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* // Call all linked-in frobnicators
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* void frob_all ( void ) {
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* struct frob *frob;
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*
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* for ( frob = frob_start ; frob < frob_end ; frob++ ) {
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* printf ( "Calling frobnicator \"%s\"\n", frob->name );
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* frob->frob ();
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* }
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* }
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*
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* @endcode
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*
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* See init.h and init.c for a real-life example.
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*
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*/
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#ifdef DOXYGEN
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#define __attribute__(x)
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#endif
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#define __table_str(x) #x
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#define __table_section(table,idx) \
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__section__ ( ".tbl." __table_str(table) "." __table_str(idx) )
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#define __table_section_start(table) __table_section(table,00)
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#define __table_section_end(table) __table_section(table,99)
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/**
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* Linker table entry.
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*
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* Declares a data structure to be part of a linker table. Use as
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* e.g.
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*
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* @code
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*
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* struct my_foo __table ( foo, 01 ) = {
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* ...
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* };
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*
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* @endcode
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*
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*/
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#define __table(table,idx) \
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__attribute__ (( __table_section(table,idx) ))
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/**
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* Linker table start marker.
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*
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* Declares a data structure (usually an empty data structure) to be
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* the start of a linker table. Use as e.g.
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*
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* @code
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*
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* static struct foo_start[0] __table_start ( foo );
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*
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* @endcode
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*
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*/
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#define __table_start(table) \
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__attribute__ (( __table_section_start(table) ))
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/**
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* Linker table end marker.
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*
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* Declares a data structure (usually an empty data structure) to be
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* the end of a linker table. Use as e.g.
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*
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* @code
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*
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* static struct foo_end[0] __table_end ( foo );
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*
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* @endcode
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*
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*/
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#define __table_end(table) \
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__attribute__ (( __table_section_end(table) ))
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#endif /* _GPXE_TABLES_H */
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