OS Specific Toolchain: Difference between revisions

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Until now you have been using a [[GCC Cross-Compiler|cross-compiler]] configured to use an existing generic bare target. This is very convenient when starting out as you get a reliable target and the compiler doesn't make any bad assumptions because it thinks it is targeting an existing operating system. However, when you proceed it becomes useful if the compiler knows it is targeting your operating system and what its customs are. For instance, you can make the compiler define a <tt>__myos__</tt> preprocessor macro, know which directories to search for include files in, what special <tt>crt*.o</tt> files are used when linking against libc, and so on. It also becomes much easier to cross-compile software to your OS when you simply have to invoke <tt>x86_64-myos-gcc hello.c -o hello</tt> to cross-compile a program. Additionally part of the instructions here can be applied to other software packages that also use the GNU build system, which will help you [[Cross-Porting Software|port existing software]].

 

 

== Introduction ==

 

* A build environment that can successfully build a [[GCC Cross-Compiler]].

* libtool

* Knowledge of autoconf and automake.

 

Additionally you will need a [[C Library]] as described in a later section. As detailed in [[Hosted GCC Cross-Compiler]], it doesn't need to support much and the functionality can be stubbed, but libgcc will need to believe you have a libc.

 

 

This tutorial currently only have instructions for adding a new x86 and x86_64 target for myos, but it serves as a good enough example that it should be trivial to add more processors by basing it on these instructions and what other operating systems have done.

 

== Modifying Binutils ==

=== config.sub ===

 

 

=== bfd/config.bfd ===

 

 

i[3-7]86-*-myos*)

targ_selvecs=

;;

#ifdef BFD64

x86_64-*-myos*)

want64=true

;;

#endif

 

 

 

=== gas/configure.tgt ===

This file tells the gnu assembler what type of output to generate for each target. It automatically matches the i686 part of your target and generates the correct output for that. We just need to tell it what type of object file to generate for myos. In the section starting '<tt>Assign object format ... case ${generic_target} in</tt>' you need to add a line like

 

i386-*-myos*) fmt=elf ;;

 

You should use '<tt>i386</tt>' in this line even if you are targeting x86_64. This is the only file where you should do it. It is basically because the variable 'generic_target' is not your canonical target name, but rather a variable generated further up in the configure.tgt file, and it sets the first part to <tt>i386</tt> for any <tt>i[3-7]86</tt> or <tt>x86_64</tt>.

 

=== ld/configure.tgt ===

 

 

i[3-7]86-*-myos*)

targ_emul=elf_i386_myos

x86_64-*-myos*)

targ_emul=elf_x86_64_myos

;;

 

* '''elf_i386_myos''' is a 32-bit target for your OS.

Now we need to actually define our emulation. There is a generic file called <tt>ld/genscripts.sh</tt> which creates the required linker scripts for our target (you need more than one, depending on shared object usage and the like: I have 13 for a single target). It uses a linker script template (from the ld/scripttempl directory) to do this, and it creates the actual emulation C file from an emulation template (from the ld/emultempl directory). These templates are customised by running a script in the ld/emulparams directory which sets various variables. You are welcome to define your own emulation and linker templates, but I find the ELF ones adequate, given that they can be customised by simply adding a file to the emulparams directory. This is what we are going to do now. The content of the file could be something like:

 

 

This script is included by <tt>ld/genscripts.sh</tt> to customize its behavior through shell variables. We include the base <tt>elf_i386.sh</tt> script as it sets reasonable defaults. Finally, we override the variables whose defaults we disagree with.

There are a large number of variables that can be set here to customize your toolchain. Read the documentation and look at existing emulations for further information. These are some of the variables that can be set:

 

* '''SCRIPT_NAME'''=''name'' Controls which ld/scripttempl/''name''.sc script generates our linker scripts.

* '''TEMPLATE_NAME'''=''name'' Controls which ld/emultempl/''name''.em script generates our bfd emulation C implementation.

This file is just like the above <tt>ld/emulparams/elf_i386_myos.sh</tt> but for x86_64.

 

 

=== ld/Makefile.am ===

 

 

'''Note''': You ''must'' run <tt>automake</tt> in the <tt>ld</tt> directory after you modify <tt>Makefile.am</tt> to regenerate <tt>Makefile.in</tt>.

=== config.sub ===

 

 

=== gcc/config.gcc ===

This file defines what needs to be built for each particular target and what to include in the final executable. There are two main sections: one which defines generic options for your operating system, and those which define options specific to your operating system on each individual machine type.

 

 

*-*-myos*)

gas=yes

gnu_ld=yes

default_use_cxa_atexit=yes

;;

 

 

 

i[34567]86-*-myos*)

;;

x86_64-*-myos*)

;;

 

This defines which target configuration header files gets used. You can make <tt>i386/myos32.h</tt> and <tt>i386/myos64.h</tt> files if desired.

This header allows you to customize your toolchain using preprocessor macros. The relevant parts of GCC will include this header (as controlled by <tt>gcc/config.gcc</tt>) and modify the behavior according to your customizations.

 

#undef TARGET_MYOS

#define TARGET_MYOS 1

/* Default arguments you want when running your

i686-myos-gcc/x86_64-myos-gcc toolchain */

 

 

/* Additional predefined macros. */

builtin_assert ("system=posix"); \

} while(0);

 

=== libstdc++-v3/crossconfig.m4 ===

This file describes how the libstdc++ configure file will examine your operating system and adjust the provided features of libstdc++ accordingly. Add a case similar to

 

*-myos*)

GLIBCXX_CHECK_COMPILER_FEATURES

GLIBCXX_CHECK_STDLIB_SUPPORT

;;

 

'''Note''': You need to run <tt>autoconf</tt> in the <tt>libstdc++-v3</tt> directory.

=== libgcc/config.host ===

 

 

i[34567]86-*-myos*)

tmake_file="$tmake_file i386/t-crtstuff t-crtstuff-pic t-libgcc-pic"

;;

x86_64-*-myos*)

tmake_file="$tmake_file i386/t-crtstuff t-crtstuff-pic t-libgcc-pic"

;;

 

=== fixincludes/mkfixinc.sh ===

You should disable fixincludes for your operating system. Find the case statement and add a pattern for your operating system. For instance:

 

# Check for special fix rules for particular targets

case $machine in

*-myos* | \

i?86-*-cygwin* | \

# (... snip ...)

(echo "#! /bin/sh" ; echo "exit 0" ) > ${target}

;;

 

 

This is rather inconvenient as your libc will likely happen to trigger these patterns (and false positives often happens). Any time you change your system headers, you have to rebuild your compiler so the fixed versions get updated. The first time you encounter this, it will show up as a system header that does nothing different even though you edit it.

=== Changing the Default Include Directory ===

 

 

=== Changing the Default Library Directory ===

 

 

=== Start Files Directory ===

 

#undef STANDARD_STARTFILE_PREFIX

 

== Selecting a C Library ==

* [[GCC]]

 

[[Category:Tutorials]]