Porting Newlib: Difference between revisions

 

Porting newlib is one of the easiest ways to get a simple C library into your operating system without an excessive amount of effort. As an added bonus, once complete you can port the toolchain (GCC/binutils) to your OS - and who wouldn't want to do that?

 

== Introduction ==

== Preparation ==

 

 

 

== System Calls ==

 

 

 

== Porting Newlib ==

Same as for binutils in [[OS Specific Toolchain]].

 

 

Tell newlib which system-specific directory to use for our particular target. In the section starting 'Get the source directories to use for the host ... case "${host}" in', add a section:

 

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

sys_dir=myos

;;

 

=== newlib/libc/sys/configure.in ===

Tell the newlib build system that it also needs to configure our myos-specific host directory. In the <tt>case ${sys_dir} in</tt> list, simply add

 

myos) AC_CONFIG_SUBDIRS(myos) ;;

 

'''Note:''' After this, you need to run <tt>autoconf (precisely version 2.64)</tt> in the libc/sys directory.

This file creates crt0.o, which is included in every application. It should define the symbol _start, and then call the main() function, possibly after setting up process-space segment selectors and pushing argc and argv onto the stack. A simple implementation is:

 

#include <fcntl.h>

 

exit(ex);

}

 

'''Note:''' add in argc and argv support based on how you handle them in your OS

This file should contain the implementations for each glue function newlib requires.

 

/* note these headers are all provided by newlib - you don't need to provide them */

#include <sys/stat.h>

int write(int file, char *ptr, int len);

int gettimeofday(struct timeval *p, struct timezone *z);

 

'''Note''': You may split this up into multiple files, just don't forget to link against all of them in Makefile.am.

Configure script for our system directory.

 

AC_PREREQ(2.59)

AC_INIT([newlib], [NEWLIB_VERSION])

AC_CONFIG_FILES([Makefile])

AC_OUTPUT

 

=== newlib/libc/sys/myos/Makefile.am ===

A Makefile template for this directory:

 

AUTOMAKE_OPTIONS = cygnus

INCLUDES = $(NEWLIB_CFLAGS) $(CROSS_CFLAGS) $(TARGET_CFLAGS)

ACLOCAL_AMFLAGS = -I ../../..

CONFIG_STATUS_DEPENDENCIES = $(newlib_basedir)/configure.host

 

'''Note''': After this, you need to run <tt>autoconf</tt> in the newlib/libc/sys/ directory, and <tt>autoreconf</tt> in the newlib/libc/sys/myos directory.

 

=== Signal handling ===

Alternatively, you can provide your own implementation. To do this, you need to define your own version of signal() in syscalls.c. A typical implementation would register the handler somewhere in kernel space, so that issuing a signal from another process causes the corresponding function to be called in the receiving process (this will also require some nifty stack-playing in the receiving process, as you are basically interrupting the program flow in the middle). You then need to provide a kill() function in syscalls.c which actually sends signals to another process. Newlib will still define a raise() function for you, but it is just a stub which calls kill() with the current process id. To switch newlib to this mode, you need to #define the SIGNAL_PROVIDED macro when compiling. A simple way to do this is to add the line:

 

newlib_cflags="${newlib_cflags} -DSIGNAL_PROVIDED"

 

to your host's entry in <tt>configure.host</tt>. It would probably also make sense to provide sigaction(), and provide signal() as a wrapper for it. Note that [http://pubs.opengroup.org/onlinepubs/9699919799/functions/sigaction.html the Open Group's] definition of sigaction states that 1) sigaction supersedes signal, and 2) an application designed shouldn't use both to manipulate the same signal.

'''Note:''' there must be a better way then this.

 

# newlib setup

CURRDIR=$(pwd)

 

cd ~/cross/bin/ # this is where the bootstrapped generic cross compiler toolchain (i686-elf-xxx) is installed in,

# change this based on your development environment.

# return

cd $CURRDIR

 

Then run the following commands to build newlib

 

mkdir build-newlib

cd build-newlib

make all

make DESTDIR=${SYSROOT} install

 

'''Note:''' SYSROOT is where all your OS-specific toolchains will be installed in. It will look like a miniature version of the Linux filesystem, but have your OS-specific toolchains in; I am using ~/myos as my SYSROOT directory.

 

For some reason, the newer versions of newlib (at least for me) didn't put the libraries in a location where other utilities like binutils could find.

So here's another hack to fix this:

 

 

After building all of this, your freshly built libc will be installed in your SYSROOT directory! Now you can progress to building your own [[OS Specific Toolchain]].

'''Important Note:''' I found that for newlib to properly work, you have to link against libc, libg, libm, and libnosys - hence when porting gcc, in

 

#define LIB_SPEC ...

 

 

make sure you put

 

#define LIB_SPEC "-lc -lg -lm -lnosys"

 

at the bare minimum.

 

'''Note:''' I used a lot of hacks in this article, if you find a better way to do something, please contribute to the page. Thank you.

 

== See Also ==

* [[OS Specific Toolchain]]

 

[[Category:Tutorials]]