Re: porting newlib

[Jeff Johnston <jjohnstn at redhat dot com>]

Matt Broadstone wrote:
> When the build process builds for say "arc" (for example), the defined
> syscall_dir is the directory "syscalls" in "libc" which are all stubs,
> correct? However there is another file called "syscalls.c" inside the arc
> sys_dir. I have tried to look into what was done for linux, but linux has
> nothing to do with syscalls.... I am very lost here obviously, but there
> seems to be no documentation regarding this stuff? Please excuse my
> ignorance :)

Matt,

  A basic port needs to alter a number of files and add some directories.

  1. Add a subdirectory to the newlib/libc/machine directory for your platform.
     In this directory you need to have a setjmp/longjmp implementation.  This
     is required because setjmp/longjmp usually is assembler.  Look at the
     libc/machine/fr30 directory and copy/modify the files in there.

  2. Edit newlib/libc/include/machine/ieeefp.h
     This defines the ieee endianness for your platform.  The compiler should
     be defining something that identifies your machine.  In some cases, the
     endianness may be a compiler-option so you may have to check another
     define in addition to your platform identifier.  See examples in the
     file.

  3. Edit newlib/libc/include/machine/setjmp.h
     You need to specify the setjmp buffer characteristics to match up with
     your setjmp/longjmp implementation.  This is just the size of the
     setjmp buffer.  See file for examples.

  4. Edit newlib/libc/include/sys/config.h
     This has various defines as needed.  Mostly, it defines some max
     values.  There are defaults that may apply to your platform in which case
     you needn't do anything.

  5. Edit configure.host
     You need to add your configuration so newlib can recognize it.  You should
     specify your new machine directory for your platform via the machine_dir
     variable.  If needed, you can add special newlib compile flags.  The
     sys_dir is for OS stuff so you won't need to alter that.  Older platforms
     used the sys_dir to implement syscalls but this is not correct and is
     a historical nuisance.  The syscall_dir is a choice, but I recommend as a
     default to specify syscall_dir=syscalls. Read the comments in
     newlib/libc/include/reent.h for an explanation of choices.

  6. Add a machine subdirectory to libgloss
     You need to add a bsp for your platform.  This is the minimum set of
     syscalls needed by newlib and any linker scripts needed.  This varies
     from board to board (it can also be a simulator).  See the
     mn10300 or fr30 for examples. You will need
     to edit configure.in and regenerate configure so it will build your
     new files.  By default you get libnosys which gives you a set of
     default syscall stubs.  The majority of the stubs just return failure.
     You still need to supply an __exit routine.  This can be as simple as
     generating an exception to stop the program.

  7. Possibly override header files
     If you need to override any default machine header files, you can
     add a machine directory to newlib/libc/machine/<YOUR_MACHINE_DIR>
     Header files in that subdirectory will overwrite the defaults found
     in newlib/libc/include/machine.  You will likely not need to do this.

  This assumes you have already handled adding your new configuration to the 
top directory files.

  Now linux is a different animal.  It is an OS that has an extensive set of 
syscalls.  If you look in the newlib/libc/sys/linux directory, you will find a 
number of syscalls there (e.g. see io.c).  There is a set of basic syscall 
macros that are defined for the particular platform.  For the x86, you will find 
these macros defined in newlib/libc/sys/linux/machine/i386/syscall.h file.  At 
the moment, linux support is only for x86.  To add another platform, the 
syscall.h file would have to be supplied for the new platform plus some other 
platform-specific files would need to be ported as well.

  I hope this helps.

-- Jeff J.