Fortran rules?

[Austin Schuh]

I'm looking to build Slycot from scratch.  To do that, it looks like I need to call f2py to generate a .c file.  That will then be used in a shared library that python imports along with the original fortran files.

Has anyone built fortran code with bazel before and linked it into a c/c++ shared library?

Thanks!

  Austin

[Han-Wen Nienhuys]

Not directly, but I've seen the Google fortran rules, and IIRC, they
just run $(CC) on the .f files in a genrule.

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Han-Wen Nienhuys
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[Austin Schuh]

Success!  Thanks!

(I cut a couple corners that I'd like to clean up, but it successfully builds libraries that I can build into a shared library).

I'm working on building https://github.com/avventi/Slycot so I can use it in a python script in a genrule to do codegen.  I'm doing controller pole placement and kalman filter design with it.  The last remaining issue is that Slycot expects the shared library to be in @slycot_repo//slycot:_wrapper.so, and I can only generate a BUILD file which puts it in @slycot_repo//:_wrapper.so since I'm using new_git_repository.

I've tried a couple ways to get around that.  I've tried generating a symlink in @slycot_repo//slycot/ pointing to ../_wrapper.so.  When I do that, I get a lovely 'libexternal_Sslycot_Urepo_Slibslycot_Uc.so: cannot open shared object file: No such file or directory' error, and using strace, I see that it is searching ../../_solib_k8/, which is missing 1 ../.  To me, that means that the library needs to have that exact path, so it can't be moved (or linked to).

cc_binary(

  name = '_wrapper.so',

  deps = [

    ':fortran_files',

    ':slycot_c',

  ],

  linkopts = ['-shared', '-lblas', '-llapack'],

  linkstatic=0,

)

If I try linkstatic=1, it tries to pull the .o files to link into the .so from the .a generated by the fortran_library, which doesn't have .o's.  Even if it had .o's, would be compiled without -fPIC, which doesn't work.  Ideally, if this worked, the _wrapper.so library could be moved since it would be pretty self-sufficient.

It looks like my best option might be to fork the repository and add the BUILD files in that way.  That means that I need to keep rebasing if upstream adds fixes, and maintain a separate repository.  I don't really like that solution.

Any suggestions?

Austin

For reference, Fortran library rules.  I'm not sure my approach is general enough to submit upstream.  If there is interest and I can get a bit of guidance on how to improve them, I'd be happy to submit them.

def _single_fortran_object_impl(ctx):

  toolchain_cflags = (ctx.fragments.cpp.compiler_options([]) +

      ctx.fragments.cpp.c_options +

      ctx.fragments.cpp.unfiltered_compiler_options([]) + ['-fPIC'])

  cmd = [ctx.fragments.cpp.compiler_executable] + toolchain_cflags + ['-c', ctx.file.src.path, '-o', ctx.outputs.pic_o.path]

  filtered_cmd = []

  exclude_flags = ['-fcolor-diagnostics',

                   '-Wswitch-enum',

                   '-Wpointer-arith',

                   '-Wcast-qual',

                   '-Wwrite-strings',

                   '-Wsign-compare',

                   '-Wformat=2',

                   '-Werror',

                   '-Wno-builtin-macro-redefined']

  for flag in cmd:

    if flag not in exclude_flags:

      filtered_cmd.append(flag)

  ctx.action(

    inputs = [ctx.file.src],

    outputs = [ctx.outputs.pic_o],

    mnemonic = "Fortran",

    command = ' '.join(filtered_cmd),

    progress_message = 'Building %s' % ctx.outputs.pic_o.short_path,

  )

def _define_fortran_output(attrs):

  if not attrs.src.name.endswith('.f'):

    fail('Fortran files must end in \'.f\'', 'src')

  fortran_file_base = attrs.src.name[:-2]

  return {

    'pic_o': fortran_file_base + '.pic.o',

  }

_single_fortran_object = rule(

  implementation = _single_fortran_object_impl,

  attrs = {

    'src': attr.label(single_file=True, allow_files=FileType(['.f'])),

    'cc_libs': attr.label_list(providers=['cc']),

  },

  outputs = _define_fortran_output,

  fragments = [

    'cpp',

  ],

)

def fortran_library(name, srcs, deps = [], visibility = None):

  pic_o_files = []

  for src in srcs:

    pic_o_file = src[:-2] + '.pic.o'

    _single_fortran_object(name=name + '_' + pic_o_file,

                           src=src,

                           visibility=['//visibility:private'])

    pic_o_files.append(pic_o_file)

  native.cc_library(

    name = name,

    deps = deps,

    srcs = pic_o_files,

    linkopts = [

      '-lgfortran', 

    ],

    visibility = visibility,

  )