How to invoke CROSSTOOL tools from Bazel macros / rules? - bazel

Question

I'm building ARM Cortex-M firmware from Bazel with a custom CROSSTOOL. I'm successfully building elf files and manually objcopying them to binary files with the usual:
path/to/my/objcopy -o binary hello.elf hello.bin
I want to make a Bazel macro or rule called cc_firmware that:
Adds the -Wl,-Map=hello.map flags to generate a mapfile
Changes the output elf name from hello to hello.elf
Invokes path/to/my/objcopy to convert the elf to a bin.
I don't know how to get the name of a CROSSTOOL tool (objcopy) to invoke it, and it feels wrong to have the rule know the path to the tool executable.
Is there a way to use the objcopy that I've already told Bazel about in my CROSSTOOL file?

Answer 1

You can actually access this from a custom rule. Basically you need to tell Bazel that you want access to the cpp configuration information (fragments = ["cpp"]) and then access its path via ctx.fragments.cpp.objcopy_executable, e.g.,:
def _impl(ctx):
print("path: {}".format(ctx.fragments.cpp.objcopy_executable))
# TODO: actually do something with the path...
cc_firmware = rule(
implementation = _impl,
fragments = ["cpp"],
attrs = {
"src" : attr.label(allow_single_file = True),
"map" : attr.label(allow_single_file = True),
},
outputs = {"elf" : "%{name}.elf"}
)
Then create the output you want with something like (untested):
def _impl(ctx):
src = ctx.attr.src.files.to_list()[0]
m = ctx.attr.map.files.to_list()[0]
ctx.action(
command = "{objcopy} -Wl,-Map={map} -o binary {elf_out} {cc_bin}".format(
objcopy=ctx.fragments.cpp.objcopy_executable,
map=m,
elf_out=ctx.outputs.elf.path,
cc_bin=src,
),
outputs = [ctx.outputs.elf],
inputs = [src, m],
)