Failure compiling glog with gflags support using Bazel - bazel

Question

I'm getting a failure when I try to compile glog with gflags support using Bazel. A github repo reproducing this problem and showing the compilation error message is here: https://github.com/dionescu/bazeltrunk.git
I suspect that the problem occurs because glog is finding and using the "config.h" file published by gflags. However, I do not understand why this happens and why the current structure of the build files results in such errors. One solution I found was to provide my own BUILD file for gflags where the config was in a separate dependency (just how glog does it in my example).
I would appreciate any help with understanding the issue in this example.

Answer 1

The problem is that gflag's BUILD file is including its own config. Adding -H to glog.BUILD's copts yields:
. external/glog_archive/src/utilities.h
.. external/glog_archive/src/base/mutex.h
... bazel-out/local-fastbuild/genfiles/external/com_github_gflags_gflags/config.h
In file included from external/glog_archive/src/utilities.h:73:0,
from external/glog_archive/src/utilities.cc:32:
external/glog_archive/src/base/mutex.h:147:3: error: #error Need to implement mutex.h for your architecture, or #define NO_THREADS
# error Need to implement mutex.h for your architecture, or #define NO_THREADS
^
If you take a look at gflag's config.h, it went with a not-very-helful approach of commenting out most of the config:
// ---------------------------------------------------------------------------
// System checks
// Define if you build this library for a MS Windows OS.
//cmakedefine OS_WINDOWS
// Define if you have the <stdint.h> header file.
//cmakedefine HAVE_STDINT_H
// Define if you have the <sys/types.h> header file.
//cmakedefine HAVE_SYS_TYPES_H
...
So nothing is defined.
Options:
The easiest way is probably to generate the config.h in your glog.BUILD:
genrule(
name = "config",
outs = ["config.h"],
cmd = "cd external/glog_archive; ./configure; cd ../..; cp external/glog_archive/src/config.h $#",
srcs = glob(["**"]),
)
# Then add the generated config to your glog target.
cc_library(
name = "glog",
srcs = [...],
hdrs = [
":config.h",
...
This puts the .h file at a higher-precedence location than the gflags version.
Alternatively, you could do something like this in the genrule, if you want to use your //third_party/glog/config.h (#// is shorthand for your project's repository):
genrule(
name = "config",
outs = ["config.h"],
cmd = "cp $(location #//third_party/glog:config.h) $#",
srcs = ["#//third_party/glog:config.h"],
)
You'll have to add exports_files(['config.h']) to the third_party/glog/BUILD file, too.