Discussion on: June 16th, 2022: What did you learn this week?

[Ben Lovy]

Cute trick to inspect your C compiler configuration:

$ gcc -x c -v -E /dev/null
Using built-in specs.
COLLECT_GCC=/usr/bin/gcc
OFFLOAD_TARGET_NAMES=nvptx-none
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-redhat-linux
Configured with: ../configure --enable-bootstrap --enable-languages=c,c++,fortran,objc,obj-c++,ada,go,d,lto --prefix=/usr --mandir=/usr/share/man --infodir=/usr/share/info --with-bugurl=http://bugzilla.redhat.com/bugzilla --enable-shared --enable-threads=posix --enable-checking=release --enable-multilib --with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions --enable-gnu-unique-object --enable-linker-build-id --with-gcc-major-version-only --enable-libstdcxx-backtrace --with-linker-hash-style=gnu --enable-plugin --enable-initfini-array --with-isl=/builddir/build/BUILD/gcc-12.1.1-20220507/obj-x86_64-redhat-linux/isl-install --enable-offload-targets=nvptx-none --without-cuda-driver --enable-offload-defaulted --enable-gnu-indirect-function --enable-cet --with-tune=generic --with-arch_32=i686 --build=x86_64-redhat-linux --with-build-config=bootstrap-lto --enable-link-serialization=1
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 12.1.1 20220507 (Red Hat 12.1.1-1) (GCC) 
COLLECT_GCC_OPTIONS='-v' '-E' '-mtune=generic' '-march=x86-64'
 /usr/libexec/gcc/x86_64-redhat-linux/12/cc1 -E -quiet -v /dev/null -mtune=generic -march=x86-64 -dumpbase null
ignoring nonexistent directory "/usr/lib/gcc/x86_64-redhat-linux/12/include-fixed"
ignoring nonexistent directory "/usr/lib/gcc/x86_64-redhat-linux/12/../../../../x86_64-redhat-linux/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/lib/gcc/x86_64-redhat-linux/12/include
 /usr/local/include
 /usr/include
End of search list.
# 0 "/dev/null"
# 0 "<built-in>"
# 0 "<command-line>"
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 0 "<command-line>" 2
# 1 "/dev/null"
COMPILER_PATH=/usr/libexec/gcc/x86_64-redhat-linux/12/:/usr/libexec/gcc/x86_64-redhat-linux/12/:/usr/libexec/gcc/x86_64-redhat-linux/:/usr/lib/gcc/x86_64-redhat-linux/12/:/usr/lib/gcc/x86_64-redhat-linux/
LIBRARY_PATH=/usr/lib/gcc/x86_64-redhat-linux/12/:/usr/lib/gcc/x86_64-redhat-linux/12/../../../../lib64/:/lib/../lib64/:/usr/lib/../lib64/:/usr/lib/gcc/x86_64-redhat-linux/12/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-E' '-mtune=generic' '-march=x86-64'

You can get just the configuration with a plain gcc -v, but by invoking the preprocessor with -E and pointing it at some nonsense /dev/null, you additionally get to inspect the paths it will use for looking up libraries. Because you passed in something nonsensical, it can't determine what sort of compilation you're doing, so you have to explicitly specify with -x c. This forces it to attempt to preprocess the input as a C program which provides the most complete set of information. Works with clang too.

Handy!

[Nick Taylor]

Awesome!