When I compile a very simple source file with gcc I don't have to specify the path to standard include files such as stdio or stdlib.
How does GCC know how to find these files?
Does it have the /usr/include path hardwired inside, or it will get the paths from other OS components?
In order to figure out the default paths used by gcc/g++, as well as their priorities, you need to examine the output of the following commands:
echo | gcc -xc -E -v -
echo | gcc -xc++ -E -v -
The credit goes to Qt Creator team.
Here's a breakdown of the flags:
-x selects the language, C or C++ respectively
-E makes gcc to run the preprocessor only, so no compilation takes place
-v prints all the commands run, which is the key to dumping the standard paths
- is the "input file" to preprocess, as a convention - stands for stdin (or stdout, depending on the context);
echo | feeds an empty string to gcc so effectively we preprocess an empty file generated on the fly
Here's a nice explaining it in more detail: https://explainshell.com/explain?cmd=echo+%7C+gcc+-xc+-E+-v+-
cpp -v /dev/null -o /dev/null
- refers to stdin here. So you could also do: echo | gcc -xc++ -E -v - and echo | gcc -xc++ -E -v -
include ... search starts here piece which is the one we are looking for. Running on Ubuntu 22.04, GCC 11.3.0There is a command with a shorter output, which allows to automatically cut the include pathes from lines, starting with a single space:
$ echo | gcc -Wp,-v -x c++ - -fsyntax-only
ignoring nonexistent directory "/usr/lib/gcc/x86_64-redhat-linux/4.8.2/include-fixed"
ignoring nonexistent directory "/usr/lib/gcc/x86_64-redhat-linux/4.8.2/../../../../x86_64-redhat-linux/include"
#include "..." search starts here:
#include <...> search starts here:
/usr/lib/gcc/x86_64-redhat-linux/4.8.2/../../../../include/c++/4.8.2
/usr/lib/gcc/x86_64-redhat-linux/4.8.2/../../../../include/c++/4.8.2/x86_64-redhat-linux
/usr/lib/gcc/x86_64-redhat-linux/4.8.2/../../../../include/c++/4.8.2/backward
/usr/lib/gcc/x86_64-redhat-linux/4.8.2/include
/usr/local/include
/usr/include
End of search list.
The credit goes to the libc++ front-page.
/dev/null instead of -, and you'll need no pipe
-E, no need to run the compiler.To summarise the other answers:
For C++:
c++ -xc++ /dev/null -E -Wp,-v 2>&1 | sed -n 's,^ ,,p'
For C:
cc -xc /dev/null -E -Wp,-v 2>&1 | sed -n 's,^ ,,p'
Though I agree with Ihor Kaharlichenko’s answer for considering C++ and with abyss.7’s answer for the compactness of its output, they are still incomplete for the multi-arch versions of gcc because input processing depends on the command line parameters and macros.
Example:
echo | /opt/gcc-arm-none-eabi-9-2019-q4-major/bin/arm-none-eabi-g++ -specs=nano.specs -mcpu=cortex-m4 -march=armv7e-m -mthumb -mfloat-abi=soft -x c++ -E -Wp,-v\
- -fsyntax-only yields
⋮
/opt/gcc-arm-none-eabi-9-2019-q4-major/bin/../arm-none-eabi/include/newlib-nano
/opt/gcc-arm-none-eabi-9-2019-q4-major/bin/../lib/gcc/arm-none-eabi/9.2.1/../../../../arm-none-eabi/include/c++/9.2.1
/opt/gcc-arm-none-eabi-9-2019-q4-major/bin/../lib/gcc/arm-none-eabi/9.2.1/../../../../arm-none-eabi/include/c++/9.2.1/arm-none-eabi/thumb/v7e-m/nofp
/opt/gcc-arm-none-eabi-9-2019-q4-major/bin/../lib/gcc/arm-none-eabi/9.2.1/../../../../arm-none-eabi/include/c++/9.2.1/backward
/opt/gcc-arm-none-eabi-9-2019-q4-major/bin/../lib/gcc/arm-none-eabi/9.2.1/include
/opt/gcc-arm-none-eabi-9-2019-q4-major/bin/../lib/gcc/arm-none-eabi/9.2.1/include-fixed
/opt/gcc-arm-none-eabi-9-2019-q4-major/bin/../lib/gcc/arm-none-eabi/9.2.1/../../../../arm-none-eabi/include
⋮
whereas echo | /opt/gcc-arm-none-eabi-9-2019-q4-major/bin/arm-none-eabi-g++ -x c++ -E -Wp,-v - -fsyntax-only yields
⋮
/opt/gcc-arm-none-eabi-9-2019-q4-major/bin/../lib/gcc/arm-none-eabi/9.2.1/../../../../arm-none-eabi/include/c++/9.2.1
/opt/gcc-arm-none-eabi-9-2019-q4-major/bin/../lib/gcc/arm-none-eabi/9.2.1/../../../../arm-none-eabi/include/c++/9.2.1/arm-none-eabi
/opt/gcc-arm-none-eabi-9-2019-q4-major/bin/../lib/gcc/arm-none-eabi/9.2.1/../../../../arm-none-eabi/include/c++/9.2.1/backward
/opt/gcc-arm-none-eabi-9-2019-q4-major/bin/../lib/gcc/arm-none-eabi/9.2.1/include
/opt/gcc-arm-none-eabi-9-2019-q4-major/bin/../lib/gcc/arm-none-eabi/9.2.1/include-fixed
/opt/gcc-arm-none-eabi-9-2019-q4-major/bin/../lib/gcc/arm-none-eabi/9.2.1/../../../../arm-none-eabi/include
⋮
The former invocation utilizes newlib (see lines 1 and 3 of the output), the latter goes with the standard includes. The common files at the end of the list are an example for the usage of include_next.
Bottom line: Always consider all macros and compiler options when printing the include directories.
Just run the following to list the default search paths:
$(gcc -print-prog-name=cc1) -v
cc1 usually can't be called directly because it is an internal command which is not on the PATH. Maybe you want to edit your answer to describe where to find the file.
echo | gcc -Wp,-v -x c++ - -fsyntax-only