Why is fmt library not found after installing fmt and gcc compiler with brew?

Question

I am using MACOS Mojave version 10.14.3 and need to use GNU compiler and not clang.

I installed gcc compiler using brew install gcc. Then I installed fmt library using brew install fmt.

I put #include <fmt/format.h> at the top of my C++ script

Then I type:

/usr/local/bin/g++-8 -O0 -g3 -Wall -c -fmessage-length=0 -std=c++17 -MMD -MP -MF"src/trail2.d" -MT"src/trail2.o" -o "src/trail2.o" "../src/trail2.cpp"

However I get this error:

fatal error: fmt/format.h: No such file or directory

Same happens when I try using the boost library.

When I use clang, fmt and boost are found without problem.

Just in case I am using Eclipse IDE for C/C++ Developers. Version: 2018-12 (4.10.0).

To be specific, how can I get fmt library to work with the gcc I installed with brew?

What I have in my Mac at /usr/local/Cellar is:

drwxr-xr-x   3 aaa  staff    96 Feb 22 22:07 gcc
drwxr-xr-x   3 aaa  staff    96 Feb 23 01:58 fmt

What I have in my Mac at /usr/local/Cellar is:

    lrwxr-xr-x    1 aaa  admin        29 Feb 22 22:07 c++-8 -> ../Cellar/gcc/8.2.0/bin/c++-8
    lrwxr-xr-x    1 aaa  admin        29 Feb 22 22:07 cpp-8 -> ../Cellar/gcc/8.2.0/bin/cpp-8
    lrwxr-xr-x    1 aaa  admin        29 Feb 22 22:07 g++-8 -> ../Cellar/gcc/8.2.0/bin/g++-8
    lrwxr-xr-x    1 aaa  admin        29 Feb 22 22:07 gcc-8 -> ../Cellar/gcc/8.2.0/bin/gcc-8

Thanks for the help in advanced

Cabasset

This is not an issue with GCC, it's an issue where the library is installed. If it's not installed in a "standard" location, you must tell the g++ frontend program where to find the header and library files. Use the -I (upper-case i) option to specify where the header files are installed, and the -L and -l (lower-case L) options for any linker libraries. – Deas

I'd guess that's /usr/local/include and /usr/local/lib – Lofton

Considering the contents of /usr/local/Cellar my guess is that you need to add -I/usr/local/Cellar. Since I don't know how brew organizes its packages when installed, perhaps something like -I/usr/local/Cellar/fmt/include? You can add the directory in your Eclipse project settings somewhere (Preprocessor settings tab or similar?), but then usually without the actual -I option. – Deas

Hi. I am new user in C++. Could you please help me with how my g++ command line should look like? In /usr/local/lib I have this: lrwxr-xr-x 1 aaa admin 32 Feb 23 01:58 libfmt.a -> ../Cellar/fmt/5.3.0/lib/libfmt.a – Cabasset

Answer 1

The way homebrew works is that it puts everything in

/usr/local/Cellar/PACKAGE-NAME/PACKAGE-VERSION

and then it creates links to those things for binaries in /usr/local/bin, e.g.

/usr/local/bin/grep -> /usr/local/Cellar/grep/4.17/bin/grep

so that you just need to put /usr/local/bin in your PATH and all homebrew programs will be runnable by name, e.g. grep in the above example.

---

It does the same thing for compiling, it puts the actual headers and libraries in:

/usr/local/Cellar/PACKAGE-NAME/PACKAGE-VERSION/include
/usr/local/Cellar/PACKAGE-NAME/PACKAGE-VERSION/lib

and also creates links to those in

/usr/local/include
/usr/local/lib

So, your gcc command will be:

g++-8 -I /usr/local/include -L /usr/local/lib -lfmt <PROGRAM.CPP> -o <PROGRAM>

---

You can see what files belong to your fmt package and where they are with:

brew ls fmt --verbose

---

If you install pkg-config, using:

brew install pkg-config

it will use the file fmt.pc and can tell you the correct switches for compiling if you type:

pkg-config --libs --cflags fmt

Sample Output

-I/usr/local/Cellar/fmt/5.3.0/include -L/usr/local/Cellar/fmt/5.3.0/lib -lfmt

That means you can simplify your gcc command to:

g++-8 $(pkg-config --libs --cflags fmt) <PROGRAM.CPP> -o <PROGRAM>

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