experimental::filesystem linker error

Question

I try to use the new c++1z features actually on the head of development within gcc 6.0.

If I try this little example:

#include <iostream>
#include <experimental/filesystem>
namespace fs = std::experimental::filesystem;
int main()
{
    fs::path p1 = "/home/pete/checkit";

    std::cout << "p1 = " << p1 << std::endl;
}

I got:

/opt/linux-gnu_6-20151011/bin/g++ --std=c++1z main.cpp -O2 -g -o go
/tmp/ccaGzqFO.o: In function \`std::experimental::filesystem::v1::__cxx11::path::path(char const (&) [36])':
/opt/linux-gnu_6-20151011/include/c++/6.0.0/experimental/bits/fs_path.h:167: undefined reference to `std::experimental::filesystem::v1::__cxx11::path::_M_split_cmpts()'
collect2: error: ld returned 1 exit status

gcc version is the snapshot linux-gnu_6-20151011

Any hints how to link for the new c++1z features?

Puccini

Answer 1

The Filesystem TS is nothing to do with C++1z support, it is a completely separate specification not part of the C++1z working draft. GCC's implementation (in GCC 5.3 and later) is even available in C++11 mode.

You just need to link with -lstdc++fs to use it.

(The relevant library, libstdc++fs.a, is a static library, so as with any static library it should come after any objects that depend on it in the linker command.)

Update Nov 2017: as well as the Filesystem TS, GCC 8.x also has an implementation of the C++17 Filesystem library, defined in <filesystem> and in namespace std::filesystem (N.B. no "experimental" in those names) when using -std=gnu++17 or -std=c++17. GCC's C++17 support is not complete or stable yet, and until it's considered ready for prime time use you also need to link to -lstdc++fs for the C++17 Filesystem features.

Update Jan 2019: starting with GCC 9, the C++17 std::filesystem components can be used without -lstdc++fs (but you still need that library for std::experimental::filesystem).

Update Apr 2024: starting with GCC 13.3 the std::experimental::filesystem symbols are also available in -lstdc++exp (along with the other experimental definitions, such as a contract violation handler, and std::stacktrace symbols).

Pester

Is this documented anywhere, I tried to determine this myself and came up w/ nothing, did I miss some resource here? – Feudalize

@ShafikYaghmour, I've just updated the docs: gcc.gnu.org/onlinedocs/libstdc++/manual/… gcc.gnu.org/onlinedocs/libstdc++/manual/… gcc.gnu.org/onlinedocs/libstdc++/manual/… (at the bottom) – Pester

When I try to use this I get the same linker error. c++ -lstd++fs main.cpp. I am using gcc version 5.3.1 20151207 (Red Hat 5.3.1-2) (GCC) – Herra

ok, -lstdc++fs has to be at the end of the line (after the source file at least). I don't understand why some -lxxx need to be at the end and others don't. – Herra

@Herra because that's how linkers work. References are resolved left to right, so you need to list static libraries after the objects that use them. – Pester

No problem, the idea was to improve the answer. These comments will be deleted soon. – Herra

As reported here, std::filesystem is part of C++17. – Newbold

@Newbold but this question is about std::experimental::filesystem (the TS) which is not the same thing. I repeat, The Filesystem TS is nothing to do with C++1z support, it is a completely separate specification not part of the C++1z working draft. – Pester

Are you saying that, in trunk, -stdc++fs implements both Filesystem TS, and C++17 filesystem? – Antipathy

How about GCC 8.2.1? I just tried, and it seems like GCC 8 still requires -lstdc++fs but it is kind of odd because GCC 8 support full set of C++17. No? – Loricate

@HCSF, No, GCC 8 does not support full C++17, and what part of "starting with GCC 9, the C++17 std::filesystem components can be used without -lstdc++fs" is not clear? – Pester

My distro doesn't have GCC 9 yet. And I thought GCC 8 supports full C++17, and so I felt strange that -lstdc++fs was still required for a compiler that supports full C++17. That's why I asked. Thanks for clarifying. – Loricate

how do I add the -lstdc++fs to my CMakeLists.txt? – Operculum

Note that with g++ version 7.5.x and with -std=c++17, it's marked as experimental. So you need the -lstdc++fs trick and the experimental namespace. (that's the default compile on Ubuntu 18.04) – Didymous

@AlexisWilke I've clarified the answer to say that the C++17 std::filesystem code is only available from GCC 8, so yes, you need to use std::experimental::filesystem with GCC 7 (whether or not you compile with -std=c++17). – Pester

Answer 2

If you are using cmake, add the following line to CMakeLists.txt:

link_libraries(stdc++fs)

So that cmake can link against the corresponding library.

Dialectologist

I did target_link_libraries(hello_world_ stdc++fs) and it compiled. – Yenyenisei

target_link_libraries did not work for me in g++-8. This worked – Adamok

Answer 3

With clang 4.0+, you need to link against libc++experimental.a

Make sure you're building with libc++ (not libstdc++) with the -stdlib=libc++ (as mentioned in the comments)

Rover

I also needed -stdlib=libc++ because my clang version was unexpectedly using libstdc++. – Chiliad

@BowieOwens thanks, updated answer to make that clear. – Rover

When you say "make sure you're building with libc++", how do I do this? (Solution preferably with CMake.) . Thanks. – Argolis

@Argolis -stdlib=libc++ or set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -stdlib=libc++") – Rover

Answer 4

Here is a demo that might be helpful to someone in the future:

env: el6, gcc/5.5.0

#include <iostream>
#include <string>
#include <experimental/filesystem>

int main()
{
    std::string path = std::experimental::filesystem::current_path();

    std::cout << "path = " << path << std::endl;
}

The following are compiling and testing. The flags are -std=c++17 -lstdc++fs:

$ g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/apps/gcc-5.5.0/bin/../libexec/gcc/x86_64-unknown-linux-gnu/5.5.0/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: ../configure --prefix=/apps/gcc-5.5.0 --disable-multilib --enable-shared --enable-threads=posix --enable-__cxa_atexit --enable-clocale=gnu --enable-languages=all
Thread model: posix
gcc version 5.5.0 (GCC)

$ ls -lrt /apps/gcc-5.5.0/lib64 | grep libstdc
-rwxr-xr-x. 1 root root  11272436 Jun 25 10:51 libstdc++.so.6.0.21
-rw-r--r--. 1 root root      2419 Jun 25 10:51 libstdc++.so.6.0.21-gdb.py
-rwxr-xr-x. 1 root root       976 Jun 25 10:51 libstdc++.la
-rwxr-xr-x. 1 root root  11272436 Jun 25 10:51 libstdc++.so
-rw-r--r--. 1 root root  10581732 Jun 25 10:51 libstdc++fs.a
-rw-r--r--. 1 root root  28985412 Jun 25 10:51 libstdc++.a
-rwxr-xr-x. 1 root root       916 Jun 25 10:51 libstdc++fs.la
-rwxr-xr-x. 1 root root  11272436 Jun 25 10:51 libstdc++.so.6

$ g++ filesystem-testing.cpp -lstdc++fs -std=c++17
$ ./a.out

$ g++ -std=c++17 filesystem-testing.cpp -lstdc++fs
$ ./a.out
path = /home/userid/projects-c++/filesystem-testing

It also works with flags: -std=c++11

$ g++ -std=c++11 filesystem-testing.cpp -lstdc++fs
$ ./a.out
path = /home/userid/projects-c++/filesystem-testing

The follows had compiling error _ZNSt12experimental10filesystem2v112current_pathB5cxx11Ev

$ g++ -std=c++17 -lstdc++fs filesystem-testing.cpp
/tmp/ccA6Q9oF.o: In function `main':
filesystem-testing.cpp:(.text+0x11): undefined reference to `_ZNSt12experimental10filesystem2v112current_pathB5cxx11Ev'
collect2: error: ld returned 1 exit status

Extra Notes:

The following link might be helpful

How to install gcc8 using devtoolset-8-gcc

Pythia

Answer 5

For

dyld: lazy symbol binding failed: Symbol not found: 
__ZNSt3__14__fs10filesystem4path17replace_extensionERKS2_

and

Undefined symbols for architecture x86_64:   
"std::__1::__fs::filesystem::__current_path(std::__1::error_code*)", 
referenced from:      
sys::path::get_cwd() in Path.cc.o
ld: symbol(s) not found for architecture x86_64

.. try the following:

For LLVM clang >= 10, link with libc++.1.0.dylib supplied by LLVM.

add_link_options("-Wl,-rpath,location_of_llvm_install/lib;location_of_llvm_install/lib/libc++.1.0.dylib")

This is not for Apple Clang, but for LLVM clang installed from official https://releases.llvm.org or by a package manager.

Xcode < 11 doesn't have filesystem header. macOS < 10.15 doesn't have std::filesystem::path symbols in the system dylib at /usr/lib/libc++.1.0.dylib

Trier

Answer 6

You can easily try my code online.

//  currentPath.cpp
// https://stackoverflow.com/questions/33149878#65128193
#include <experimental/filesystem>
#include <iostream>
using namespace std;

int main() {
  cout << "path = " << experimental::filesystem::current_path() << endl;
}

Compile and run:

clang++ currentPath.cpp -lstdc++fs && ./a.out # Linux
clang++ currentPath.cpp -lstdc++fs && ./a.exe # MSYS2, Windows

Note: -lstdc++fs is a linker flag, not a compiler flag. (Important when you write a makefile.)

Expected output:

path = "/path/to/the/current/directory"

Livi

If you try to compile and run my code, I believe you will see a good reason to upvote my answer. If you do compile and run, and still don't want to upvote, please consider letting my know why in a comment below. - Admittedly, my answer does have similarities with the answer by @caot, but there are also some significant differences. This is why I prefer to publish an answer of my own rather than suggesting edits to caot's answer. – Jodeejodhpur