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<p class="MsoNormal">Traditional Unix linkers only pull symbols from archives (static libraries) if an undefined reference to that symbol has already been seen by the time the static library is encountered on the command line. In other words, if libfoo.a defines
foo, and bar.o has an undefined reference to foo, ordering them on the link line as bar.o libfoo.a will work, but libfoo.a bar.o won’t, since the undefined reference to foo appears after the static library defining it. This is the reason why static libraries
are sometimes repeated on the command line or --start-group --end-group exist.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">This is specific to traditional Unix linkers. LLD for ELF chooses is agnostic to the order of libraries; see “Efficient archive file handling” on
<a href="http://lld.llvm.org/NewLLD.html">http://lld.llvm.org/NewLLD.html</a>. ld64 and Microsoft’s linker are also agnostic to the ordering (though Microsoft’s linker has its own peculiarities related to library symbol lookup, which again aren’t emulated by
LLD for COFF; see <a href="https://docs.microsoft.com/en-us/cpp/build/reference/link-input-files?view=vs-2019">
https://docs.microsoft.com/en-us/cpp/build/reference/link-input-files?view=vs-2019</a>, specifically “Libraries are searched in command line order as well, with the following caveat: Symbols that are unresolved when bringing in an object file from a library
are searched for in that library first, and then the following libraries from the command line and
<a href="https://docs.microsoft.com/en-us/cpp/build/reference/defaultlib-specify-default-library?view=vs-2019">
/DEFAULTLIB (Specify Default Library)</a> directives, and then to any libraries at the beginning of the command line.”).<o:p></o:p></p>
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<p class="MsoNormal"><b><span style="font-size:12.0pt;color:black">From: </span></b><span style="font-size:12.0pt;color:black">libcxx-dev <libcxx-dev-bounces@lists.llvm.org> on behalf of Louis Dionne via libcxx-dev <libcxx-dev@lists.llvm.org><br>
<b>Reply-To: </b>Louis Dionne <ldionne@apple.com><br>
<b>Date: </b>Tuesday, April 2, 2019 at 2:37 PM<br>
<b>To: </b>cfe-dev <cfe-dev@lists.llvm.org><br>
<b>Cc: </b>TheAspiringHacker <TheAspiringHacker@protonmail.com>, "libcxx-dev@lists.llvm.org" <libcxx-dev@lists.llvm.org><br>
<b>Subject: </b>Re: [libcxx-dev] Is the C++ filesystem object file installed via LLVM's APT repository missing symbols?<o:p></o:p></span></p>
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<p class="MsoNormal">On Apr 2, 2019, at 17:21, Louis Dionne via libcxx-dev <<a href="mailto:libcxx-dev@lists.llvm.org">libcxx-dev@lists.llvm.org</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">On Apr 1, 2019, at 19:01, Eric Fiselier via libcxx-dev <<a href="mailto:libcxx-dev@lists.llvm.org">libcxx-dev@lists.llvm.org</a>> wrote:<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica"><o:p> </o:p></span></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt"><span style="font-size:9.0pt;font-family:Helvetica"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">On Mon, Apr 1, 2019 at 9:46 AM TheAspiringHacker via libcxx-dev <<a href="mailto:libcxx-dev@lists.llvm.org">libcxx-dev@lists.llvm.org</a>> wrote:<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">Hello,<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">I have downloaded LLVM and Clang 8 from the APT repository (deb<span class="apple-converted-space"> </span><a href="https://urldefense.proofpoint.com/v2/url?u=http-3A__apt.llvm.org_xenial_&d=DwMFAg&c=5VD0RTtNlTh3ycd41b3MUw&r=o3kDXzdBUE3ljQXKeTWOMw&m=6c1v3c4C7T7kyJ5MxPQqPPRPSvxC9FTOOJvF9OKPhgI&s=Fvn2T3HkYgEDOQLMyFlM5ngp1xc1hI_AfCb7Jd6ggpo&e=" target="_blank">http://apt.llvm.org/xenial/</a><span class="apple-converted-space"> </span>llvm-toolchain-xenial-8
main). When I compile and link<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">#include <filesystem><o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">namespace fs = std::filesystem;<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">int main()<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">{<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica"> <span class="apple-converted-space"> </span>fs::path wd = fs::current_path();<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica"> <span class="apple-converted-space"> </span>return 0;<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">}<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">with clang-8 -std=c++17 -stdlib=libc++ -lc++ -lc++abi -lc++fs test.cpp -v<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">I get the output:<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">clang version 8.0.0-svn356034-1~exp1~20190313094121.53 (branches/release_80)<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">Target: x86_64-pc-linux-gnu<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">Thread model: posix<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">InstalledDir: /usr/bin<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/5<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/5.5.0<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/6<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/6.5.0<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/7<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/7.4.0<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/8<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/5<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/5.5.0<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/6<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/6.5.0<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7.4.0<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/8<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">Selected GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/5.5.0<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">Candidate multilib: .;@m64<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">Selected multilib: .;@m64<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">"/usr/lib/llvm-8/bin/clang" -cc1 -triple x86_64-pc-linux-gnu -emit-obj -mrelax-all -disable-free -disable-llvm-verifier -discard-value-names -main-file-name test.cpp -mrelocation-model
static -mthread-model posix -mdisable-fp-elim -fmath-errno -masm-verbose -mconstructor-aliases -munwind-tables -fuse-init-array -target-cpu x86-64 -dwarf-column-info -debugger-tuning=gdb -v -resource-dir /usr/lib/llvm-8/lib/clang/8.0.0 -internal-isystem /usr/lib/llvm-8/bin/../include/c++/v1
-internal-isystem /usr/include/clang/8.0.0/include/ -internal-isystem /usr/local/include -internal-isystem /usr/lib/llvm-8/lib/clang/8.0.0/include -internal-externc-isystem /usr/include/x86_64-linux-gnu -internal-externc-isystem /include -internal-externc-isystem
/usr/include -std=c++17 -fdeprecated-macro -fdebug-compilation-dir /home/<username> -ferror-limit 19 -fmessage-length 0 -fobjc-runtime=gcc -fcxx-exceptions -fexceptions -fdiagnostics-show-option -o /tmp/test-b088e9.o -x c++ test.cpp -faddrsig<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">clang -cc1 version 8.0.0 based upon LLVM 8.0.0 default target x86_64-pc-linux-gnu<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">ignoring nonexistent directory "/include"<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">ignoring duplicate directory "/usr/include/clang/8.0.0/include"<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">#include "..." search starts here:<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">#include <...> search starts here:<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">/usr/lib/llvm-8/bin/../include/c++/v1<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">/usr/include/clang/8.0.0/include<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">/usr/local/include<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">/usr/include/x86_64-linux-gnu<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">/usr/include<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">End of search list.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">"/usr/bin/ld" --hash-style=both --build-id --eh-frame-hdr -m elf_x86_64 -dynamic-linker /lib64/ld-linux-x86-64.so.2 -o a.out /usr/bin/../lib/gcc/x86_64-linux-gnu/5.5.0/../../../x86_64-linux-gnu/crt1.o
/usr/bin/../lib/gcc/x86_64-linux-gnu/5.5.0/../../../x86_64-linux-gnu/crti.o /usr/bin/../lib/gcc/x86_64-linux-gnu/5.5.0/crtbegin.o -L/usr/bin/../lib/gcc/x86_64-linux-gnu/5.5.0 -L/usr/bin/../lib/gcc/x86_64-linux-gnu/5.5.0/../../../x86_64-linux-gnu -L/lib/x86_64-linux-gnu
-L/lib/../lib64 -L/usr/lib/x86_64-linux-gnu -L/usr/bin/../lib/gcc/x86_64-linux-gnu/5.5.0/../../.. -L/usr/lib/llvm-8/bin/../lib -L/lib -L/usr/lib -lc++ -lc++abi -lc++fs /tmp/test-b088e9.o -lgcc --as-needed -lgcc_s --no-as-needed -lc -lgcc --as-needed -lgcc_s
--no-as-needed /usr/bin/../lib/gcc/x86_64-linux-gnu/5.5.0/crtend.o /usr/bin/../lib/gcc/x86_64-linux-gnu/5.5.0/../../../x86_64-linux-gnu/crtn.o<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">/tmp/test-b088e9.o: In function `std::__1::__fs::filesystem::current_path()':<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">test.cpp:(.text._ZNSt3__14__fs10filesystem12current_pathEv[_ZNSt3__14__fs10filesystem12current_pathEv]+0x14): undefined reference to `std::__1::__fs::filesystem::__current_path(std::__1::error_code*)'<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">clang: error: linker command failed with exit code 1 (use -v to see invocation)<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">I ran:<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">nm -C /usr/lib/llvm-8/lib/libc++fs.a | fgrep 'std::__1::__fs::filesystem::__current_path'<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">and got:<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">0000000000002540 T std::__1::__fs::filesystem::__current_path(std::__1::error_code*)<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">0000000000002880 T std::__1::__fs::filesystem::__current_path(std::__1::__fs::filesystem::path const&, std::__1::error_code*)<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">It seems that the `current_path` functions, as defined in the libc++fs.a file that I installed, do not match the overloads as documented at<span class="apple-converted-space"> </span><a href="https://urldefense.proofpoint.com/v2/url?u=https-3A__en.cppreference.com_w_cpp_filesystem_current-5Fpath&d=DwMFAg&c=5VD0RTtNlTh3ycd41b3MUw&r=o3kDXzdBUE3ljQXKeTWOMw&m=6c1v3c4C7T7kyJ5MxPQqPPRPSvxC9FTOOJvF9OKPhgI&s=E0gAesL6qFbjGV1xY1Z5CJpYnf455FJ2JbQ1_NLfQQI&e=" target="_blank">https://en.cppreference.com/w/cpp/filesystem/current_path</a><span class="apple-converted-space"> </span>or
specified in the filesystem header file of libc++. According to<span class="apple-converted-space"> </span><a href="https://urldefense.proofpoint.com/v2/url?u=http-3A__libcxx.llvm.org_ts1z-5Fstatus.html&d=DwMFAg&c=5VD0RTtNlTh3ycd41b3MUw&r=o3kDXzdBUE3ljQXKeTWOMw&m=6c1v3c4C7T7kyJ5MxPQqPPRPSvxC9FTOOJvF9OKPhgI&s=l8wb7lcPuHZoJXnCYsaVRM_qmC7avUrrsjaRI18MPGc&e=" target="_blank">http://libcxx.llvm.org/ts1z_status.html</a>,
as of June 17, 2016, the filesystem library is complete. For comparison, the APT repository was last updated on March 27, 2019.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">I do not know if perhaps I made a mistake in linking the code or installing the library.<span class="apple-converted-space"> </span><a href="https://urldefense.proofpoint.com/v2/url?u=https-3A__libcxx.llvm.org_&d=DwMFAg&c=5VD0RTtNlTh3ycd41b3MUw&r=o3kDXzdBUE3ljQXKeTWOMw&m=6c1v3c4C7T7kyJ5MxPQqPPRPSvxC9FTOOJvF9OKPhgI&s=C0te1DBSsmWIERl3PcPtdRW03Y0iwrp1-Z2qr2thXks&e=" target="_blank">https://libcxx.llvm.org/</a><span class="apple-converted-space"> </span>directed
me to ask here if I'm not sure whether I found a bug.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">Is this an issue with the files I downloaded from the APT repository, or is it my own mistake?<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">Your invocation of Clang puts -lc++fs before the source file that depends on it.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">You need to put the source file before the libraries. Ex:<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica"> clang-8 -std=c++17 -stdlib=libc++ test.cpp -lc++fs -v<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Helvetica">Isn't that something that Clang should handle?</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">To clarify, what I meant is that from a very naive point of view, the order in which you provide `-l`'s on the command line shouldn't matter -- it should work as long as you provide the right set of `-l`'s. I'm puzzled every time a tool
(e.g. `ld`) behaves differently based on the order of command line arguments.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Maybe there's a good reason for this, though. Maybe the cfe-dev folks can explain?<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Louis<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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