How to link C++ object files with ld

Question

I'm trying to link the output of C++ using ld and not g++. I'm only doing this to learn how to do it, not for practical purposes, so please don't suggest just to do it with g++.

Looking at this question, the person gets the same error when they run the ld command:

$ ld test.o -o test.out ld: warning: cannot find entry symbol _start; defaulting to 00000000004000e8 test.o: In function `main': test.cpp:(.text+0x1c): undefined reference to `strcasecmp' test.cpp:(.text+0x23): undefined reference to `std::cout' test.cpp:(.text+0x28): undefined reference to `std::ostream::operator<<(int)' test.cpp:(.text+0x2d): undefined reference to `std::basic_ostream<char, std::char_traits<char> >& std::endl<char, std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&)' test.cpp:(.text+0x35): undefined reference to `std::ostream::operator<<(std::ostream& (*)(std::ostream&))' test.o: In function `__static_initialization_and_destruction_0(int, int)': test.cpp:(.text+0x75): undefined reference to `std::ios_base::Init::Init()' test.cpp:(.text+0x7a): undefined reference to `__dso_handle' test.cpp:(.text+0x84): undefined reference to `std::ios_base::Init::~Init()' test.cpp:(.text+0x89): undefined reference to `__cxa_atexit' ld: test.out: hidden symbol `__dso_handle' isn't defined ld: final link failed: Bad value 

The answers in the linked post suggest that adding the C++ library as a linker argument will fix the problem, so I tried

ld test.o -o test.out -llibstd++ 

which is what they suggested, and I also tried a lot of other library names like libstdc++ or stdc++. But I'll always get an error that looks like

ld: cannot find -llibstd++ 

What am I doing wrong and how can I link my object files using ld?

Answer 1

If you run g++ with the -v flag, you'll see the link line it uses. Here's a simple example program:

#include <iostream>  int main(void) {     std::cout << "Hello, world!" << std::endl;     return 0; } 

And the output from running g++ -v -o example example.cpp:

Using built-in specs. Target: x86_64-linux-gnu Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro 4.4.4-14ubuntu5.1' --with-bugurl=file:///usr/share/doc/gcc-4.4/README.Bugs --enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-4.4 --enable-shared --enable-multiarch --enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.4 --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-objc-gc --disable-werror --with-arch-32=i686 --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu Thread model: posix gcc version 4.4.5 (Ubuntu/Linaro 4.4.4-14ubuntu5.1)  COLLECT_GCC_OPTIONS='-v' '-o' 'example' '-shared-libgcc' '-mtune=generic'  /usr/lib/gcc/x86_64-linux-gnu/4.4.5/cc1plus -quiet -v -D_GNU_SOURCE example.cpp -D_FORTIFY_SOURCE=2 -quiet -dumpbase example.cpp -mtune=generic -auxbase example -version -fstack-protector -o /tmp/ccV8qjvd.s ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu" ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/4.4.5/../../../../x86_64-linux-gnu/include" ignoring nonexistent directory "/usr/include/x86_64-linux-gnu" #include "..." search starts here: #include <...> search starts here:  /usr/include/c++/4.4  /usr/include/c++/4.4/x86_64-linux-gnu  /usr/include/c++/4.4/backward  /usr/local/include  /usr/lib/gcc/x86_64-linux-gnu/4.4.5/include  /usr/lib/gcc/x86_64-linux-gnu/4.4.5/include-fixed  /usr/include End of search list. GNU C++ (Ubuntu/Linaro 4.4.4-14ubuntu5.1) version 4.4.5 (x86_64-linux-gnu)     compiled by GNU C version 4.4.5, GMP version 4.3.2, MPFR version 3.0.0-p3. GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072 Compiler executable checksum: d92fbc2d715a3b7e0f4133f0c40053e4 COLLECT_GCC_OPTIONS='-v' '-o' 'example' '-shared-libgcc' '-mtune=generic'  as -V -Qy -o /tmp/ccGHR0pc.o /tmp/ccV8qjvd.s GNU assembler version 2.20.51 (x86_64-linux-gnu) using BFD version (GNU Binutils for Ubuntu) 2.20.51-system.20100908 COMPILER_PATH=/usr/lib/gcc/x86_64-linux-gnu/4.4.5/:/usr/lib/gcc/x86_64-linux-gnu/4.4.5/:/usr/lib/gcc/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/4.4.5/:/usr/lib/gcc/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/4.4.5/:/usr/lib/gcc/x86_64-linux-gnu/ LIBRARY_PATH=/usr/lib/gcc/x86_64-linux-gnu/4.4.5/:/usr/lib/gcc/x86_64-linux-gnu/4.4.5/:/usr/lib/gcc/x86_64-linux-gnu/4.4.5/../../../../lib/:/lib/../lib/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-linux-gnu/4.4.5/../../../:/lib/:/usr/lib/:/usr/lib/x86_64-linux-gnu/ COLLECT_GCC_OPTIONS='-v' '-o' 'example' '-shared-libgcc' '-mtune=generic'  /usr/lib/gcc/x86_64-linux-gnu/4.4.5/collect2 --build-id --eh-frame-hdr -m elf_x86_64 --hash-style=gnu -dynamic-linker /lib64/ld-linux-x86-64.so.2 -o example -z relro /usr/lib/gcc/x86_64-linux-gnu/4.4.5/../../../../lib/crt1.o /usr/lib/gcc/x86_64-linux-gnu/4.4.5/../../../../lib/crti.o /usr/lib/gcc/x86_64-linux-gnu/4.4.5/crtbegin.o -L/usr/lib/gcc/x86_64-linux-gnu/4.4.5 -L/usr/lib/gcc/x86_64-linux-gnu/4.4.5 -L/usr/lib/gcc/x86_64-linux-gnu/4.4.5/../../../../lib -L/lib/../lib -L/usr/lib/../lib -L/usr/lib/gcc/x86_64-linux-gnu/4.4.5/../../.. -L/usr/lib/x86_64-linux-gnu /tmp/ccGHR0pc.o -lstdc++ -lm -lgcc_s -lgcc -lc -lgcc_s -lgcc /usr/lib/gcc/x86_64-linux-gnu/4.4.5/crtend.o /usr/lib/gcc/x86_64-linux-gnu/4.4.5/../../../../lib/crtn.o 

Wow, what a mess. Conveniently the link line is the last one there, so you can see what's happening pretty easily.

As you noticed in your comment below, the front-end is using collect2 rather than ld. Luckily, collect2 is just an alias for ld. Here's an example using it:

First let's generate an object file:

$ ls example.cpp $ c++ -c example.cpp $ ls example.cpp example.o 

Then we'll use the front-end to link it to see the link line:

$ c++ -v -o example example.o Using built-in specs. Target: x86_64-linux-gnu Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro 4.4.4-14ubuntu5.1' --with-bugurl=file:///usr/share/doc/gcc-4.4/README.Bugs --enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-4.4 --enable-shared --enable-multiarch --enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.4 --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-objc-gc --disable-werror --with-arch-32=i686 --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu Thread model: posix gcc version 4.4.5 (Ubuntu/Linaro 4.4.4-14ubuntu5.1)  COMPILER_PATH=/usr/lib/gcc/x86_64-linux-gnu/4.4.5/:/usr/lib/gcc/x86_64-linux-gnu/4.4.5/:/usr/lib/gcc/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/4.4.5/:/usr/lib/gcc/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/4.4.5/:/usr/lib/gcc/x86_64-linux-gnu/ LIBRARY_PATH=/usr/lib/gcc/x86_64-linux-gnu/4.4.5/:/usr/lib/gcc/x86_64-linux-gnu/4.4.5/:/usr/lib/gcc/x86_64-linux-gnu/4.4.5/../../../../lib/:/lib/../lib/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-linux-gnu/4.4.5/../../../:/lib/:/usr/lib/:/usr/lib/x86_64-linux-gnu/ COLLECT_GCC_OPTIONS='-v' '-o' 'example' '-shared-libgcc' '-mtune=generic'  /usr/lib/gcc/x86_64-linux-gnu/4.4.5/collect2 --build-id --eh-frame-hdr -m elf_x86_64 --hash-style=gnu -dynamic-linker /lib64/ld-linux-x86-64.so.2 -o example -z relro /usr/lib/gcc/x86_64-linux-gnu/4.4.5/../../../../lib/crt1.o /usr/lib/gcc/x86_64-linux-gnu/4.4.5/../../../../lib/crti.o /usr/lib/gcc/x86_64-linux-gnu/4.4.5/crtbegin.o -L/usr/lib/gcc/x86_64-linux-gnu/4.4.5 -L/usr/lib/gcc/x86_64-linux-gnu/4.4.5 -L/usr/lib/gcc/x86_64-linux-gnu/4.4.5/../../../../lib -L/lib/../lib -L/usr/lib/../lib -L/usr/lib/gcc/x86_64-linux-gnu/4.4.5/../../.. -L/usr/lib/x86_64-linux-gnu example.o -lstdc++ -lm -lgcc_s -lgcc -lc -lgcc_s -lgcc /usr/lib/gcc/x86_64-linux-gnu/4.4.5/crtend.o /usr/lib/gcc/x86_64-linux-gnu/4.4.5/../../../../lib/crtn.o 

Then throw away the binary, and link ourselves (normally, I would have just copy/pasted the line, but to make it easier to read I did it the multiline way with \s):

$ ls example  example.cpp  example.o $ rm example $ ls example.cpp example.o $ ld                                                              \ > --build-id                                                      \ > --eh-frame-hdr                                                  \ > -m elf_x86_64                                                   \ > --hash-style=gnu                                                \ > -dynamic-linker                                                 \ > /lib64/ld-linux-x86-64.so.2                                     \ > -o example                                                      \ > -z relro                                                        \ > /usr/lib/gcc/x86_64-linux-gnu/4.4.5/../../../../lib/crt1.o      \ > /usr/lib/gcc/x86_64-linux-gnu/4.4.5/../../../../lib/crti.o      \ > /usr/lib/gcc/x86_64-linux-gnu/4.4.5/crtbegin.o                  \ > -L/usr/lib/gcc/x86_64-linux-gnu/4.4.5                           \ > -L/usr/lib/gcc/x86_64-linux-gnu/4.4.5                           \ > -L/usr/lib/gcc/x86_64-linux-gnu/4.4.5/../../../../lib           \ > -L/lib/../lib                                                   \ > -L/usr/lib/../lib                                               \ > -L/usr/lib/gcc/x86_64-linux-gnu/4.4.5/../../..                  \ > -L/usr/lib/x86_64-linux-gnu                                     \ > example.o                                                       \ > -lstdc++                                                        \ > -lm                                                             \ > -lgcc_s                                                         \ > -lgcc                                                           \ > -lc                                                             \ > -lgcc_s                                                         \ > -lgcc                                                           \ > /usr/lib/gcc/x86_64-linux-gnu/4.4.5/crtend.o                    \ > /usr/lib/gcc/x86_64-linux-gnu/4.4.5/../../../../lib/crtn.o 

Finally, run it!

$ ls example example.cpp example.o $ ./example  Hello, world! 

You can probably significantly shorten that link line by removing some arguments. Here's the minimal set I came up with after some experimentation:

$ ld                                                              \ > -dynamic-linker                                                 \ > /lib64/ld-linux-x86-64.so.2                                     \ > -o example                                                      \ > /usr/lib/gcc/x86_64-linux-gnu/4.4.5/../../../../lib/crt1.o      \ > /usr/lib/gcc/x86_64-linux-gnu/4.4.5/../../../../lib/crti.o      \ > /usr/lib/gcc/x86_64-linux-gnu/4.4.5/crtbegin.o                  \ > example.o                                                       \ > -L/usr/lib/gcc/x86_64-linux-gnu/4.4.5                           \ > -lstdc++                                                        \ > -lc                                                             \ > /usr/lib/gcc/x86_64-linux-gnu/4.4.5/crtend.o                    \ > /usr/lib/gcc/x86_64-linux-gnu/4.4.5/../../../../lib/crtn.o 

This set of flags and libraries will of course depend on what library functions and language features your program uses.