Come collegare file oggetto C++ con ld

Question

Sto provando a collegare l'output di C++ usando ld e non g ++. Sto solo facendo questo per imparare come farlo, non per scopi pratici, quindi per favore non suggerire solo di farlo con g ++.

Guardando this question, la persona ottiene lo stesso errore quando corrono il comando ld:

$ 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 

Le risposte nel post legata suggeriscono che l'aggiunta la libreria C++ come argomento linker risolvere il problema, in modo da ho provato

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

che è quello che hanno suggerito, e ho anche provato un sacco di altri nomi di libreria come libstdC++ o STDC++. Ma io sarò sempre ottengo un errore che assomiglia

ld: cannot find -llibstd++ 

Che cosa sto facendo di sbagliato e come posso collegare i miei file oggetto utilizzando ld?

Answer 1

Se si esegue g++ con il flag -v, verrà visualizzata la linea di collegamento utilizzata. Ecco un semplice programma di esempio:

#include <iostream> 

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

E l'uscita dalla corsa 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, che casino. Convenientemente la linea di collegamento è l'ultima lì, in modo da poter vedere cosa sta succedendo abbastanza facilmente.

Come hai notato nel tuo commento qui sotto, il front-end utilizza collect2 anziché ld. Fortunatamente, collect2 è solo un alias per ld. Ecco un esempio di usarlo:

Prima di tutto generare un file oggetto:

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

Poi useremo il front-end per collegarlo a vedere la linea di collegamento:

$ 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 

Poi buttare via il binario e collegarci (normalmente, avrei solo copiato/incollato la riga, ma per renderlo più facile da leggere l'ho fatto in modo multilinea con \ 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 

Infine, eseguilo!

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

Probabilmente si può ridurre in modo significativo quella linea di collegamento rimuovendo alcuni argomenti. Ecco il set minimo mi si avvicinò con dopo qualche sperimentazione:

$ 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 

Questo insieme di bandiere e biblioteche sarà ovviamente dipenderà da quello che funzioni di libreria e la lingua sono disponibili il programma utilizza.