I have the following source code in main.cpp:
#include <iostream>
#include <iomanip>
int main() {
std::cout << "Hi" << std::endl;
return 0;
}
Using this command works, and creates the executable file:
g++ -o main main.cpp
But this commands don't work:
g++ -c main.cpp
ld -o main main.o
The second one errors with:
ld: warning: cannot find entry symbol _start; defaulting to 00000000004000e8
main.o: In function `main':
main.cpp:(.text+0xa): undefined reference to `std::cout'
main.cpp:(.text+0xf): undefined reference to `std::basic_ostream<char, std::char_traits<char> >& std::operator<< <std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*)'
main.cpp:(.text+0x14): 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> >&)'
main.cpp:(.text+0x1c): undefined reference to `std::ostream::operator<<(std::ostream& (*)(std::ostream&))'
main.o: In function `__static_initialization_and_destruction_0(int, int)':
main.cpp:(.text+0x4a): undefined reference to `std::ios_base::Init::Init()'
main.cpp:(.text+0x4f): undefined reference to `std::ios_base::Init::~Init()'
main.cpp:(.text+0x54): undefined reference to `__dso_handle'
main.cpp:(.text+0x61): undefined reference to `__cxa_atexit'
GNU C++ Compiler ( g++ ) is a compiler in Linux which is used to compile C++ programs. It compiles both files with extension . c and . cpp as C++ files. The following is the compiler command to compile C++ program.
An “Undefined Reference” error occurs when we have a reference to object name (class, function, variable, etc.) in our program and the linker cannot find its definition when it tries to search for it in all the linked object files and libraries.
GCC internally links against a few additional libraries and object files. To see what those are, execute -###
, which will print the tool commands it would execute, including the linker. The linker command used by my GCC is (I made the actual .o
file I invoked g++
on bold, so you can spot it easily).
/usr/lib/gcc/i686-pc-linux-gnu/4.6.0/collect2 --build-id --eh-frame-hdr -m elf_i386 "--hash-style=both" -dynamic-linker /lib/ld-linux.so.2 /usr/lib/gcc/i686-pc-linux-gnu/4.6.0/../../../crt1.o /usr/lib/gcc/i686-pc-linux-gnu/4.6.0/../../../crti.o /usr/lib/gcc/i686-pc-linux-gnu/4.6.0/crtbegin.o -L/usr/lib/gcc/i686-pc-linux-gnu/4.6.0 -L/usr/lib/gcc/i686-pc-linux-gnu/4.6.0/../../.. main1.o "-lstdc++" -lm -lgcc_s -lgcc -lc -lgcc_s -lgcc /usr/lib/gcc/i686-pc-linux-gnu/4.6.0/crtend.o /usr/lib/gcc/i686-pc-linux-gnu/4.6.0/../../../crtn.o
You can substitute the collect2
path by ld
(if I remember correctly, collect2
is only needed as a proxy for the real ld
by GCC backends that don't support modern binary formats, to collect constructor and destructor functions according to their mangled name. The ELF format has native sections support for that).
Executing that command, with ld
substituted literally, successfully links the executable.
I think if you use ld
directly it does not include the C++ libraries by default. You can use g++
to do the linking as well, it will call ld
with the correct settings.
g++ -c main.cpp
g++ -o main main.o
When you use g++
, it also links the libstd++
library which linker uses to resolve the names. But using g++ -c
and then ld
do not link the library automatically. You've to link the library manually now.
Try (untested):
ld -o main main.o -llibstd++
Or you can look into the correct syntax by reading this manual:
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