Combining multiple .o files into an executable

Question

I'm trying to combine object files created from C++ files into an executable using gcc. Unfortunately, gcc is giving me thousands of undefined reference errors to strings, arrays, etc.

I am doing this on a Windows machine, so no terminal commands; only cmd commands.

I'm simply doing:

gcc a.o b.o c.o -o prgm.exe

What am I missing/doing wrong?

EDIT:

I recreated the .o files with g++ doing: g++ a.cpp -g -c -Wall -std=c++0x -lSDLmain -lSDL -lSDL_image -lSDL_ttf -IC:\SDL-1.2.14\include -o a.o, where a.cpp and a.o are the directories where i keep the files, not the g++ directory

Then, I did g++ a.o b.o c.o -o prgm.exe. This gave dozens (I guess that's an improvement?) errors like

undefined reference to `_SDL_SetColorKey'

but I included SDL didnt I?

The final error from this is:

c:/program files (x86)/codeblocks/mingw/bin/../lib/gcc/mingw32/4.7.0/../../../li
bmingw32.a(main.o):main.c:(.text.startup+0xa7): undefined reference to `_WinMain
@16'
collect2.exe: error: ld returned 1 exit status

int main(int argc, char * argv[]) is in the code

Answer 1

Based upon your updates then I think you'd need to do the following:

g++ a.cpp b.cpp c.cpp -g -Wall -IC:\SDL-1.2.14\include -LC:\SDL-1.2.14\lib -std=c++0x -lSDLmain -lSDL -lSDL_image -lSDL_ttf  -o prgm.exe

I'm guessing C:\SDL-1.2.14\lib exists based upon where the headers are located.

Answer 2

You are trying to link a C++ program with the C linker. You need to use g++ instead of gcc.

Generally speaking gcc is for compiling/linking C, while g++ is for C++. IIRC compiling C++-code with gcc works by virtue of dispatching according to the file extension. Linking C++ code with gcc however does not work, since it won't link the C++ standard libraries, resulting in your undefined reference errors.

If this does not solve your problem, you might want to give us a more concrete description of your errors and your system.

Answer 3

GCC is the C compiler. Your code is C++ so you need to use G++ to do the linking:

g++ a.o b.o c.o -o prgm.exe

This automatically adds the C++ libraries to the link line, resolving many if not all of your missing references.