g++ linking against static library

Question

my main.cpp:

#include <iostream>

int foo(int arg);

using namespace std;

int main()
{
    int x = foo(22);
    cout << x;
    return 0;
}

compile command line (Ubuntu 13.10):

g++-4.8 -L. -lfoo main.cpp -o main_app

libfoo.a contains int foo(int) but I always get the same compiler error:

/tmp/cciAyTSP.o: In function `main':
main.cpp:(.text+0x19): undefined reference to `foo(int)'
collect2: error: ld returned 1 exit status

`foo` is not **defined** inside your code, or in better terms it's not defined or `extern` inside your translation unit. — user2485710, Jan 06 '14 at 21:24
How do you build libfoo.a? Could you give us a [minimal complete example](http://sscce.org)? — Beta, Jan 06 '14 at 21:27
@CaptainObvlious I didn't said that ... for example this code lacks of any header or inclusions that could have helped to solve the problem. `extern` is implicit, I know that . — user2485710, Jan 06 '14 at 21:30
@Beta if you want to avoid the definition of a function inside a translation unit, yes it is ... — user2485710, Jan 06 '14 at 21:31
@user2485710: If you mean avoid having the definition of `foo` in `main.cpp`, you're wrong. What do you think the `#include` directive does? — Beta, Jan 06 '14 at 21:43
@user2485710: I think it is now safe to say that neither of us knows what you're talking about. — Beta, Jan 06 '14 at 21:49
@Beta nevermind, the op can either define the function used inside this piece of code, right in the source file, or remove the definition and include an header to an external library that will be solved when linking. Problem is that the combination of the source code and the command line doesn't make any sense. — user2485710, Jan 06 '14 at 21:52
I don't have any .h files but I don't need them as long as I know function signatures. — iamnp, Jan 07 '14 at 07:05

score 3 · Accepted Answer · answered Jan 06 '14 at 21:55

3

Of course it's impossible to be sure without a reproducible case, but a common error is that if the function foo is written in C then you need to put

extern "C" { int foo(int); }

in the .h file for the C++ program to let it know that the function was not written in C++.

To write a cross-language header file that will be good for both C and C++ the common approach is

#ifdef __cplusplus
extern "C" {
#endif

... C declarations ...

#ifdef __cplusplus
}
#endif

answered Jan 06 '14 at 21:55

6502

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2

also, move -lfoo to the end of the command. – Xephon Jan 06 '14 at 21:57
@Xephon no, it doesn't help – iamnp Jan 07 '14 at 07:03
1

@iamnp Not moving the linkage statement may cause other issues. Not trying to answer the question but introducing a better practice per se. – Xephon Jan 08 '14 at 14:38

score 0 · Answer 2 · answered Mar 28 '14 at 08:00

In addition to 6502's answer and Xephon's suggestion, also be aware that the order of the options matter. Instead of:

g++-4.8 -L. -lfoo main.cpp -o main_app

You should write:

g++-4.8 main.cpp -o main_app -L. -lfoo

That's because ld is a single pass linker. It won't revisit library libfoo to use a symbol from it for main_app.o.

g++ linking against static library

2 Answers2