[Bug c++/106483] New: undefined reference to function implemented in shared library if putting main.cpp after options, but non of any error when putting before options

[Bug c++/106483] New: undefined reference to function implemented in shared library if putting main.cpp after options, but non of any error when putting before options

[xyang619 at 126 dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106483

            Bug ID: 106483
           Summary: undefined reference to function implemented in shared
                    library if putting main.cpp after options, but non of
                    any error when putting before options
           Product: gcc
           Version: og10 (devel/omp/gcc-10)
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: xyang619 at 126 dot com
  Target Milestone: ---

GCC version and OS: gcc version 10.2.1 20210110 (Debian 10.2.1-6)
Code example to reproduce the bugs:
#add.h
int add(int, int);

#add.cpp

#include "add.h"
int add(int a, int b) {
        return a+b;
}

#main.cpp
#include "add.h"
#include <cstdio>
int main() {
        int a=2;
        int b=3;
        int c=add(a,b);
        printf("%d\n", c);
        return 0;
}

#first, compile add.cpp to shared library, it works well
g++ -Wall -fPIC -shared -o libadd.so add.cpp

#second, compile main.cpp and link with the libadd.so, it will produce error as
followed:
g++ -Wall -L. -ladd -o add_main main.cpp
/usr/bin/ld: /tmp/ccCzqGbU.o: in function `main':
main.cpp:(.text+0x21): undefined reference to `add(int, int)'
collect2: error: ld returned 1 exit status

#however, if I put the main.cpp before the options, it works well
g++ main.cpp -Wall -L. -ladd -o add_main

[Bug c++/106483] undefined reference to function implemented in shared library if putting main.cpp after options, but non of any error when putting before options

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106483

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Firstly, this is the linker doing this, not GCC. GCC just passes the arguments
to the linker in the order you specify them.

Secondly, this is the correct behaviour. Your linker is set up to default to
--as-needed which means that shared libraries on the command line will only get
used if they satisfy a non-weak undefined reference from a regular object file
that has already been processed by the linker. If you put -ladd before main.cpp
then there are no undefined symbols yet, so the library is just ignored.

https://web.archive.org/web/20210609020437/c-faq.com/lib/libsearch.html

Either link with -Wl,--no-as-needed to change the linker's behaviour so that
shared libraries are always used, or put the libraries in the right order.