how to avoid g++ linker error "undefined reference"

Question

if function definition is not available and only the function prototype is externed in header files, then g++ compiler wont give any errors. but during linking, g++ linker will identify the error and says "undefined reference" to corresponding fn. i don't care abt fn definition. i just want to get the binary image and whenever corresponding fn called from that image, then that time only i want to crash. may i know the linker flags to suppress this "undefined reference" linker error?. In VC++, there is a options called /FORCE. is there any similar flags?

What do you expect to happen for undefined symbols which aren't functions? — Flexo
Thanks for looking into it. It will be great if symbols other than functions shows the same weak symbol behaviour. but it is not mandatory. — siva

Jonathan Wakely Jonathan Wakely · Accepted Answer · 2012-08-23T13:04:10

It's completely undefined behaviour, but if you want a crash ...

If you have an undefined symbol such as a function void h() then its mangled name will be _Z1hv, so if you define a symbol that name and external linkage it will be found, even if the type is wrong:

int _Z1hv = 0;
void h();

int main()
{
  h();
}

This will link, but crash at run-time because the call to h() will try to "run" a function at the address of the integer variable.

Doing this is wrong and disgusting and kills kittens.

how to avoid g++ linker error "undefined reference"

1 Answers