Easy check for unresolved symbols in shared libraries?

Question

I am writing a fairly large C++ shared-object library, and have run into a small issue that makes debugging a pain:

If I define a function/method in a header file, and forget to create a stub for it (during development), since I am building as a shared object library rather than an executable, no errors appear at compile-time telling me I have forgotten to implement that function. The only way I find out something is wrong is at runtime, when eventually an application linking against this library falls over with an 'undefined symbol' error.

I am looking for an easy way to check if I have all the symbols I need at compile time, perhaps something I can add to my Makefile.

One solution I did come up with is to run the compiled library through nm -C -U to get a demangled list of all undefined references. The problem is this also comes up with the list of all references that are in other libraries, such as GLibC, which of course will be linked against along with this library when the final application is put together. It would be possible to use the output of nm to grep through all my header files and see if any of the names corresponding.. but this seems insane. Surely this is not an uncommon issue and there is a better way of solving it?

Answer 1

Check out the linker option -z defs / --no-undefined. When creating a shared object, it will cause the link to fail if there are unresolved symbols.

If you are using gcc to invoke the linker, you'll use the compiler -Wl option to pass the option to the linker:

gcc -shared ... -Wl,-z,defs

As an example, consider the following file:

#include <stdio.h>

void forgot_to_define(FILE *fp);

void doit(const char *filename)
{
    FILE *fp = fopen(filename, "r");
    if (fp != NULL)
    {
        forgot_to_define(fp);
        fclose(fp);
    }
}

Now, if you build that into a shared object, it will succeed:

> gcc -shared -fPIC -o libsilly.so silly.c && echo succeeded || echo failed
succeeded

But if you add -z defs, the link will fail and tell you about your missing symbol:

> gcc -shared -fPIC -o libsilly.so silly.c -Wl,-z,defs && echo succeeded || echo failed
/tmp/cccIwwbn.o: In function `doit':
silly.c:(.text+0x2c): undefined reference to `forgot_to_define'
collect2: ld returned 1 exit status
failed