Is an executable built differently if linked against a library that's not used?

Question

Apart from a longer compile time, is there any downside to linking against an unused library?

for example, is there any difference in the executable of a program that is compiled one of two ways:

g++ -o main main.cpp
g++ -o main main.cpp -llib1 -llib2 -llib3 -lmore

*no library files were actually needed to build main.

I believe it makes no difference because the file sizes are the same, but I'm asking for confirmation.

Answer 1

It depends.

1. If liblib1.a, liblib2.a, and liblib3.a are static libraries, and no symbols are used from them, then there will be no difference.

2. If liblib1.so, liblib2.so, or liblib3.so are shared libraries, then they will be loaded at runtime whether or not they are used. You can use the linker flag --as-needed to change this behavior, and this flag is recommended.

To check which shared libraries your binary directly loads at runtime, on an ELF system you can use readelf.

$ cat main.c
int main()
{
    return 0;
}
$ gcc main.c
$ readelf -d a.out | grep NEEDED
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.6]
$ gcc -lpng main.c
$ readelf -d a.out | grep NEEDED
 0x0000000000000001 (NEEDED)             Shared library: [libpng12.so.0]
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.6]

You can see that on my system, -lpng links against libpng12.so.0, whether or not symbols from it are actually used. The --as-needed linker flag fixes this:

$ gcc -Wl,--as-needed -lpng main.c
$ readelf -d a.out | grep NEEDED
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.6]

Notes

1. The --as-needed flag must be specified before the libraries. It only affects libraries which appear after it. So gcc -lpng -Wl,--as-needed doesn't work.

2. The ldd command lists not only the libraries your binary directly links against, but also all the indirect dependencies. This can change depending on how those libraries were compiled. Only readelf will show you your direct dependencies, and only ldd will show you indirect dependencies.