When to use --dynamic option in nm

Question

Sometimes when I do nm on a .so file (for example, libstdc++.so.6), it says no symbols, and I need to use nm --dynamic. But for some other .so files, I can see the symbols with

Answer 1

You need to use --dynamic or -D option on a shared library if it is stripped and thus only contains a dynamic symbol table.

You may want to use this option for other shared libraries to explicitly display the dynamic symbol table as this is the table that is consulted by the dynamic linker.

The file utility indicates whether a shared library is stripped or not. Example:

$ file /usr/lib64/libcrypt-nss-2.26.so
[..] ELF 64-bit LSB shared object, x86-64 [..], not stripped
$ file /usr/lib64/libxml2.so.2.9.7
[..] ELF 64-bit LSB shared object, x86-64 [..], stripped

Example for how the different symbol tables may contain different symbols:

$ nm -D /usr/lib64/libcrypt-nss-2.26.so | wc -l
39
$ nm /usr/lib64/libcrypt-nss-2.26.so | wc -l 
112