nm reports symbol is defined but ldd reports symbol is undefined

Question

I'm having a linking problem. I need to link against a shared library libfoo.so that depends on a function read which I would like to define myself in the file read.c.

I compile and link everything together but at runtime I get the error

/home/bar/src/libfoo.so: undefined symbol: sread.

nm reports the symbol is defined

$nm baz | grep sread
  00000000000022f8 t sread

but ldd reports the symbol is undefined

$ldd -r baz | grep sread 
undefined symbol: sread (/home/bar/src/libfoo.so)

What gives? Is there some isse with the fact that libfoo.so is a shared library?

Answer 1

When you build your shared library you need to resolve all undefined symbols from either within the same library or another (shared) library. The linker will not resolve an undefined symbol from a library with a symbol from your application.

Answer 2

First, defining a function called 'read' is a bad idea(TM), because it is a standard libc function on all UNIXen. The behavior of your program is undefined when you do this.

Second, the read function you defined in libbaz.so is marked with a 't' in nm output. This means that this function is local (not visible outside libbaz.so). Global functions are marked with 'T' by nm.

Did you use 'static int read(...)' when you defined it in read.c? If not, did you use a linker script, or attribute((visibility(hidden))), or perhaps -fvisibility=hidden on command line when you compiled and linked libbaz.so?