Dumb linker/loader question

Max Okumoto okumoto at ucsd.edu
Sat Jan 15 13:31:15 PST 2005

Simon 'corecode' Schubert wrote:
On 15.01.2005, at 20:52, Chris Pressey wrote:

I'm clearly lacking some basic understanding of how this whole
thing works.  How can code that uses the header files from a
library compile *and* link successfully without that library?
In brief: because linking to shared objects (.so's) happens only when
the program is loaded, i.e. at runtime.  It doesn't even try to link in
.so's at compile-time - it just takes it on faith that the functions
that you've said will be there (by including a header file) will in fact
be there - so it doesn't know there are unresolved symbols (missing
dependencies) until later, so everything builds without a hitch.

I think that's not 100% true. It won't work with executables because the 
linker indeed looks for the functions, if they exist in the shared 
object you're linking to. But it can't do so if you build a shared 
objects yourself. Then it just takes it for granted that the functions 
you are referring to will be present at load time.

The linker (/usr/bin/ld) need either a foo.a or a foo.so file.  The 
header is not sufficent.  There must be a library laying around in your
link path.  Try and run ldd on the binary and see what it thinks it is


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