Soruce file layout

[Daniel Richard G.]

On Mon, 2006 Jul 17 06:59:32 -0400, David Quigley wrote:
> Hello,
> 	I was looking through the modules directory for the PAM sources and it
> seems that almost all of the modules exist in one source file in which
> every function is static except for the main PAM exports. Is there a
> reason for that? Is there a reason for not breaking a large module up
> into smaller source files?
> 

I'll try a stab at this, as this is more a C thing than anything 
PAM-specific.

The reason is to produce a shared object with no other externally-visible 
symbols beyond the standard PAM functions. If you break the module up into 
multiple objects, this will usually involve module-internal functions that 
are nonetheless exported, which can be problematic.

E.g. if the pam_sm_open_session() function is in foo-session.c, but it 
calls out to a routine do_blah() in foo-common.c, then the resulting foo.so 
PAM module will show do_blah() as an export alongside the standard PAM 
entry points. This is bad if the consuming application links against 
another shared library that also has a do_blah() export.

Now, there are various ways to address this issue:

1. Prefix the module-internal-but-non-static symbols with e.g. "__foo_" 
   (__foo_do_blah() and so on), to reduce the risk of namespace collisions

2. Use Libtool's symbol-visibility functionality to hide the 
   module-internal symbols (may not work on all platforms)

3. Have various source files, but declare all module-internal functions 
   static and compile a single master source file that #includes all the 
   others

4. Implement each PAM function in a separate source file, with all other 
   functions declared static (simple but inflexible)

As for why Linux-PAM doesn't take these approaches more often... well, most 
modules aren't that big, and the single-source-file MO is simpler :-)


HTH,


--Daniel


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