Monday, January 9, 2012

A static function, an inline function, and a static variable walked into a bar....

... well, not really. Hacking around with some library code, so I thought I'd write up a quick refresher on scoping. Steve Clamage and I cover scoping in more detail in the series on libraries and linking. For the code I was working on today, the problem was much more limited.

I had a single file containing all the source code. I wanted to export only the minimal number of symbols that were needed to act as an interface for the library. You can imagine it being something like:

#include <stdio.h>

int count=0;

inline void printcount()
  printf("Count = %i\n",count);

void next()

If I compile this, and then use nm to inspect the resulting library, I can see a global symbol for count. The function printcount() is defined with local scope. However, the only interface I want to export is next().

bash-3.00$ cc -g -G -O -o t.c
bash-3.00$ nm|grep GLOB
[45]    |     66468|       4|OBJT |GLOB |0    |11     |count
[43]    |       724|      40|FUNC |GLOB |0    |5      |next
[42]    |         0|       0|FUNC |GLOB |0    |UNDEF  |printf
bash-3.00$ nm |grep count
[44]    |     66460|       4|OBJT |GLOB |0    |11     |count
[32]    |       672|      52|FUNC |LOCL |0    |5      |printcount

So I can define count as a static variable, and that reduces its scope to the file in which it is defined. However, this does not actually make it disappear, it is still there, but with name mangling:

bash-3.00$ nm|grep count
[40]    |     66476|       4|OBJT |GLOB |0    |11     |$XAS4IkBuA_CPGtc.count
[33]    |       688|      52|FUNC |LOCL |0    |5      |printcount

The reason for this is that I'm building with debug (-g). With debug, I get a local version of the routine printcount(), and I get a globalised version of the variable count. If I remove -g, I get the following output from nm:

bash-3.00$ nm|grep count
[29]    |     66316|       4|OBJT |LOCL |0    |11     |count
[36]    |         0|       0|FUNC |GLOB |0    |UNDEF  |printcount

The variable count has local scope, which is what we expected - it is no longer exported from the file, so we have avoided possible name conflicts there. However, printcount() is now no longer defined. That might be ok so long as we don't actually call the routine:

bash-3.00$ dis|grep printcount
         2e4:  7f ff ff ef  call        printcount      ! 0x2a0

Oops. We've hit the rule about needing to provide an extern version of any inline functions. Once again, I suggest parsing Douglas Walls' discussion of the topic for the gory details. Anyhow, the upshot is that this library wouldn't work. The fix is trivial, declare printcount() to be static inline, and the compiler will generate the local version of the function:

bash-3.00$ cc -G -O -o t.c
bash-3.00$ nm |grep count
[29]    |     66448|       4|OBJT |LOCL |0    |11     |count
[30]    |       664|      52|FUNC |LOCL |0    |5      |printcount

With these fixes the library no longer exports any functions but the ones I left with external linkage. This substantially reduces the risk of "undefined behaviour".