Question

I have the following program:

~/test> cat test.cc int main() {   int i = 3;   int j = __sync_add_and_fetch(&i, 1);   return 0; } 

I’m compiling this program using GCC 4.2.2 on Linux running on a multi-cpu 64-bit Intel machine:

~/test> uname --all Linux doom 2.6.9-67.ELsmp #1 SMP Wed Nov 7 13:56:44 EST 2007 x86_64 x86_64 x86_64 GNU/Linux 

When I compile the program in 64-bit mode, it compiles and links fine:

~/test> /share/tools/gcc-4.2.2/bin/g++ test.cc ~/test> 

When I compile it in 32-bit mode, I get the following error:

~/test> /share/tools/gcc-4.2.2/bin/g++ -m32 test.cc /tmp/ccEVHGkB.o(.text+0x27): In function `main': : undefined reference to `__sync_add_and_fetch_4' collect2: ld returned 1 exit status ~/test> 

Although I will never actually run on a 32-bit processor, I do need a 32-bit executable so I can link with some 32-bit libraries.

My 2 questions are:

1. Why do I get a link error when I compile in 32-bit mode?

2. Is there some way to get the program to compile and link, while still being able to link with a 32-bit library?

Answer 1

From the GCC page on Atomic Builtins:

Not all operations are supported by all target processors. If a particular operation cannot be implemented on the target processor, a warning will be generated and a call an external function will be generated. The external function will carry the same name as the builtin, with an additional suffix `_n’ where n is the size of the data type.

Judging from your compiler output, which refers to __sync_add_and_fetch_4, this is what’s happening. For some reason, GCC is not generating the external function properly.

This is likely why you’re only getting an error in 32-bit mode – when compiling for 64-bit mode, it compiles for your processor more closely. When compiling for 32-bit, it may well be using a generic arch (i386, for example) which does not natively support those features. Try specifying a specific architecture for your chip family (Xeon, Core 2, etc.) via -mcpu and see if that works.

If not, you’ll have to figure out why GCC isn’t including the appropriate function that it should be generating.