Question

I have been having these really odd problems with Visual Studio 2010. At this point, the behavior is so erratic that I really wish I did not have to use it for CUDA (I know I don’t have to, but it is hard not to use it).

One of the many problems I have been having with really basic stuff is header files being included more than once. For example:

//vars.cuh
#if !defined(VARS_cuh)
#define VARS_cuh

#include <cuda.h>
#include <cuda_runtime_api.h>

int* kern_xstart, *kern_xend, *kern_ystart, *kern_yend, *kern_zstart, *kern_zend;
/* more variable definitions */

#endif

I then include this file in most of my source files:

//source_file.cu
extern "C"{
#include "vars.cuh"
/* more includes of my own headers */
#include <cuda.h>
#include <cuda_runtime_api.h>
}

/* source file body */

The VS 2010 compiler puts out errors like this: “error LNK2005: foo already defined in other_source_file_I_wrote.cu.obj”

Why is it doing this? Also, to kill two birds with one stone, with this setup, I also have problems with writing a function in source_file.cu, and then prototyping it in vars.cuh. The problem arrises that vars.cuh can’t see the definition, even though I am clearly including vars.cuh in source_file.cu!

Thank you!

Answer 1

The header file is being compiled multiple times because, as you say, you include this header file in most of your source files. Those global variables are included in multiple source files and thus are defined in every source file that includes the header. When the linker links all of the object files together, it finds multiple definitions of those variables, hence the error.

If you want to share global variables across multiple source files, declare them as extern in the header, then define each of them once in one source file.

This isn’t a problem with Visual Studio or the Visual C++ compiler, it’s how C works.