Duplicate symbol issue with C headers
c, c++, cuda, duplicate-symbol, header-files
Solution
Header files should normally contain only declarative code. `h_n_osc` should be declared here, not defined.
extern unsigned int h_n_osc;
In at least one of your modules, or a new one of its own you will need a definition; for example:
env_vars.cu
#include "env_vars.h"
unsigned int h_n_osc;
Then link that. Alternatively you could of course place the definition in one of the existing modules multigpu.cu or adm_matrix.cu.
I am not sure of the semantics of the The CUDA `__device__` extension, while it may link, it is not necessarily correct; you may end up with each module referencing a separate copy of the device variable; it may be necessary to qualify that with `extern` as well. This question appears to deal with that issue.
Problem
This is my first time tackling a CUDA project that's slightly more complex than the simple write-single-source-file-and-compile routine. As expected, I'm facing some issues with C headers, namely duplicated symbols. According to the linker, conflicts arise over the inclusion of the following header file in multiple `.cu` files: env_vars.h ``` #ifndef ENV_VARS_H_ #define ENV_VARS_H_ /*** GLOBAL VARIABLES ***/ unsigned int h_n_osc; __device__ unsigned int d_n_osc; /*** CONSTANTS ***/ const double OMEGA_0 = 6.447421494058077e+09; /* other constants defined in the middle */ #endif ``` multigpu.cu ``` #include "env_vars.h" /* assigns h_n_osc */ ``` adm_matrix.cu ``` #include "env_vars.h" /* uses h_n_osc */ ``` Building the project in Nsight Eclipse Edition results in the linker complaining about the `h_n_osc` variable being defined twice: ``` duplicate symbol _h_n_osc in: ./adm_matrix.o ./multigpu.o ld: 1 duplicate symbol for architecture x86_64 ``` Searching through the Internet, I've realized that moving the declaration of the `h_n_osc` variable to `multigpu.cu` and re-declaring it as an `extern` variable in `adm_matrix.cu` (and wherever I might need it later) solves the problem, which in fact it does. Problem solved, but I'd like to take a deeper look into this: - Why doesn't the linker complain about the `d_n_osc` variable as well? And why are the constants (such as `OMEGA_0`) equally not a problem? - Does this mean that it is not possible to place global variables in header files? - What puzzles me most is that a number of sources over the Internet state that duplicated symbol errors should happen only when the header file contains a definition of the variable, while its simple declaration shouldn't constitute a problem. The reason I have a hard time believing this is that I'm facing the issue even though my header only contains a declaration! Am I missing something? Thanks in advance for your patience, folks!