Using a C function from a C++ program - extern keyword problem

Question

I am having a problem with a C++ program. I want to use a function which is defined in a C file in a C++ file. Here is my code for the C++ file:

#include <string>
#include <iostream>
#include <stdio.h>
extern void squre_array();
using namespace std;

int main() {
    squre_array();
}

Now here is the code for the C file in which I defined squre_array():

#include <stdio.h>
#include <cuda.h>

__global__ void square_array(float *a, int N)
{
  int idx = blockIdx.x * blockDim.x + threadIdx.x;
  if (idx<N) 
    a[idx] = a[idx] * a[idx];
}
void squre_array()
{
  float *a_h, *a_d; 
  const int N = 10;  
  size_t size = N * sizeof(float);
  a_h = (float *)malloc(size);        
  cudaMalloc((void **) &a_d, size);   
  for (int i=0; i<N; i++) a_h[i] = (float)i;
  cudaMemcpy(a_d, a_h, size, cudaMemcpyHostToDevice);
  int block_size = 4;
  int n_blocks = N/block_size + (N%block_size == 0 ? 0:1);
  square_array <<< n_blocks, block_size >>> (a_d, N);

  cudaMemcpy(a_h, a_d, sizeof(float)*N, cudaMemcpyDeviceToHost);
  // Print results
  for (int i=0; i<N; i++) printf("%d %f\n", i, a_h[i]);

  free(a_h); 
  cudaFree(a_d);
}

Now can any one tell me how to link this function to my C++ program? Every time I compile the program, I got the error message:

cpp:: undefined reference to `squre_array()'

Can anyone tell me what I was doing wrong? How can I link the squre_array() function into my C++ program?

What is square_array <<< n_blocks, block_size >>> (a_d, N); ? It doesn't look like C code. — Jim Balter
– Jim Balter, Commented Mar 16, 2011 at 6:52

Arve · Accepted Answer · 2011-03-16 05:57:24Z

6

In the C++ code you need to declare the C function as extern "C"

extern "C" void squre_array();

answered Mar 16, 2011 at 5:57

Arve

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1 Comment

user513164 Over a year ago

ya i did that it still not working if i mention void squre_array ();

Pete Wilson · Accepted Answer · 2011-03-16 06:06:25Z

3

Before the first line of your code snippet, add the three lines:

#ifdef  __cplusplus
extern "C" {
#endif

After the final line of your code snippet, add the three lines:

#ifdef  __cplusplus
}
#endif

Magic? No, it's only that the C and C++ compilers save your code's symbols in different formats; this lets the C++ linker understand the C symbols in your code snippet.

--pete

answered Mar 16, 2011 at 6:06

Pete Wilson

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3 Comments

user513164 Over a year ago

thanx for help but cant uderstand exact location to put these line or in which code c code or c++ code .please note thati,m using 2 differnt file

Jim Balter Over a year ago

@user513164 Indeed, it's an awful answer. Those before and after segments would be used in header files that declare C functions that you want to use in both C and C++ code, which is a common usage but not the usage in the code you posted. For that you can use Arve's answer.

user513164 Over a year ago

hi thank you for ur support and answer but can u explain me the process completly how to do this

Pete Wilson · Accepted Answer · 2011-03-17 10:58:31Z

Step-by-step, let's modify the OP's code as it now (Mar 17 at 0600 eastern) appears, after the OP applied a few edits:

STEP 1:
Consider snippet 1, which is the C++ int main( ) prog. The linker will try to do what you want, which is to make squre_array( ) accessible -- i.e., callable -- from main( ). In this C++ file, you must #include the header file that declares squre_array( ) to be a C-language function -- the one and only crucial point in this whole process -- rather than a C++ function. (Why? Because the compiler formats and stores C-language symbols differently from C++ symbols; and so when the linker comes along, the C-type symbol defined in the C source is not the same as the C++-type symbol referenced in main( ).) Now, is that header file named cuda.h? Let's assume it is. Remember that such a declaration makes "extern void squre_array( )" superfluous and confusing, so take that line out of this source file:

#include <string>
#include <iostream>
#include <stdio.h>
#include <cuda.h>         <-- add this line
//extern void squre_array();  <-- delete this line: we'll declare squre_array( ) in cuda.h
using namespace std;

int main() {
    squre_array();
}

STEP 2:
Now consider snippet 2, which defines the squre_array( ) function. This is plain old C code so we have to bracket all of that C code with two sets of three lines each. These six lines (total) effectively tell the linker that the symbols in the bracketed code are C-type symbols rather than C++-type"munged" symbols. When the linker is finally convinced of that, it can link the squre_array( ) function into your main program:



// insert magic three lines here, way up at the top of your .c file

#ifdef __cplusplus //if we are compiling as C++, tell

extern "C" { //the compiler that this stuff is plain old C

#endif


#include <stdio.h>

#include <cuda.h>             <-- remember this "glue" file: we'll change it in step 3


//_global_ void square_array(float *a, int N)  <-- remove the declaration,

void square_array(float *a, int N) {  <-- but retain the definition 

  int idx = blockIdx.x * blockDim.x + threadIdx.x;

  if (idx 
    a[idx] = a[idx] * a[idx];

}

void squre_array()

{

  float *a_h, *a_d;
 
  ... 

  cudaFree(a_d);

}

// close magic three lines

#ifdef __cplusplus //

} // closing curly bracket

#endif


STEP 3:
The important thing that is missing from the OP's understanding is that squre_array( ) (and square_array( ), if you want) must be declared; and that declaration(s) need to be enclosed within the same pair of magic three lines.  (OP: why must that be?) We decided in step 1 that the declaration would go in cuda.h. Or it can go in any .h file, but wherever it's declared, that .h file has to be #included in the file where main( ) resides (OP: again, why is this?). So let's fix up cuda.h:

// magic three lines again #ifdef __cplusplus extern "C" { #endif void squre_array(); void square_array(float *a, int N); // close magic three lines, just like before #ifdef __cplusplus // } // closing curly bracket #endif And that's it. Now your program will link.

-- pete

Anton Daneyko · Accepted Answer · 2012-12-05 13:21:29Z

Consider we have a function F declared in a F.h file, defined in a F.c file and a main.cpp, where F is #included and called. Your C compiler compiles F.h + F.c into an object file F.o; then the C++ compiler compiles F.h + main.cpp into main.o. However C++ compilers do name mangling, which means that function F declared in F.h would be renamed by compiler into for example F_blah. Then linker would try to combine main.o and F.o: it will find the call to F_blah in main.o, but it will not find the body of F_blah, because the C compiler compiled it as F, not F_blah. And here we are with the unresolved external symbol error (LNK2001 in VS). For the cases like this you need to tell C++ compiler to keep the name of the function as is: in your .h file you declare such function inside of the following block:

#ifdef  __cplusplus
extern "C" {
#endif

void F();

#ifdef  __cplusplus
}
#endif

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