# Call CUDA C code via ccall

**URL:** <https://discourse.julialang.org/t/call-cuda-c-code-via-ccall/3539>\
**Category:** GPU\
**Created:** [May 5, 2017, 3:05pm UTC](https://discourse.julialang.org/t/call-cuda-c-code-via-ccall/3539 "2017-05-05T15:05:20Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![WneleiGao](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/wneleigao/32/9410_2.png) [@WneleiGao](https://discourse.julialang.org/u/WneleiGao)\
**Post date:** [May 5, 2017, 3:05pm UTC](https://discourse.julialang.org/t/call-cuda-c-code-via-ccall/3539/1 "2017-05-05T15:05:20Z")

</div>

Hi everyone, I have a very simple code to perform parallel vector summation on GPU. The code is written with CUDA C, then compiled to be a shared library. When I try to call the library function via ccall in Julia, it can not find the function. The code is attached.

```julia
#include "../common/book.h" 

__global__ void add(int *a, int *b, int *c, int N){
    int id = threadIdx.x + blockIdx.x * blockDim.x;
    while (id < N){
        c[id] = a[id] + b[id];
        id = id + blockDim.x * gridDim.x;
    }
}

extern "C" void vecAdd(int *a, int *b, int *c, int N){
    
    int *d_a, *d_b, *d_c;

    HANDLE_ERROR( cudaMalloc( (void **)&d_a, sizeof(int)*N ) );
    HANDLE_ERROR( cudaMalloc( (void **)&d_b, sizeof(int)*N ) );
    HANDLE_ERROR( cudaMalloc( (void **)&d_c, sizeof(int)*N ) );

    HANDLE_ERROR( cudaMemcpy(d_a, a, sizeof(int)*N, cudaMemcpyHostToDevice) );
    HANDLE_ERROR( cudaMemcpy(d_b, b, sizeof(int)*N, cudaMemcpyHostToDevice) );

    add<<<128, 128>>>(d_a, d_b, d_c, N);
    HANDLE_ERROR( cudaMemcpy(c, d_c, sizeof(int)*N, cudaMemcpyDeviceToHost) );

    cudaFree(d_a);
    cudaFree(d_b);
    cudaFree(d_c);
  
}

```

I compiled the source code with the command

```julia
nvcc --ptxas-options=-v --compiler-options '-fPIC' -o libtest.so --shared vecAdd.cu

```

then try to call `vecAdd` from Julia with `ccall`, the julia code is

```julia
N = 123321
a = ones(Int32, N)
b = ones(Int32, N)
c = ones(zeros, N)
ccall((:vecAdd, "libtest.so"), Void, (Ptr{Int32}, Ptr{Int32}, Ptr{Int32}, Int32), a, b, c, N)

```

the error information shows that

```julia
 can not find vecAdd from library libtest.so.

```

Any suggestions how to solve this problem ? Thanks.

Wenlei

---

<div class="post-metadata">

**Author:** ![ChrisRackauckas](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/chrisrackauckas/32/77_2.png) [@ChrisRackauckas](https://discourse.julialang.org/u/ChrisRackauckas)\
**Post date:** [May 5, 2017, 3:56pm UTC](https://discourse.julialang.org/t/call-cuda-c-code-via-ccall/3539/2 "2017-05-05T15:56:51Z")

</div>

Have you seen CUDArt.jl?

[https://github.com/JuliaGPU/CUDArt.jl](https://github.com/JuliaGPU/CUDArt.jl)

---

<div class="post-metadata">

**Author:** ![WneleiGao](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/wneleigao/32/9410_2.png) [@WneleiGao](https://discourse.julialang.org/u/WneleiGao)\
**Post date:** [May 5, 2017, 4:14pm UTC](https://discourse.julialang.org/t/call-cuda-c-code-via-ccall/3539/3 "2017-05-05T16:14:13Z")

</div>

Thanks, Chris. I will take a look of that package.

---

<div class="post-metadata">

**Author:** ![maleadt](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/maleadt/32/10097_2.png) [@maleadt](https://discourse.julialang.org/u/maleadt)\
**Post date:** [May 6, 2017, 6:44am UTC](https://discourse.julialang.org/t/call-cuda-c-code-via-ccall/3539/4 "2017-05-06T06:44:25Z")

</div>

You need to ccall `./libtest.so`, eg.

```julia
ccall((:vecAdd, "./libtest.so"), Void, (Ptr{Int32}, Ptr{Int32}, Ptr{Int32}, Int32), C_NULL, C_NULL, C_NULL, 0)

```

You could have debugged this (on Linux) by running with `LD_DEBUG=libs`, in which case you would have seen:

```julia
      3031: find library=libtest.so [0]; searching
      3031: search path=/opt/julia-master/usr/bin/../lib:/opt/julia-master/usr/bin/../lib/julia (RPATH from file julia-master)
      3031: trying file=/opt/julia-master/usr/bin/../lib/libtest.so
      3031: trying file=/opt/julia-master/usr/bin/../lib/julia/libtest.so
      3031: search path=/opt/julia-master/usr/bin/../lib (RPATH from file julia-master)
      3031: trying file=/opt/julia-master/usr/bin/../lib/libtest.so
      3031: search cache=/etc/ld.so.cache
      3031: search path=/lib/x86_64-linux-gnu:/usr/lib/x86_64-linux-gnu/tls:/usr/lib/x86_64-linux-gnu:/lib:/usr/lib (system search path)
      3031: trying file=/lib/x86_64-linux-gnu/libtest.so
      3031: trying file=/usr/lib/x86_64-linux-gnu/tls/libtest.so
      3031: trying file=/usr/lib/x86_64-linux-gnu/libtest.so
      3031: trying file=/lib/libtest.so
      3031: trying file=/usr/lib/libtest.so

```

ie. clearly not looking in the current directory.

Also, in case you’re not aware of CUDAnative, you can do this [in pure Julia](https://github.com/JuliaGPU/CUDAnative.jl/blob/master/examples/vadd.jl) too!