AnsweredAssumed Answered

__global vs __constant qualifier in OpenCL

Question asked by gopal_hc on Aug 21, 2013
Latest reply on Aug 23, 2013 by himanshu.gautam

I want an array variable to have a program scope.

One way I can do this by passing it as a function pointer throughout the program, which might be complex when we have multiple functions reading/writing this array variable.

Second way to do this, is to have a global variable having program scope. As per the OpenCL specification, Global variables are declared in the program source with the __constant qualifier and are accessed as read-only variables.


//I am writing one sample program to demonstrate my problem:

__constant uint arr[2] = {0, 0};  // an array of unsigned integer

void func1 (uint tmp)


         for(int i = 0; i < 2; i ++)

               arr[i] = tmp+i;


void func2(uint tmp)


         for(int i = 0; i < 2; i++)

               tmp = arr[i];


__kernel void demoKernel(__global uint *input,

                                           __global uint *output)


        uint index = get_global_id(0);





when i compiled this i got the following error:


tmp/", line 5: error: expression must be a modifiable lvalue

                       arr[i] = tmp+i;

I searched in google for this error, i found that it is because of type of "arr" is array of 2 length (it is not a pointer).


So my questions are:

1.  What is reason for this error and How I can fix this ?

2.  secondly, my requirement is to not only to read the array, but also write on it, so how should I use __constant qualifier for that which is read-only variable?


Thanks in Advance !!