Chapter 2, example 4: Creating a program from file

[Petr Schreiber]

We have learned what platform, device and context is. The term I would like to present today is "program".

This name is a bit misleading, because in OpenCL, the "program" thing is more similar to what we know as "library of functions".
The OpenCL "program" does not get executed, does not have main function and other features you would expect.

Instead, it is collection of small functions, called kernels.

While the previous terms (platform/device/context) were managed using function calls, the OpenCL program is written in special form of something I would not hesitate call "C on steroids".
It is language based on C99 standard, but enhanced with goodies such as vector data types. The bad news for BASIC programmers is that even this version of C language still has semicolons and scary syntax for FOR cycles (gripe intented ).

The program needs to be built first to make possible to call computations stored inside (execute kernels).

As a source for building the program you can pass string, but it is more comfortable to store sources in separate file and load them from file.

I must admit in this part it was nice to see ThinBASIC does not complicate things. In C original, the code to load the program file looks like this:

program_handle = fopen(file_name[i], "r");
if(program_handle == NULL) {
  perror("Couldn't find the program file");
  exit(1);   
}
fseek(program_handle, 0, SEEK_END);
program_size[i] = ftell(program_handle);
rewind(program_handle);
program_buffer[i] = (char*)malloc(program_size[i]+1);
program_buffer[i][program_size[i]] = '\0';
fread(program_buffer[i], sizeof(char), program_size[i], program_handle);
fclose(program_handle);

In thinBASIC, this code does the job:

program_buffer(i) = FILE_Load(file_name(i)) 
program_size(i)   = Len(program_buffer(i))
      
If(program_size(i) = 0) Then
  Printl("Couldn't find the program file")
  APP_SetReturnCode(1) : WaitKey : Exit Function   
End If

The code attached to this post takes two OpenCL C source code files, and tries to build one program from them.

For demonstrative purposes, this operation fails - to show, how you can read back the OpenCL C errors. This is a thing you will experience soon, so better to get ready for it right now
If you would like to see successful compilation, just rename the kernel function in file bad.cl to something other than "good", which will fix the name conflict.

You will need the latest ThinBASIC and OpenCL headers to run it + of course modern GPU or CPU.


Petr