Read SSBO content returned by glMapBufferRange

Question

I want to retrieve the data from a storage buffer object. At first, I want to take advantage of glGetBufferSubData. But it seems that this function does not exist in opengl es, though it does exist in opengl. Then I fall back on glMapBufferRange.
It is perfect if I can interpret the information after retrieving the data from the buffer. But I think to do this I have to provide the exact layout, or even more information. For now, I do not need to do that much. What I want to do is that, under two different situations, I want to retrieve the buffer contents, and compare whether the contents are the same under the two situations.
The problem is, glMapBufferRange returns a void * type. I convert it to char * and compare each char in hex value:

char *buffercontent = (char *)(glMapBufferRange(GL_SHADER_STORAGE_BUFFER, 0, 80, GL_MAP_READ_BIT));  //just read the first 80 characters. I'm sure the buffer data length is at least 80.
for(int i=0;i<20;i++)
{
    printf("%X", *(buffercontent+i));
}
glUnmapBuffer(GL_SHADER_STORAGE_BUFFER);

I feel confused because I can only see less that 20 characters printed out. Why? What is the problem? I'm sure the buffersize is large from other infomation.

Answer 1

Make sure that you multiply 80 (the size of array) by 8, seeing as OpenGL counts via array size * size of variable type in bytes (in this case, a single char).