# GLSL function parameters

In GLSL: Common Mistakes it states:

Functions parameters must be declared with the in, out, or inout qualifiers.

but I have never declared function parameters with in, out, or inout qualifiers and I have never run into any problems. Could this be because of my specific graphics card? What are the side effects of not using the in, out, or inout qualifiers?

Info: I am using GLSL 3.3 and I have an Intel graphics card