I have a shader which works on Mac OS X but not in iOS. I am passing an array of unsigned chars as the position attribute. In the shader, it's defined like this:

in lowp vec4position;

On iOS, I get this error:

Overflow in implicit constant conversion, minimum range for lowp float is (-2,2)

How should I define this attribute? Since they are unsigned chars, I want the values to range between 0 to 255.