Framebuffer writes

I suppose this is a silly question to the initiated, but how do I write
to the framebuffer?  I can get the address fine, but I've never
programmed in a protected memory environment (I program in MacOS, where
applications are free to overwrite the system heap ;-), so when I try to
do a write of any kind (*(startAddr + offset) = color) it segfaults. 
What is the routine to write like this?
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