Why is it difficult to write portable C programs?

Question

I want to know, why is it too hard to make a program run in many OSes, like Windows and Linux, without including glue code. They all share the same architecture (x86), so I

Answer 1

Obviously, some software requires OS specific features, but it's quite interesting to think what can be created in C without using such things:

• A C or C++ compiler (though not all of the standard library)
• An assembler for any assembly language
• A linker for the above
• An interpreter for a scripting language
• Text processing tools like sed, grep etc.
• A simple editor (anyone remember ed?)

In fact, all the basic development tools can be developed (and historically were developed) in C without using OS-specific features.