cross platform - 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 think it should be simple. Furthermore, C is standardized, so why are implementations diferent in these OSes? Why is it hard to follow the standard and implement a universal library for all OSes?

I have programmed in C for almost two years, and currently I use glib for portability.

I'm not looking for a solution to this problem, I'm already using glib for this purpose. But I want to know why it is necessary, why it's dificult to make a C library without differences.

[ADD]

For example, when i write a program in C/Glib, i use the gchar/gint and so on types indeed the C primitive types, in wikipedia about glib, it says that :

"Features

On an elementary level GLib provides type definitions replacing the C primitive types char, int, float, and so on in order to improve portability"

Why C primitive types is not portable and the whole language?

Thanks for all the answers.

Answer 1

Making your basic C program that computes a little and prints results work is easy.

The problem is that any C program that does anything substantial is going to rely on a lot of services from the operating system, and those differ in many ways. File I/O tends to be similar, but when it comes to graphics, they differ a lot. And that is the crux of the problem.