C Programming
Pointers, memory and the machine: the language that operating systems and embedded devices are written in.
Beginner
Start here. No background needed beyond school maths.
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Beginner
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Beej's Guide to C Programming
A friendly, complete guide to C that covers pointers, memory and strings plainly, with many small examples. Free to read online.
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Beginner
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C Reference (cppreference)
The standard online reference for the C language and its standard library, with a page for every header and function.
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Advanced
Once the basics are comfortable.