Going to binary to hex, and back (in C)

i want to write a little pseudo assembler in C. (please don't tell me how stupid of an idea this is) Is there a built-in function in C that allows me to go from ascii chars --> binary. And then go from binary --> hex. Something along the lines of how atoi() works? I couldn't find anything, it would save me some time though.

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Learn Haskell to learn proper type notation.