What's the difference between %ul and %lu C format specifiers?

c format-specifiers
pedyram picture pedyram · May 25, 2014 · Viewed 59.9k times · Source

In an example of "C Primer Plus", the author has used %ul format specifier (in both scanf and printf) for unsigned long. When I try to generalize the problem, it seems that the %ul makes something wrong in my computer. But using %lu solved the issue.

Actually, rather than focusing on the problem and the line of codes, I want to know about the difference between %ul and %lu. Maybe I could figure out what's wrong.

Searching doesn't give me something useful (except that "they are different").

Any explanation or link/reference is appreciated.

Answer

Jonathon Reinhart picture Jonathon Reinhart · May 25, 2014

%lu is correct, while %ul is incorrect.

A printf format specifier follows the form %[flags][width][.precision][length]specifier.

u is a specifier meaning "unsigned decimal integer".

l is a length modifier meaning "long".

The length modifier should go before the conversion specifier, which means %lu is correct.

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