What's the Difference Between %Ul and %Lu C Format Specifiers?

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.

1 Answer

%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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Alexander Ross

Alexander Ross

Gaming, Esports & Interactive Media Writer

Alexander Ross has covered the video game industry for a decade, writing deep dives on game design, esports tournaments, VR developments, and gaming culture.

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