man page command: man 3 vs. man [migrated]

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http://stackoverflow.com – I'm wondering what's the difference between "man 3 command" vs "man command"? I read in wiki (http://en.wikipedia.org/wiki/Man_page) that man 3 is used in Linux in section 3 of the man pages. And section 3 refers to Library functions, covering in particular the C standard library. I tried "man 3 ls" and got this: No entry for ls in section 3 of the manual I wonder when to use man 3 and when to use man and do they serve difference purposes? Thanks. (HowTos)