It searches for a keyword in the manpages and tells you what page contains the text. $ apropos printf fmtcheck(3) - sanitizes user-supplie

It searches for a keyword in the manpages and tells you what page contains the text. $ apropos printf fmtcheck(3) - sanitizes user-supplied printf(3)-style format string format(ntcl) - Format a string in the style of sprintf fwkpfv(1) - FireWire kprintf viewer fwprintf(3), swprintf(3), vfwprintf(3), vswprintf(3), vwprintf(3), wprintf(3) - formatted wide character output conversion printf(1) - formatted output printf(3), fprintf(3), sprintf(3), snprintf(3), asprintf(3), dprintf(3), vprintf(3), vfprintf(3), vsprintf(3), vsnprintf(3), vasprintf(3), vdprintf(3) - formatted output conversion wprintf_l(3), fwprintf_l(3), swprintf_l(3), vwprintf_l(3), vfwprintf_l(3), vswprintf_l(3) - formatted wide character output conversion