You are (most likely) not including the prototype. Use the following flags to get a good balance of warning messages:
/Kbperc
For C Set++, I use these flags as the approximate equivalent
/Wall /Wext- /Wgnr- /Wgot- /Wpor- /Wppt- /Wtrd /Wuni- /Wvft-
I have these set as part of my ICC string in CONFIG.SYS, so I don't have to add them to each compile. I have found that these settings give me a good combination of thorough warning messages without too many warnings.