| Unlock | System.Posix System.Posix.IO |
| UserEntry | System.Posix System.Posix.User |
| unblockSignals | System.Posix |
| undl | System.Posix.DynamicLinker |
| unionFileModes | System.Posix System.Posix.Files |
| unsetEnv | System.Posix System.Posix.Env |
| urgentDataAvailable | System.Posix |
| userDefinedSignal1 | System.Posix |
| userDefinedSignal2 | System.Posix |
| userGroupID | System.Posix System.Posix.User |
| userID | System.Posix System.Posix.User |
| userName | System.Posix System.Posix.User |
| userShell | System.Posix System.Posix.User |
| userTime | System.Posix System.Posix.Process |
| usleep | System.Posix System.Posix.Unistd |
