| Function | Classification | Cancel | ISR | Signal | Thread |
|---|---|---|---|---|---|
| ualarm() | POSIX 1003.1 XSI | No | No | Yes | Yes |
| ultoa() | QNX 4 | No | Yes | Yes | Yes |
| ulltoa() | Unix | No | Yes | Yes | Yes |
| umask() | POSIX 1003.1 | No | No | Yes | Yes |
| umount() | QNX Neutrino | Yes | No | No | Yes |
| UNALIGNED_PUT16() | QNX Neutrino | No | Yes | Yes | Yes |
| UNALIGNED_PUT32() | QNX Neutrino | No | Yes | Yes | Yes |
| UNALIGNED_PUT64() | QNX Neutrino | No | Yes | Yes | Yes |
| UNALIGNED_RET16() | QNX Neutrino | No | Yes | Yes | Yes |
| UNALIGNED_RET32() | QNX Neutrino | No | Yes | Yes | Yes |
| UNALIGNED_RET64() | QNX Neutrino | No | Yes | Yes | Yes |
| uname() | POSIX 1003.1 | No | No | Yes | Yes |
| ungetc() | ANSI POSIX 1003.1 | Yes | No | No | Yes |
| ungetwc() | ANSI POSIX 1003.1 | Yes | No | No | Yes |
| unlink() | POSIX 1003.1 | Yes | No | Yes | Yes |
| unlockpt() | POSIX 1003.1 XSI | No | No | Yes | Yes |
| unsetenv() | POSIX 1003.1 | No | No | Yes | Yes |
| usleep() | POSIX 1003.1 XSI | Yes | No | Yes | Yes |
| utime() | POSIX 1003.1 | No | No | Yes | Yes |
| utimes() | POSIX 1003.1 XSI | No | No | Yes | Yes |
| utmpname() | Unix | Yes | No | Yes | Yes |
| utoa() | QNX 4 | No | Yes | Yes | Yes |