<unistd.h>:

lssek:   Probably should be __wur but lseek(fd,SEEK_SET,0) will succeed if
         the descriptor is fine.
lseek64: same

setuid:  will always succeed given correct privileges, so there might
         be places which don't check for it.
setreuid: same
seteuid: same
setgid:  same
setregid: same
setegid: same
setresuid: same
setresgid: same


<stdio.h>:

setvbuf:   if stream and buffer are fine and other parameters constant,
           it cannot really fail.
fseek:     see lseek
fseeko:    likewise
fgetpos:   similarly
fsetpos:   likewise