NOTES
This is a system call, and the operating system presumably knows its name, release and version. It also knows what hardware it runs on. So, four of the fields of the struct are meaningful. On the other hand, the field nodename is meaningless: it gives the name of the present machine in some undefined network, but typically machines are in more than one network and have several names. Moreover, the kernel has no way of knowing about such things, so it has to be told what to answer here. The same holds for the additional domainname field. To this end Linux uses the system calls sethostname(2) and setdomainname(2). Note that there is no standard that says that the hostname set by sethostname(2) is the same string as the nodename field of the struct returned by uname (indeed, some systems allow a 256-byte hostname and an 8-byte nodename), but this is true on Linux. The same holds for setdomainname(2) and the domainname field.
The length of the fields in the struct varies. Some operating systems or libraries use a hardcoded 9 or 33 or 65 or 257. Other systems use SYS_NMLN or _SYS_NMLN or UTSLEN or _UTSNAME_LENGTH. Clearly, it is a bad idea to use any of these constants - just use sizeof(...). Often 257 is chosen in order to have room for an internet hostname.
There have been three Linux system calls uname(). The first one used length 9, the second one used 65, the third one also uses 65 but adds the domainname field.
Part of the utsname information is also accessible via sysctl and via /proc/sys/kernel/{ostype, hostname, osrelease, version, domainname}.
"SEE ALSO"
uname(1), getdomainname(2), gethostname(2)
