Mac refusing to change hostname - macos

Question

I've been trying to figure out how to use postfix on my Mac and something has gone horribly wrong and I can't seem to fix it.
I believe the problem is related to starting Postfix.
Basically, the Mac seems to refuse to change its hostname. In bash, the user appears as "admin#(null)", if I type 'hostname' I'm given "(null)" also.
Changing the hostname in Sharing from System Preferences causes the second example to change (where it says, for example, "Other users can access shared folders on this computer, and administrators all volumes, at afp://null/ or “lion2”.") but the first stays as null.
I've even tried /etc/hostconfig manually setting hostname but nothing works.
Is there somewhere else the hostname is trying to be set but is perhaps corrupt? Or contains an invalid character or something?
This is causing Postfix not to work and report:
postfix: warning: valid_hostname: invalid character 40(decimal): (null)
postfix: fatal: unable to use my own hostname
Please, I really hope someone can help me fix this. I've been trying for hours now.
Cheers,
Scott

Answer 1

Have you tried scutil?
sudo scutil --get pref will show the current value and sudo scutil --set pref name will set the value to name. pref can be one of these:
ComputerName The user-friendly name for the system.
LocalHostName The local (Bonjour) host name.
HostName The name associated with hostname(1) and gethostname(3).
Here's what I get on my machine:
$ sudo scutil --get ComputerName
SteveBook2
$ sudo scutil --get LocalHostName
SteveBook2
$ sudo scutil --get HostName
HostName: not set

Answer 2

You should have tried running /bin/hostname directly to set the hostname on the unix/bsd layer, the values from scutil are SystemConfiguration settings which is a higher layer that unix is oblivious to.
It is perfectly normal for sudo scutil --get HostName to return not set even if running /bin/hostname shows you a hostname.
To set the hostname run sudo hostname Foo.bar (this is basically identical to sethostname() BSD call in the code given by another answer)
Optionally you could then run sudo scutil --set HostName Foo.bar to keep the SystemConfiguration settings in sync
NOTE:
The HostName in SystemConfiguration can be different from the LocalHostName and ComputerName , it can also be different from what /bin/hostname returns but it is best that they are all in sync, so you could also do :
sudo scutil --set LocalHostName Foo
sudo scutil --set ComputerName Foo

Answer 3

All other answers and help was much appreciated, however after much investigation, the problem appears to lie with my router and iMac: router not allowing the iMac to change its hostname client-side OR possibly sending a weird hostname to the iMac for it to use.

Answer 4

If you run this tiny program before starting postfix, does it work?
#include <unistd.h>
int main(int argc, char* argv[]) {
char host[] = "newhostname";
sethostname(host, sizeof(host));
return 0;
}
I don't presume to know what else might depend upon your old hostname -- do some testing on all your services after running this.
Details at: http://developer.apple.com/library/mac/#documentation/Darwin/Reference/ManPages/man3/gethostname.3.html
Update
To compile and run this little program, save its contents to a file (/tmp/newhostname.c will do) then run:
cd /tmp
make newhostname
sudo ./newhostname
At least I assume your make(1) knows how to compile from C sources to runnable binaries with default rules.
If the compiler isn't already installed maybe someone else will have a better idea.