From mboxrd@z Thu Jan  1 00:00:00 1970
Message-ID: <88ee35789aeb1fdcfb4491b2b83a95b4@plan9.jp>
To: 9fans@cse.psu.edu
From: Joel “chesky” Salomon <JoelCSalomon@Gmail.com>
Date: Sun,  8 Oct 2006 23:13:28 -0400
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Subject: [9fans] non-truncating create
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Still for the Brain-Dead SHell homework assignment...

I=E2=80=99m trying to implement output redirection, > and >>.  Truncating
redirect (>) is easy; just use create(2) whether the file exists or
no.  The non-truncing option is puzzling me, though.  If the file
exists, I need to o =3D open(oname, OWRITE), then seek(o, 0, 2); if the
file does not exist I need to create it.

How do I check for the existence of a file?  I=E2=80=99m thinking about t=
he
following code snippet:
	o =3D open(oname, OWRITE)
	rerrstr(errbuf, ERRMAX);
	if((strstr(errbuf, "file does not exist") =3D=3D 0){
		complain("cannot open %s: %r", oname);
		return;
	}else	o =3D create(oname, OWRITE, ~0);
	if(o < 0)
		...
Is there a cleaner way to accomplish this?  Some form of stat(2) that
has an explicit =E2=80=9Cfile does not exist=E2=80=9D code outside parsin=
g errstr

Of secondary interest: Not that this is production code, but should I
be concerned about race conditions where the file is created by
somebody else between the failed open(2) and the create(2)?

Of tertiary interest: Is there a general use for a library function
complain() that behaves identically to sysfatal(2) except for not
terminating the program?


--Joel