Re: [9fans] non-truncating create

["Russ Cox" <rsc@swtch.com>]

> I'm 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 = 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'm thinking about the
> following code snippet:

Rc just tries the open, and if that fails, tries the create.
It doesn't even bother looking at the error code.
If the open failed due to permissions, so will the create.
Or use create with OEXCL if you really care.

> 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)?

Nope.  This is Plan 9.  Everyone has private /tmp directories.

> 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?

Probably not.  The interesting part of sysfatal is the
terminating the program, not the printing of the error.

Russ