Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!uunet!hsi!stevens
From: stevens@hsi.UUCP (Richard Stevens)
Newsgroups: comp.unix.questions
Subject: Re: fopen ( ..., "a" ) --- how does the "a" work?
Summary: O_APPEND flag to open
Message-ID: <895@hsi86.hsi.UUCP>
Date: 7 Dec 89 02:33:14 GMT
References: <3250@hub.UUCP>
Organization: Health Systems Intl., New Haven, CT
Lines: 25

In article <3250@hub.UUCP>, 6600pete@hub.UUCP writes:
> When one opens a file under *most* flavors of UN*X (I realize this is
> the kind of thing that will be system-dependent, though it oughtn't)
> with fopen ( ..., "a" ), the file mark is supposed to be moved to EOF
> before every write. Now, how is this done? Are there two system calls,
> one to move the file mark and one to do the write, or is there one
> system call, "append"?

With System V Release 2, fopen specifies the O_APPEND flag to the
open system call if you specify the "a" mode.  This has the kernel
move the inode's read/write offset to the end of the file every
time you write to the file.  Hence only one system call is required.
I suspect the later release of System V also do this.

Interesting, however, is that the 4.3BSD source differs.  It does
an lseek to the EOF when fopen is called, and that's it.  4.3 does
have an O_APPEND option to open, but it doesn't appear to be used.
The 4.3 man page for fopen also doesn't go to the lengths that
the system V man page does specifying that "a" really means that
every write gets appended, regardless of the file's current position.

	Richard Stevens
	Health Systems International, New Haven, CT
	   stevens@hsi.com
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