File truncation

Sat Apr 15 08:24:00 GMT 2000

While testing I/O under win32 I discovered how FileDescriptor performs
truncation. Currently there is no explicit flag for truncation, and it
isn't done consistently.
Specifically, the Posix FileDescriptor::open method truncates when WRITE
is set but READ and APPEND are not:
 O_RDONLY O_WRONLY O_RDWR O_TRUNC O_APPEND O_EXCL
READ X
WRITE X X
READ|WRITE X
WRITE|APPEND X X
WRITE|EXCL X X
This happens to work because no other combination of flags is ever used
in java.io. But the implicit truncate isn't clear (at least to me) and
new ports are likely to get it wrong (as win32 did). Moreover, somebody
might sometime use FileDescriptor from native code in a way that it
isn't yet used, and get unintended side effects.
I propose adding a new flag to FileDescriptor.java:
 static final int TRUNC = 16;
and set it explicitly in FileOutputStream:
 fd = new FileDescriptor (path, (append
	? FileDescriptor.APPEND : FileDescriptor.TRUNC)
	| FileDescriptor.WRITE);
Any objections?
--
Jeff Sturm
jsturm@sigma6.com