fix stderr, fdopen

[Eric Blake]

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POSIX requires "The stderr stream is expected to be open for reading and
writing".  This implements that (and if fd 2 is a write-only pipe instead
of a read-write tty, fgetc(stderr) will correctly fail later on); ok to
apply?

Also, for fdopen, POSIX states "Although not explicitly required by this
volume of IEEE Std 1003.1-2001, a good implementation of append (a) mode
would cause the O_APPEND flag to be set."  OK to apply?

I'm not quite sure whether POSIX requires the following example to remain
in append mode, or if it can go to seekable writes; but I'm assuming that
a stream must remain in append mode even if the underlying fd is altered.

remove("foo");
FILE *f = fopen("foo", "a");
fputs("hi", f);
fflush(f);
fcntl(fileno(f), F_SETFL, fcntl(fileno(f), F_GETFL) & ~O_APPEND);
fseek(f, 0, SEEK_SET);
fputs("bye", f);
fclose(f);
/* Does "foo" now contain "hibye", or just "bye"?  */

2007-07-13  Eric Blake  <ebb9@byu.net>

	More POSIX stream corner cases.
	* libc/stdio/findfp.c (__sinit): Open stderr read/write.
	* libc/stdio/fdopen.c (_fdopen_r): Set O_APPEND on fd when
	requested.
	* libc/stdio64/fdopen64.c (_fdopen64_r): Likewise.

- --
Don't work too hard, make some time for fun as well!

Eric Blake             ebb9@byu.net
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