major stdio overhaul, using readv/writev, plus other changes

the biggest change in this commit is that stdio now uses readv to fill
the caller's buffer and the FILE buffer with a single syscall, and
likewise writev to flush the FILE buffer and write out the caller's
buffer in a single syscall.

making this change required fundamental architectural changes to
stdio, so i also made a number of other improvements in the process:

- the implementation no longer assumes that further io will fail
  following errors, and no longer blocks io when the error flag is set
  (though the latter could easily be changed back if desired)

- unbuffered mode is no longer implemented as a one-byte buffer. as a
  consequence, scanf unreading has to use ungetc, to the unget buffer
  has been enlarged to hold at least 2 wide characters.

- the FILE structure has been rearranged to maintain the locations of
  the fields that might be used in glibc getc/putc type macros, while
  shrinking the structure to save some space.

- error cases for fflush, fseek, etc. should be more correct.

- library-internal macros are used for getc_unlocked and putc_unlocked
  now, eliminating some ugly code duplication. __uflow and __overflow
  are no longer used anywhere but these macros. switch to read or
  write mode is also separated so the code can be better shared, e.g.
  with ungetc.

- lots of other small things.

1 files changed, 5 insertions, 47 deletions

diff --git a/src/stdio/__overflow.c b/src/stdio/__overflow.c
index e35104de..3bb37923 100644
--- a/src/stdio/__overflow.c
+++ b/src/stdio/__overflow.c
@@ -1,52 +1,10 @@
 #include "stdio_impl.h"
 
-static int overflow(FILE *f, int c)
+int __overflow(FILE *f, int _c)
 {
-	/* Initialize if we're not already writing */
-	if (!f->wend) {
-		/* Fail if we're in error state or unwritable. */
-		if (f->flags & (F_ERR|F_NOWR)) return EOF;
-
-		/* Set byte orientation -1,0=>-1; 1=>1 */
-		f->mode |= f->mode-1;
-
-		/* Clear read buffer (easier than summoning nasal demons) */
-		f->rpos = f->rend = f->rstop = 0;
-
-		/* Activate write through the buffer */
-		f->wpos = f->wbase = f->buf;
-		f->wend = f->buf + f->buf_size;
-		f->wstop = (f->lbf < 0) ? f->wend - 1 : 0;
-	}
-
-	/* Buffer can always hold at least 1 byte... */
-	if (c != EOF) {
-		*f->wpos++ = c;
-		if (f->wpos <= f->wstop && c != f->lbf) return c;
-	}
-	/* ...since if the next call fails, buffer is empty. */
-	if (f->write(f, f->wbase, f->wpos - f->wbase) < 0) {
-		f->flags |= F_ERR;
-		f->wpos = f->wbase = f->wend = f->wstop = 0;
-		return EOF;
-	}
-
-	/* Buffer is empty so reset position to beginning */
-	f->wpos = f->wbase;
-
+	unsigned char c = _c;
+	if (!f->wend && __towrite(f)) return EOF;
+	if (f->wpos < f->wend && c != f->lbf) return *f->wpos++ = c;
+	if (f->write(f, &c, 1)!=1) return EOF;
 	return c;
 }
-
-int __overflow(FILE *f, int c)
-{
-	return overflow(f, c & 0xff);
-}
-
-int __oflow(FILE *f)
-{
-	overflow(f, EOF);
-	return (f->flags & F_ERR) ? EOF : 0;
-}
-
-/* Link flush-on-exit code iff any stdio write functions are linked. */
-int (*const __fflush_on_exit)(FILE *) = fflush;