libio: Update number of written bytes in dprintf implementation

The __printf_buffer_flush_dprintf function needs to record that
the buffer has been written before reusing it.  Without this
accounting, dprintf always returns zero.

Fixes commit 8ece45e4f5
("libio: Convert __vdprintf_internal to buffers").

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Tested-by: Carlos O'Donell <carlos@redhat.com>

libio/iovdprintf.c

@@ -54,6 +54,7 @@ __printf_buffer_flush_dprintf (struct __printf_buffer_dprintf *buf)
 	}
       p += ret;
     }
+  buf->base.written += buf->base.write_ptr - buf->base.write_base;
   buf->base.write_ptr = buf->buf;
 }
 

stdio-common/Makefile

@@ -180,6 +180,7 @@ tests := \
   tst-bz11319 \
   tst-bz11319-fortify2 \
   tst-cookie \
+  tst-dprintf-length \
   tst-fdopen \
   tst-ferror \
   tst-fgets \

stdio-common/tst-dprintf-length.c

@@ -0,0 +1,45 @@
+/* Test that dprintf returns the expected length.
+   Copyright (C) 2023 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <stdio.h>
+#include <string.h>
+#include <support/check.h>
+#include <sys/socket.h>
+#include <unistd.h>
+
+static int
+do_test (void)
+{
+  /* Use a datagram socket to check that everything arrives in one packet.
+     The dprintf function should perform a single write call.  */
+  int fds[2];
+  TEST_VERIFY_EXIT (socketpair (AF_LOCAL, SOCK_DGRAM, 0, fds) == 0);
+
+  TEST_COMPARE (dprintf (fds[0], "(%d)%s[%d]", 123, "---", 4567), 14);
+
+  char buf[32];
+  ssize_t ret = read (fds[1], buf, sizeof (buf));
+  TEST_VERIFY_EXIT (ret > 0);
+  TEST_COMPARE_BLOB (buf, ret, "(123)---[4567]", strlen ("(123)---[4567]"));
+
+  close (fds[1]);
+  close (fds[0]);
+  return 0;
+}
+
+#include <support/test-driver.c>