[PATCH 09/11] stdio-common: Allow skipping initial bytes in __printf_buffer for %n

[Florian Weimer <fweimer@redhat.com>]

Making the written field signed allows subtraction of a prefix count
in case multiple writes happen to the same buffer.
---
 include/printf_buffer.h          |  8 +++++---
 stdio-common/tst-printf_buffer.c | 18 ++++++++++++++++++
 2 files changed, 23 insertions(+), 3 deletions(-)

diff --git a/include/printf_buffer.h b/include/printf_buffer.h
index a5483e8a97..2400371b94 100644
--- a/include/printf_buffer.h
+++ b/include/printf_buffer.h
@@ -92,8 +92,10 @@ struct __printf_buffer
      buffer).  Potentially updated on flush.  The actual number of
      written bytes also includes the unflushed-but-written buffer
      part, write_ptr - write_base.  A 64-bit value is used to avoid
-     the need for overflow checks.  */
-  uint64_t written;
+     the need for intermediate overflow checks.  Negative values can
+     be used to ignore leading parts of the buffer for %n computations
+     (for repeated __printf_buffer calls to the same buffer).  */
+  int64_t written;
 
   /* Identifies the flush callback.  */
   enum __printf_buffer_mode mode;
@@ -225,7 +227,7 @@ struct __wprintf_buffer
   wchar_t *write_base;
   wchar_t *write_ptr;
   wchar_t *write_end;
-  uint64_t written;
+  int64_t written;
   enum __wprintf_buffer_mode mode;
 };
 
diff --git a/stdio-common/tst-printf_buffer.c b/stdio-common/tst-printf_buffer.c
index d12da8c939..2a3ee511d0 100644
--- a/stdio-common/tst-printf_buffer.c
+++ b/stdio-common/tst-printf_buffer.c
@@ -132,6 +132,24 @@ do_test (void)
     TEST_VERIFY (!__printf_buffer_has_failed (&buf.base));
     TEST_COMPARE (__printf_buffer_asprintf_free (&buf), -1);
   }
+
+  /* Test %n with initial skip.  */
+  {
+    struct __printf_buffer_asprintf buf;
+    __printf_buffer_asprintf_init (&buf);
+    __printf_buffer (&buf.base, "abc");
+    buf.base.written = -2;
+    int n = -1;
+    __printf_buffer (&buf.base, "D%nEF", &n);
+    TEST_COMPARE (n, 2);        /* Two characters were skipped.  */
+    TEST_VERIFY (buf.base.write_base == buf.direct);
+    TEST_COMPARE_BLOB (buf.base.write_base,
+                       buf.base.write_ptr - buf.base.write_base, "abcDEF", 6);
+    TEST_VERIFY (!__printf_buffer_has_failed (&buf.base));
+    TEST_COMPARE (__printf_buffer_done (&buf.base), 4); /* Two skipped.  */
+    TEST_COMPARE (__printf_buffer_asprintf_free (&buf), -1);
+  }
+
   return 0;
 }
 
-- 
2.43.0