diff mbox series

[PULL,11/33] string-output-visitor: Fix to use sufficient precision

Message ID 20201219105532.1734134-12-armbru@redhat.com
State New
Headers show
Series [PULL,01/33] rocker: Revamp fp_port_get_info | expand

Commit Message

Markus Armbruster Dec. 19, 2020, 10:55 a.m. UTC 
The string output visitor should serialize numbers so that the string
input visitor deserializes them back to the same number.  It fails to
do so.

print_type_number() uses format %f.  This is prone to nasty rounding
errors.  For instance, numbers between 0 and 0.0000005 get flushed to
zero.

We currently use this visitor only for HMP info migrate, info network,
info qtree, and info memdev.  No double values occur there as far as I
can tell.

Fix anyway by formatting with %.17g.  17 decimal digits always suffice
for IEEE double.

See also recent commit "qobject: Fix qnum_to_string() to use
sufficient precision".

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20201210161452.2813491-9-armbru@redhat.com>
---
 qapi/string-output-visitor.c       | 2 +-
 tests/test-string-output-visitor.c | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)
diff mbox series

Patch

diff --git a/qapi/string-output-visitor.c b/qapi/string-output-visitor.c
index b74aa4d44c..5506c933de 100644
--- a/qapi/string-output-visitor.c
+++ b/qapi/string-output-visitor.c
@@ -258,7 +258,7 @@  static bool print_type_number(Visitor *v, const char *name, double *obj,
                               Error **errp)
 {
     StringOutputVisitor *sov = to_sov(v);
-    string_output_set(sov, g_strdup_printf("%f", *obj));
+    string_output_set(sov, g_strdup_printf("%.17g", *obj));
     return true;
 }
 
diff --git a/tests/test-string-output-visitor.c b/tests/test-string-output-visitor.c
index cec20848ea..0dae04b960 100644
--- a/tests/test-string-output-visitor.c
+++ b/tests/test-string-output-visitor.c
@@ -136,7 +136,7 @@  static void test_visitor_out_number(TestOutputVisitorData *data,
     visit_type_number(data->ov, NULL, &value, &error_abort);
 
     str = visitor_get(data);
-    g_assert_cmpstr(str, ==, "3.141593");
+    g_assert_cmpstr(str, ==, "3.1415926535897931");
 }
 
 static void test_visitor_out_string(TestOutputVisitorData *data,