From 54addb01d8c2511ef96b0f0ca6b695d120dd8363 Mon Sep 17 00:00:00 2001
From: Markus Armbruster <armbru@redhat.com>
Date: Thu, 10 Dec 2020 17:14:50 +0100
Subject: [PATCH] string-output-visitor: Fix to use sufficient precision
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 --git a/qapi/string-output-visitor.c b/qapi/string-output-visitor.c
index b74aa4d44c4..5506c933dee 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 cec20848ea2..0dae04b9603 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,
--
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