Merge pull request #793 from cppalliance/to_chars_10s

Fix handling of trailing 0 in fixed format to_chars with unspecified precision

Author: mborland

fuzzing/Jamfile

@@ -36,7 +36,7 @@ toolset.flags $(__name__).make-corpus PYTHON <python.interpreter> ;
 
 for local fuzzer in $(all_fuzzers)
 {
-    local fuzz_time = 60 ;
+    local fuzz_time = 30 ;
     local corpus = /tmp/corpus/$(fuzzer) ;
     local min_corpus = /tmp/mincorpus/$(fuzzer) ;
     local seed_corpus = $(HERE)/seedcorpus/$(fuzzer) ;

include/boost/decimal/charconv.hpp

@@ -542,16 +542,16 @@ BOOST_DECIMAL_CONSTEXPR auto to_chars_fixed_impl(char* first, char* last, const
         {
             append_trailing_zeros = true;
         }
+    }
 
-        // In general formatting we remove trailing 0s
-        if (fmt == chars_format::general)
-        {
-
-            const auto zeros_removal {remove_trailing_zeros(significand)};
-            significand = zeros_removal.trimmed_number;
-            exponent += static_cast<int>(zeros_removal.number_of_removed_zeros);
-            num_dig -= static_cast<int>(zeros_removal.number_of_removed_zeros);
-        }
+    // In general formatting we remove trailing 0s
+    // Same with unspecified precision fixed formatting
+    if ((precision == -1 && fmt == chars_format::fixed) || fmt == chars_format::general)
+    {
+        const auto zeros_removal {remove_trailing_zeros(significand)};
+        significand = zeros_removal.trimmed_number;
+        exponent += static_cast<int>(zeros_removal.number_of_removed_zeros);
+        num_dig -= static_cast<int>(zeros_removal.number_of_removed_zeros);
     }
 
     // Make sure the result will fit in the buffer
@@ -569,25 +569,32 @@ BOOST_DECIMAL_CONSTEXPR auto to_chars_fixed_impl(char* first, char* last, const
             *first++ = '0';
             return {first, std::errc()};
         }
-        else if (num_leading_zeros > precision)
+        else if (precision != -1 && num_leading_zeros > precision)
         {
             *first++ = '0';
             *first++ = '.';
-            std::memset(first, '0', static_cast<std::size_t>(precision));
+            boost::decimal::detail::memset(first, '0', static_cast<std::size_t>(precision));
             return {first + precision, std::errc()};
         }
         else
         {
             *first++ = '0';
             *first++ = '.';
-            std::memset(first, '0', static_cast<std::size_t>(num_leading_zeros));
+            boost::decimal::detail::memset(first, '0', static_cast<std::size_t>(num_leading_zeros));
             first += num_leading_zeros;
 
             // We can skip the rest if there's nothing more to do for the required precision
             if (significand == 0)
             {
-                std::memset(first, '0', static_cast<std::size_t>(precision - num_leading_zeros));
-                return {first + precision, std::errc()};
+                if (precision - num_leading_zeros > 0)
+                {
+                    boost::decimal::detail::memset(first, '0', static_cast<std::size_t>(precision - num_leading_zeros));
+                    return {first + precision, std::errc()};
+                }
+                else
+                {
+                    return {first, std::errc()};
+                }
             }
         }
     }

include/boost/decimal/detail/memcpy.hpp

@@ -23,6 +23,7 @@
 #if defined(__GNUC__) && __GNUC__ >= 10
 #  pragma GCC diagnostic push
 #  pragma GCC diagnostic ignored "-Wstringop-overflow"
+#  pragma GCC diagnostic ignored "-Warray-bounds"
 #  define BOOST_DECIMAL_STRINGOP_OVERFLOW_DISABLED
 #endif
 

test/Jamfile

@@ -45,6 +45,7 @@ project : requirements
 run-fail benchmarks.cpp ;
 run compare_dec128_and_fast.cpp ;
 compile-fail concepts_test.cpp ;
+run crash_report_1.cpp ;
 run github_issue_426.cpp ;
 run github_issue_448.cpp ;
 run-fail github_issue_519.cpp ;

test/crash_report_1.cpp

@@ -0,0 +1,27 @@
+// Copyright 2025 Matt Borland
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/decimal.hpp>
+#include <boost/core/lightweight_test.hpp>
+#include <iostream>
+
+using namespace boost::decimal;
+
+int main()
+{
+    char buffer[64]{};
+    const auto print_val {0.000001_df};
+
+    const auto r = boost::decimal::to_chars(
+            buffer,
+            buffer + sizeof(buffer),
+            print_val,
+            boost::decimal::chars_format::fixed
+    );
+    *r.ptr = '\0';
+
+    BOOST_TEST_CSTR_EQ(buffer, "0.000001");
+
+    return boost::report_errors();
+}

test/test_to_chars.cpp

@@ -23,7 +23,7 @@ static constexpr auto N = static_cast<std::size_t>(1024U >> 4U); // Number of tr
 #if !defined(BOOST_DECIMAL_DISABLE_CLIB) && !(defined(__GNUC__) && __GNUC__ >= 13 && !defined(__aarch64__))
 
 template <typename T>
-void test_value(T val, const char* result, chars_format fmt, int precision = -1)
+void test_value(T val, const char* result, chars_format fmt, int precision)
 {
     char buffer[256] {};
     auto r = to_chars(buffer, buffer + sizeof(buffer), val, fmt, precision);
@@ -32,6 +32,16 @@ void test_value(T val, const char* result, chars_format fmt, int precision = -1)
     BOOST_TEST_CSTR_EQ(result, buffer);
 }
 
+template <typename T>
+void test_value(T val, const char* result, chars_format fmt)
+{
+    char buffer[256] {};
+    auto r = to_chars(buffer, buffer + sizeof(buffer), val, fmt);
+    *r.ptr = '\0';
+    BOOST_TEST(r);
+    BOOST_TEST_CSTR_EQ(result, buffer);
+}
+
 template <typename T>
 void test_value(T val, const char* result)
 {
@@ -764,6 +774,25 @@ void test_777()
     test_value(value3, "-2111000000", chars_format::fixed, 0);
 }
 
+template <typename T>
+void test_more_powers_10()
+{
+    test_value(T{1, -6}, "0.000001", chars_format::fixed);
+    test_value(T{1, -5}, "0.00001", chars_format::fixed);
+    test_value(T{1, -4}, "0.0001", chars_format::fixed);
+    test_value(T{1, -3}, "0.001", chars_format::fixed);
+    test_value(T{1, -2}, "0.01", chars_format::fixed);
+    test_value(T{1, -1}, "0.1", chars_format::fixed);
+    test_value(T{1, 0}, "1", chars_format::fixed);
+    test_value(T{1, 1}, "10", chars_format::fixed);
+    test_value(T{1, 2}, "100", chars_format::fixed);
+    test_value(T{1, 3}, "1000", chars_format::fixed);
+    test_value(T{1, 4}, "10000", chars_format::fixed);
+    test_value(T{1, 5}, "100000", chars_format::fixed);
+    test_value(T{1, 6}, "1000000", chars_format::fixed);
+    test_value(T{1, 7}, "10000000", chars_format::fixed);
+}
+
 int main()
 {
     test_non_finite_values<decimal32>();
@@ -874,6 +903,10 @@ int main()
     test_777<decimal64_fast>();
     test_777<decimal128_fast>();
 
+    test_more_powers_10<decimal32>();
+    test_more_powers_10<decimal64>();
+    test_more_powers_10<decimal128>();
+
     return boost::report_errors();
 }