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Remove JSON_HEDLEY_NON_NULL(2) from write_characters() overrides

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output_vector_adapter, output_stream_adapter and output_string_adapter
declared their write_characters(const CharType*, std::size_t) override
JSON_HEDLEY_NON_NULL(2), but binary_writer legitimately calls it with
a null pointer and length 0 for an empty string or binary value; the
type-erased call path only stayed silent under UBSan because the
static callee at those call sites is the unattributed virtual base.
A nonnull attribute on a definition lets GCC and Clang assume the
parameter is non-null inside the function body even when the call is
virtual, so this was latent undefined behavior, not just style.

Drop the attribute from the three overrides and document the
(nullptr, 0) contract on output_adapter_protocol::write_characters.
unit-cbor, unit-msgpack, unit-bson and unit-bon8 (which all exercise
empty binary/string payloads through the stream and vector/string
adapters) pass under -fsanitize=address,undefined,nonnull-attribute.

Part of #5709

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-09-30 10:00:12 +02:00
parent a0fe86d087
commit 83b3bc6831
2 changed files with 8 additions and 6 deletions
Download Patch File Download Diff File Expand all files Collapse all files
include/nlohmann/detail/output/output_adapters.hpp
+4 -3
View File
@@ -29,6 +29,10 @@ namespace detail
template<typename CharType> struct output_adapter_protocol
{
virtual void write_character(CharType c) = 0;
/// @param[in] s pointer to the characters to write; binary_writer legitimately
/// passes a null pointer together with length 0 for an empty
/// string or binary value, so implementations must tolerate that
/// @param[in] length number of characters at @a s
virtual void write_characters(const CharType* s, std::size_t length) = 0;
virtual ~output_adapter_protocol() = default;
@@ -95,7 +99,6 @@ class output_vector_adapter : public output_adapter_protocol<CharType>
sink.write_character(c);
}
JSON_HEDLEY_NON_NULL(2)
void write_characters(const CharType* s, std::size_t length) override
{
sink.write_characters(s, length);
@@ -120,7 +123,6 @@ class output_stream_adapter : public output_adapter_protocol<CharType>
stream.put(c);
}
JSON_HEDLEY_NON_NULL(2)
void write_characters(const CharType* s, std::size_t length) override
{
stream.write(s, static_cast<std::streamsize>(length));
@@ -145,7 +147,6 @@ class output_string_adapter : public output_adapter_protocol<CharType>
str.push_back(c);
}
JSON_HEDLEY_NON_NULL(2)
void write_characters(const CharType* s, std::size_t length) override
{
str.append(s, length);
single_include/nlohmann/json.hpp
+4 -3
View File
@@ -20168,6 +20168,10 @@ namespace detail
template<typename CharType> struct output_adapter_protocol
{
virtual void write_character(CharType c) = 0;
/// @param[in] s pointer to the characters to write; binary_writer legitimately
/// passes a null pointer together with length 0 for an empty
/// string or binary value, so implementations must tolerate that
/// @param[in] length number of characters at @a s
virtual void write_characters(const CharType* s, std::size_t length) = 0;
virtual ~output_adapter_protocol() = default;
@@ -20234,7 +20238,6 @@ class output_vector_adapter : public output_adapter_protocol<CharType>
sink.write_character(c);
}
JSON_HEDLEY_NON_NULL(2)
void write_characters(const CharType* s, std::size_t length) override
{
sink.write_characters(s, length);
@@ -20259,7 +20262,6 @@ class output_stream_adapter : public output_adapter_protocol<CharType>
stream.put(c);
}
JSON_HEDLEY_NON_NULL(2)
void write_characters(const CharType* s, std::size_t length) override
{
stream.write(s, static_cast<std::streamsize>(length));
@@ -20284,7 +20286,6 @@ class output_string_adapter : public output_adapter_protocol<CharType>
str.push_back(c);
}
JSON_HEDLEY_NON_NULL(2)
void write_characters(const CharType* s, std::size_t length) override
{
str.append(s, length);