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Document that a NUL byte in the input is treated as end of input (#5534)

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* docs: document that a NUL byte in the input is treated as end of input

A NUL byte anywhere in the input - trailing, or embedded ahead of more
otherwise well-formed JSON - is currently treated the same as genuine
end of input, so parsing silently stops there instead of raising the
parse_error.101 any other unexpected byte triggers. This mirrors the
NUL-terminated-C-string convention already used when no explicit input
length is given (json::parse(const char*) already stops at strlen()),
just applied uniformly rather than only when a length is genuinely
unavailable.

This behavior predates this change and is not being altered here -
changing it would be an observable, backwards-incompatible behavior
change for any caller that (knowingly or not) depends on it, which is
not something to do silently in a patch. Documenting the current,
verified behavior as a new FAQ entry instead, so it's an intentional
and discoverable part of the contract rather than a surprise.

Fixes #5530.

Signed-off-by: Niels Lohmann <niels.lohmann@gmail.com>
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N4RQ1Ahan5YAGbnAQGjZTY

* Add JSON_STRICT_NUL_HANDLING opt-in macro for issue #5530

A NUL byte anywhere in the input is currently treated the same as real
end of input, rather than raising parse_error.101 like any other
unexpected byte (documented in the previous commit's FAQ entry). A full
unconditional fix was tried in PR #5532 but rejected as too risky to
ship by default: any caller could depend on the current behavior, even
unknowingly (e.g. a zero-padded buffer). On PR #5534, gregmarr proposed
a compile-time opt-in flag instead, and the maintainer agreed, wanting
it available now and defaulting to the corrected behavior in 4.0.0.

This mirrors the existing JSON_BRACE_INIT_COPY_SEMANTICS precedent as
closely as sensible:
- JSON_STRICT_NUL_HANDLING defaults to 0 (off); the three lexer sites
  that treat '\0' as EOF/comment-terminator are gated with
  `#if !JSON_STRICT_NUL_HANDLING` so the default-off behavior is
  byte-for-byte identical to today's.
- input_adapters.hpp's `T (&array)[N]` overload additionally trims a
  single trailing '\0' from a `char` array (e.g. a string literal like
  `json::parse("123")`) when the macro is on, so that case keeps
  working; every other element type (unsigned char, std::uint8_t, ...)
  always keeps its full extent. This intentionally does *not* reuse the
  existing strlen()-based pointer overload via SFINAE-excluding `char`
  from the array overload, as originally sketched for this change: that
  approach is ambiguous against the newer generic container overload
  added since PR #5532, and even where it compiles, strlen()-scanning a
  `char` array that is not NUL-terminated within its bounds reads past
  the end of the array (confirmed with AddressSanitizer). Trimming only
  a single trailing byte, without scanning, avoids both problems.
- Documented via docs/mkdocs/docs/api/macros/json_strict_nul_handling.md,
  linked from the macros index/nav/features page, the FAQ entry, and
  the parse/accept/operator>> reference pages.
- Tested in unit-class_parser.cpp and unit-deserialization.cpp, default
  state unguarded and opt-in state guarded. Since the library itself
  #undefs the macro at the end of json.hpp (as JSON_BRACE_INIT_COPY_SEMANTICS
  already does), a plain `#if defined(JSON_STRICT_NUL_HANDLING)` guard
  after the include never actually triggers; the tests instead capture
  the command-line value into a test-local macro before including the
  header. A few pre-existing fixtures elsewhere (std::array<uint8_t, N>
  sized one larger than their literal, relying on value-initialization
  to silently add a trailing zero byte) needed the same one-byte
  adjustment to keep passing under the opt-in behavior.

Unlike the precedent, this adds a proper `JSON_StrictNulHandling` CMake
option (rather than a raw -DCMAKE_CXX_FLAGS injection) and wires its
ci_test_strict_nul_handling target into the ci_cmake_options job matrix
in .github/workflows/ubuntu.yml, so the opt-in build is actually
exercised in CI -- closing the one gap in the precedent's own CI setup
(ci_test_brace_init_copy_semantics is defined but never referenced by
any workflow, so it has never actually run).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Clarify where JSON_STRICT_NUL_HANDLING does not reject NUL bytes

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

---------

Signed-off-by: Niels Lohmann <niels.lohmann@gmail.com>
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>
This commit is contained in:
Niels Lohmann
2026-09-24 06:55:21 +02:00
committed by GitHub
co-authored by Claude Sonnet 5
parent f751547a81
commit ed513715a8
20 changed files with 475 additions and 11 deletions
Download Patch File Download Diff File Expand all files Collapse all files
docs/mkdocs/docs/api/basic_json/accept.md
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@@ -90,6 +90,10 @@ Linear in the length of the input. The parser is a predictive LL(1) parser.
A UTF-8 byte order mark is silently ignored.
By default, a `'\0'` (NUL) byte anywhere in the input is treated as end of input, rather than as an ordinary (and,
outside of a string, invalid) byte; see the [FAQ entry](../../home/faq.md#nul-bytes-in-the-input) for details and the
[`JSON_STRICT_NUL_HANDLING`](../macros/json_strict_nul_handling.md) macro to opt into rejecting it instead.
## Examples
??? example
@@ -111,6 +115,8 @@ A UTF-8 byte order mark is silently ignored.
- [parse](parse.md) - deserialize from a compatible input
- [sax_parse](sax_parse.md) - parse input using the SAX interface
- [operator>>](../operator_gtgt.md) - deserialize from stream
- [`JSON_STRICT_NUL_HANDLING`](../macros/json_strict_nul_handling.md) - opt in to rejecting a NUL byte in the input
instead of treating it as end of input
## Version history
@@ -120,6 +126,8 @@ A UTF-8 byte order mark is silently ignored.
- Added `ignore_trailing_commas` in version 3.13.0.
- Extended container support (1) to include types with lvalue-only ADL `begin`/`end` (matching `std::begin`/`std::end` semantics) in version 3.13.0.
- Extended overload (2) to accept heterogeneous iterator+sentinel pairs (C++20 ranges support) in version 3.13.0.
- `JSON_STRICT_NUL_HANDLING` added in version 3.13.0 to optionally reject a NUL byte in the input instead of treating
it as end of input; planned to become the default in version 4.0.0.
!!! warning "Deprecation"
docs/mkdocs/docs/api/basic_json/parse.md
+8
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@@ -103,6 +103,10 @@ A UTF-8 byte order mark is silently ignored.
Invalid Unicode escapes and unpaired surrogates in the input are reported as
[`parse_error.101`](../../home/exceptions.md#jsonexceptionparse_error101) with a detailed message.
By default, a `'\0'` (NUL) byte anywhere in the input is treated as end of input, rather than as an ordinary (and,
outside of a string, invalid) byte; see the [FAQ entry](../../home/faq.md#nul-bytes-in-the-input) for details and the
[`JSON_STRICT_NUL_HANDLING`](../macros/json_strict_nul_handling.md) macro to opt into rejecting it instead.
## Examples
??? example "Parsing from a character array"
@@ -236,6 +240,8 @@ Invalid Unicode escapes and unpaired surrogates in the input are reported as
- [accept](accept.md) - check if the input is valid JSON
- [sax_parse](sax_parse.md) - parse input using the SAX interface
- [operator>>](../operator_gtgt.md) - deserialize from stream
- [`JSON_STRICT_NUL_HANDLING`](../macros/json_strict_nul_handling.md) - opt in to rejecting a NUL byte in the input
instead of treating it as end of input
## Version history
@@ -246,6 +252,8 @@ Invalid Unicode escapes and unpaired surrogates in the input are reported as
- Added `ignore_trailing_commas` in version 3.13.0.
- Extended container support (1) to include types with lvalue-only ADL `begin`/`end` (matching `std::begin`/`std::end` semantics) in version 3.13.0.
- Extended overload (2) to accept heterogeneous iterator+sentinel pairs (C++20 ranges support) in version 3.13.0.
- `JSON_STRICT_NUL_HANDLING` added in version 3.13.0 to optionally reject a NUL byte in the input instead of treating
it as end of input; planned to become the default in version 4.0.0.
!!! warning "Deprecation"
docs/mkdocs/docs/api/macros/index.md
+5
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@@ -14,6 +14,11 @@ header. See also the [macro overview page](../../features/macros.md).
- [**JSON_DIAGNOSTIC_POSITIONS**](json_diagnostic_positions.md) - access positions of elements
- [**JSON_NOEXCEPTION**](json_noexception.md) - switch off exceptions
## Parsing
- [**JSON_STRICT_NUL_HANDLING**](json_strict_nul_handling.md) - opt in to rejecting a NUL byte in the input instead of
treating it as end of input
## Language support
- [**JSON_HAS_CPP_11**<br>**JSON_HAS_CPP_14**<br>**JSON_HAS_CPP_17**<br>**JSON_HAS_CPP_20**](json_has_cpp_11.md) - set supported C++ standard
docs/mkdocs/docs/api/macros/json_strict_nul_handling.md
+126
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@@ -0,0 +1,126 @@
# JSON_STRICT_NUL_HANDLING
```cpp
#define JSON_STRICT_NUL_HANDLING /* value */
```
When defined to `1`, a `'\0'` (NUL) byte in JSON text input is rejected with `parse_error.101`, like any other
unexpected byte, instead of being silently treated as end of input.
The macro only affects the JSON text parser ([`parse`](../basic_json/parse.md), [`accept`](../basic_json/accept.md),
[`sax_parse`](../basic_json/sax_parse.md), and [`operator>>`](../operator_gtgt.md)). There are three cases where a NUL
byte is still not rejected:
- The binary formats ([`from_bjdata`](../basic_json/from_bjdata.md), [`from_bson`](../basic_json/from_bson.md),
[`from_cbor`](../basic_json/from_cbor.md), [`from_msgpack`](../basic_json/from_msgpack.md),
[`from_ubjson`](../basic_json/from_ubjson.md)) are never affected: there, `0x00` is ordinary data.
- A bare `const char*` pointer has no length of its own, so its length is still determined with `strlen()`. The first
NUL byte therefore still marks the end of the input, and nothing after it is read.
- One trailing `'\0'` at the end of a `char` array (e.g., a string literal) is trimmed; see the warning below.
## Default definition
The default value is `0` (disabled — existing behavior is preserved).
```cpp
#define JSON_STRICT_NUL_HANDLING 0
```
## Notes
!!! note "Background"
By default, a `'\0'` byte anywhere in the input is treated the same as the real end of the input, rather than as
an ordinary (and, outside of a string, invalid) byte. Everything from that byte onward is silently ignored,
without a parse error - including further, otherwise well-formed JSON:
```cpp
json::parse(std::string("123") + '\0'); // == 123, no error
json::parse(std::string("123") + '\0' + "true"); // == 123, the "true" is silently ignored too
```
This falls out of the same convention used when no explicit input length is given at all: parsing from a
`const char*` already stops at the first NUL byte via `strlen()`, since a bare pointer has no length of its own.
The library applies that same NUL-terminated-C-string convention uniformly, rather than only when a length is
genuinely unavailable - so a `std::string`, iterator range, or container whose content happens to include a NUL
byte is affected the same way a raw `const char*` would be (see the
[FAQ entry](../../home/faq.md#nul-bytes-in-the-input) for a fuller explanation).
This was not fixed unconditionally, because doing so is backwards-incompatible for any caller who happens to
depend on the current behavior - even unknowingly, for instance because their input already contains trailing
padding they never noticed was being discarded (see [#5530](https://github.com/nlohmann/json/issues/5530)).
This macro instead offers an opt-in path to the corrected behavior ahead of version 4.0.0, where it is planned to
become the default.
!!! warning "Opt-in only"
This macro must be defined **before** including `<nlohmann/json.hpp>`. Defining it after the include has no
effect.
Enabling it also changes how a `char` array (including a string literal, e.g. `json::parse("123")`) is read: such
an array normally carries a trailing `'\0'` contributed by the compiler, not by the source text. With this macro
enabled, that one trailing byte is trimmed if present so that parsing a string literal keeps working; every other
byte in the array - including any `'\0'` that is not the very last element - is read as real data and rejected
like any other unexpected byte. Arrays of any other element type (`unsigned char`, `std::uint8_t`, ...), as used
for CBOR or MessagePack, are never affected by this trimming; their full extent - including a genuine trailing
`0x00` - is always preserved, in both states of this macro.
!!! tip "Workaround without the macro"
To reject a NUL byte without enabling this macro, trim your input yourself before calling `parse()`:
```cpp
s.resize(s.find('\0')); // drop everything from the first NUL onward, if any
json::parse(s);
```
## Examples
??? example "Default behavior (macro not defined)"
Without the macro, a NUL byte silently ends parsing at that point:
```cpp
#include <nlohmann/json.hpp>
using json = nlohmann::json;
int main()
{
json j = json::parse(std::string("123") + '\0' + "true");
// j is 123 -- the '\0' and everything after it is silently ignored
}
```
??? example "Opt-in strict handling (macro defined to 1)"
With the macro, a NUL byte is rejected like any other unexpected byte:
```cpp
#define JSON_STRICT_NUL_HANDLING 1
#include <nlohmann/json.hpp>
using json = nlohmann::json;
int main()
{
json j = json::parse(std::string("123") + '\0' + "true");
// throws parse_error.101 -- the NUL byte is now invalid input,
// exactly like any other unexpected trailing byte
json ok = json::parse("123");
// ok is 123 -- parsing from a string literal still works
}
```
## See also
- [FAQ: NUL bytes in the input](../../home/faq.md#nul-bytes-in-the-input)
- [**parse**](../basic_json/parse.md) - deserialize from a compatible input
- [**accept**](../basic_json/accept.md) - check if the input is valid JSON
- [**operator>>**](../operator_gtgt.md) - deserialize from stream
## Version history
- Added in version 3.13.0.
- Planned to become the default (with the macro removed) in version 4.0.0.
docs/mkdocs/docs/api/operator_gtgt.md
+11
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@@ -72,6 +72,13 @@ input >> j2; // parses the next value
Note that reading concatenated values does **not** work for [JSON Lines](../features/parsing/json_lines.md)
(newline-delimited JSON) input -- see that page for why and for the recommended alternative.
By default, a `'\0'` (NUL) byte encountered while reading a value is treated as end of input, rather than as an
ordinary (and, outside of a string, invalid) byte; see the [FAQ entry](../home/faq.md#nul-bytes-in-the-input) for
details and the [`JSON_STRICT_NUL_HANDLING`](macros/json_strict_nul_handling.md) macro to opt into rejecting it
instead. Because `operator>>` only parses a single value and does not require the rest of the stream to be consumed,
a NUL byte *after* a complete value has no effect on `operator>>` either way; it only matters while a value is still
being read.
!!! warning "Deprecation"
This function replaces function `#!cpp std::istream& operator<<(basic_json& j, std::istream& i)` which has
@@ -98,7 +105,11 @@ Note that reading concatenated values does **not** work for [JSON Lines](../feat
- [accept](basic_json/accept.md) - check if the input is valid JSON
- [parse](basic_json/parse.md) - deserialize from a compatible input
- [`JSON_STRICT_NUL_HANDLING`](macros/json_strict_nul_handling.md) - opt in to rejecting a NUL byte in the input
instead of treating it as end of input
## Version history
- Added in version 1.0.0.
- `JSON_STRICT_NUL_HANDLING` added in version 3.13.0 to optionally reject a NUL byte in the input instead of treating
it as end of input; planned to become the default in version 4.0.0.
docs/mkdocs/docs/features/macros.md
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@@ -105,6 +105,19 @@ using the library with compilers that do not fully support C++11 and may only wo
See [full documentation of `JSON_SKIP_UNSUPPORTED_COMPILER_CHECK`](../api/macros/json_skip_unsupported_compiler_check.md).
## `JSON_STRICT_NUL_HANDLING`
When defined to `1`, a `'\0'` (NUL) byte anywhere in the input is rejected with `parse_error.101`, like any other
unexpected byte, instead of being silently treated as end of input (see the
[FAQ entry](../home/faq.md#nul-bytes-in-the-input) for background). The default value is `0`, which preserves the
existing behavior; this is planned to become the default in version 4.0.0.
The strict handling can also be enabled with the CMake option
[`JSON_StrictNulHandling`](../integration/cmake.md#json_strictnulhandling) (`OFF` by default) which sets
`JSON_STRICT_NUL_HANDLING` accordingly.
See [full documentation of `JSON_STRICT_NUL_HANDLING`](../api/macros/json_strict_nul_handling.md).
## `JSON_THROW_USER(exception)`
This macro overrides `#!cpp throw` calls inside the library. The argument is the exception to be thrown.
docs/mkdocs/docs/home/faq.md
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@@ -90,6 +90,54 @@ The library supports **Unicode input** as follows:
In most cases, the parser is right to complain, because the input is not UTF-8 encoded. This is especially true for Microsoft Windows, where Latin-1 or ISO 8859-1 is often the standard encoding.
### NUL bytes in the input
!!! question "Questions"
- Why does `json::parse()` silently ignore part of my input?
- Why does a `std::string`/buffer with extra data after the JSON text parse without error, while a similar-looking string with extra text does not?
A `'\0'` (NUL) byte anywhere in the input is treated the same as the real end of the input, rather than as an ordinary (and, outside of a string, invalid) byte. Everything from that byte onward is silently ignored, without a parse error — including further, otherwise well-formed JSON:
```cpp
json::parse(std::string("123") + '\0'); // == 123, no error
json::parse(std::string("123") + '\0' + "true"); // == 123, the "true" is silently ignored too
```
This is different from any other unexpected trailing byte, which *does* raise [`parse_error.101`](../home/exceptions.md#jsonexceptionparse_error101):
```cpp
json::parse("123x"); // throws parse_error.101: unexpected additional data
```
This falls out of the same convention used when no explicit input length is given at all: `json::parse(const char*)` already stops at the first NUL byte via `strlen()`, since a bare pointer has no length of its own. The library applies that same NUL-terminated-C-string convention uniformly, rather than only when a length is genuinely unavailable — so a `std::string`, iterator range, or container whose content happens to include a NUL byte is affected the same way a raw `const char*` would be.
If your input may contain a trailing or embedded NUL that is **not** meant to signal the end of the JSON text — for instance, a fixed-size, zero-padded buffer — trim it yourself before calling `parse()`, since the library will otherwise silently stop there instead of raising an error:
```cpp
s.resize(s.find('\0')); // drop everything from the first NUL onward, if any
json::parse(s);
```
**Opt-in strict handling (since version 3.13.0)**
Manually trimming every input is easy to forget. If you define [`JSON_STRICT_NUL_HANDLING`](../api/macros/json_strict_nul_handling.md) to `1` before including the library, a `'\0'` byte is instead rejected like any other unexpected byte and raises `parse_error.101`, instead of being treated as end of input:
```cpp
#define JSON_STRICT_NUL_HANDLING 1
#include <nlohmann/json.hpp>
json::parse(std::string("123") + '\0'); // throws parse_error.101 instead of silently returning 123
```
This macro defaults to `0` (disabled, preserving the behavior described above) to avoid breaking existing code that may depend on it, even unknowingly; it is planned to become the default in version 4.0.0. See [its documentation](../api/macros/json_strict_nul_handling.md) for details, including how it also affects `char` arrays such as string literals.
Note that this is unrelated to an *unescaped* NUL byte occurring **inside** a quoted JSON string, which is a different, already-invalid case and is correctly rejected either way:
```cpp
json::parse(std::string("\"") + '\0' + "\""); // throws parse_error.101: control character U+0000 (NUL) must be escaped to \u0000
```
### Wide string handling
!!! question
docs/mkdocs/docs/integration/cmake.md
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@@ -198,6 +198,11 @@ Use the non-amalgamated version of the library. This option is `ON` by default.
Treat the library headers like system headers (i.e., adding `SYSTEM` to the [`target_include_directories`](https://cmake.org/cmake/help/latest/command/target_include_directories.html) call) to check for this library by tools like Clang-Tidy. This option is `OFF` by default.
### `JSON_StrictNulHandling`
Reject a `'\0'` (NUL) byte in the input instead of treating it as end of input, by defining the macro
[`JSON_STRICT_NUL_HANDLING`](../api/macros/json_strict_nul_handling.md). This option is `OFF` by default.
### `JSON_Valgrind`
Execute the test suite with [Valgrind](https://valgrind.org). This option is `OFF` by default. Depends on `JSON_BuildTests`.
docs/mkdocs/mkdocs.yml
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@@ -294,6 +294,7 @@ nav:
- 'JSON_NO_IO': api/macros/json_no_io.md
- 'JSON_SKIP_LIBRARY_VERSION_CHECK': api/macros/json_skip_library_version_check.md
- 'JSON_SKIP_UNSUPPORTED_COMPILER_CHECK': api/macros/json_skip_unsupported_compiler_check.md
- 'JSON_STRICT_NUL_HANDLING': api/macros/json_strict_nul_handling.md
- 'JSON_USE_GLOBAL_UDLS': api/macros/json_use_global_udls.md
- 'JSON_USE_IMPLICIT_CONVERSIONS': api/macros/json_use_implicit_conversions.md
- 'JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON': api/macros/json_use_legacy_discarded_value_comparison.md