Use fseek instead of rewind in the UBJSON FILE* test

clang-tidy (bugprone-unsafe-functions) rejects rewind() because it has no
error detection. Check the result of fseek() instead.

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

docs/mkdocs/docs/api/basic_json/dump.md

@@ -58,9 +58,7 @@ is not UTF-8 encoded and `error_handler` is set to `strict`
 
 ## Complexity
 
-Linear in the size of the result. With an `indent`, every line starts with up to `indent` characters per nesting level,
-so the result can be larger than the value by a factor of its nesting depth (see
-[Parsing untrusted input](../../features/parsing/untrusted_input.md#nesting-depth)).
+Linear.
 
 ## Notes
 

docs/mkdocs/docs/api/basic_json/flatten.md

@@ -18,8 +18,7 @@ Strong exception safety: if an exception occurs, the original value stays intact
 
 ## Complexity
 
-Linear in the size of the result. Every key repeats the path of its value, so the keys can be larger than the value by a
-factor of its nesting depth (see [Parsing untrusted input](../../features/parsing/untrusted_input.md#nesting-depth)).
+Linear in the size of the JSON value.
 
 ## Notes
 

docs/mkdocs/docs/api/basic_json/from_ubjson.md

@@ -89,8 +89,10 @@ Strong guarantee: if an exception is thrown, there are no changes in the JSON va
 - Throws [out_of_range.406](../../home/exceptions.md#jsonexceptionout_of_range406) if a high-precision number, or a
   float64 when `number_float_t` is `float`, does not fit into `number_float_t`
 - Throws [out_of_range.408](../../home/exceptions.md#jsonexceptionout_of_range408) if the count of an optimized
-  container exceeds the capacity of the container (or of `std::size_t`), or if an optimized array of `Z`, `T`, or `F` has
-  more than 1,048,576 elements
+  container exceeds the capacity of the container (or of `std::size_t`; an array of type `N` is empty, so only the
+  latter applies to it), if an optimized array of `Z`, `T`, or `F` has more than 1,048,576 elements, or if the
+  optimized arrays of `Z`, `T`, and `F` in the input together have more than 1,048,576 elements plus 4 for every byte
+  read so far
 
 ## Complexity
 
@@ -132,6 +134,9 @@ Linear in the size of the input.
 - 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.
 - Added `error_handler` parameter in version 3.13.0.
+- Report `start_array(0)` to a SAX handler for an optimized array of type `N` (`[$N#n`), whatever its count, in version
+  3.13.0.
+- Limited the total number of elements of optimized arrays of `Z`, `T`, and `F` in one input in version 3.13.0.
 
 !!! warning "Deprecation"
 

docs/mkdocs/docs/api/basic_json/to_ubjson.md

@@ -117,5 +117,8 @@ Linear in the size of the JSON value `j`.
 - Added `error_handler` parameter in version 3.13.0. Its default, `keep`, writes the bytes of a string or object key
   that is not valid UTF-8 unchanged, as before; `strict` (the default if
   [`JSON_STRICT_BINARY_UTF8`](../macros/json_strict_binary_utf8.md) is enabled) throws `type_error.316`.
+- Writes arrays of `Z`, `T`, or `F` without the `$` type annotation (`[#n Z Z ...`) once their elements in one call
+  add up to more than 1,048,576, in version 3.13.0, so that `from_ubjson` can read the output back. Such output is
+  larger: 270,000 rows of 100 `null` values take 26.8 MB instead of 1.35 MB.
 - Throws `type_error.321` for a discarded value since version 3.13.0; previously, a discarded value nested in an
   array or object was silently skipped, producing invalid UBJSON.

docs/mkdocs/docs/api/ordered_map.md

@@ -89,21 +89,19 @@ This differs from `#!cpp std::map`, where the same operations are O(log n).
 !!! warning "Quadratic cost of building large objects"
 
     Because every insertion scans all elements inserted so far, building an object of `n` distinct keys costs
-    **O(n²)** in total. This applies to filling an [`ordered_json`](ordered_json.md) object key by key (with
-    `operator[]`, `emplace`, or `insert`) and, for now, to reading one with a binary reader such as
-    [`from_cbor`](basic_json/from_cbor.md) ([#6104](https://github.com/nlohmann/json/issues/6104)). Parsing JSON text
-    is not affected: the parser finds the keys of an object with many members in a hash table and takes linear time.
+    **O(n²)** in total. This applies to filling an [`ordered_json`](ordered_json.md) object key by key as well as to
+    parsing one, since the parser inserts each key as it is read.
 
     The cost is negligible for the object sizes typically found in configuration files or API payloads, but it grows
-    steeply for machine-generated objects with many thousands of keys. Measured with Apple clang 21, `-O2 -DNDEBUG`,
-    arm64, for a flat object of `n` keys (best of seven runs):
+    steeply for machine-generated objects with many thousands of keys. Measured with `-O2 -DNDEBUG` for parsing a flat
+    object of `n` keys, relative to `#!cpp nlohmann::json` (which uses `#!cpp std::map`):
 
-    | `n`    | `json::parse` | `ordered_json::parse` | `ordered_json`, key by key | `ordered_json::from_cbor` |
-    |--------|---------------|-----------------------|----------------------------|---------------------------|
-    | 2000   | 0.3 ms        | 0.1 ms                | 4.5 ms                     | 4.4 ms                    |
-    | 4000   | 0.5 ms        | 0.2 ms                | 17.1 ms                    | 16.8 ms                   |
-    | 8000   | 0.9 ms        | 0.3 ms                | 67.0 ms                    | 66.8 ms                   |
-    | 16 000 | 2.0 ms        | 0.7 ms                | 282.4 ms                   | 282.4 ms                  |
+    | `n`    | `json` | `ordered_json` | factor |
+    |--------|--------|----------------|--------|
+    | 2000   | 0.7 ms | 3.6 ms         | 5×     |
+    | 4000   | 0.8 ms | 14.0 ms        | 19×    |
+    | 8000   | 1.6 ms | 67.8 ms        | 43×    |
+    | 16 000 | 3.3 ms | 181.6 ms       | 54×    |
 
     If key order matters for objects of that size, consider a container with a lookup index, such as
     [`nlohmann::fifo_map`](https://github.com/nlohmann/fifo_map)

docs/mkdocs/docs/features/binary_formats/ubjson.md

@@ -122,9 +122,18 @@ The library uses the following mapping from JSON values types to UBJSON types ac
     already is the value. Its declared count is therefore the only thing that decides how much memory the receiving side
     allocates, and a handful of bytes can describe billions of elements. `from_ubjson` rejects such an array with
     [`out_of_range.408`](../../home/exceptions.md#jsonexceptionout_of_range408) when the count exceeds 1,048,576
-    (`1 << 20`), and `to_ubjson` writes longer arrays of these types in the counted but untyped form: the `$` type marker is
-    dropped, the `#` count is kept, and every element is written with its own marker (`[#l 1048577 Z Z ...`), so any value
-    it produces can be read back. Objects and untyped counted arrays are not subject to this limit.
+    (`1 << 20`), or when the counts of all such arrays in the input add up to more than 1,048,576 plus 4 for every byte
+    read so far. `to_ubjson` therefore writes an array of these types in the counted but untyped form when it is longer
+    than that, or when it would bring the total of the typed arrays written so far above 1,048,576: the `$` type marker
+    is dropped, the `#` count is kept, and every element is written with its own marker (`[#l 1048577 Z Z ...`), so
+    everything it produces can be read back. Only input from another encoder, or written by version 3.12.0 and earlier of
+    this library, can exceed the limit: for example, 1,100 rows of 1,000 `null` values or 210,000 rows of 25, each with
+    a `$Z` annotation. Version 3.12.0 writes such a row in 5 bytes (6 above 255 elements), so rows of 20 or fewer such
+    values are never rejected. Objects and untyped counted arrays are not subject to this limit.
+
+    An optimized array of type `N` (no-op) is empty whatever its count is, because a no-op is not a value. Its count is
+    not passed on: `sax_parse` reports `start_array(0)` (versions before 3.13.0 reported the declared count), and the DOM
+    parser reserves nothing for it.
 
 !!! info "Binary values"
 
@@ -180,10 +189,11 @@ The library maps UBJSON types to JSON value types as follows:
 
     Every well-formed UBJSON value can be converted to a JSON value, with these exceptions: a high-precision number
     outside the range of `number_float_t` ([`out_of_range.406`](../../home/exceptions.md#jsonexceptionout_of_range406)),
-    an optimized array of `Z`, `T`, or `F` with more than 1,048,576 elements, and a count that exceeds the capacity of the
-    container ([`out_of_range.408`](../../home/exceptions.md#jsonexceptionout_of_range408)). High-precision numbers that
-    do not fit an integer type are stored as floating-point numbers and may lose precision. Ill-formed input is rejected
-    with a parse error.
+    optimized arrays of `Z`, `T`, or `F` with more elements than the limit allows (1,048,576 plus 4 per byte read),
+    and a count that exceeds the capacity of the container
+    ([`out_of_range.408`](../../home/exceptions.md#jsonexceptionout_of_range408)). High-precision numbers that do not
+    fit an integer type are stored as floating-point numbers and may lose precision. Ill-formed input is rejected with a
+    parse error.
 
 !!! info "Duplicate object keys"
 

docs/mkdocs/docs/features/parsing/untrusted_input.md

@@ -81,20 +81,6 @@ result. See the [documentation of the macro](../../api/macros/json_recursion_dep
 the speed, and the rules for choosing a value. Destroying a value (its destructor) recurses over the same number of
 levels and then continues without the call stack, and needs 32 KiB or less at every limit.
 
-!!! warning "Indented output and flattened keys grow with the depth"
-
-    Some results are larger than the value by a factor of its nesting depth.
-    [`dump`](../../api/basic_json/dump.md) with an `indent` of `n` (and likewise `operator<<` with `std::setw(n)` and
-    [`basic_json_view::dump`](../../api/basic_json_view/dump.md)) writes up to `n` characters per nesting level in
-    front of every line, and [`flatten`](../../api/basic_json/flatten.md) repeats the path of every value in its key.
-    For the 80,001-byte array `[1,[1,...[1,1]...]]` with 20,000 levels, `dump(1)` returns 600 MB, `dump(4)` 2.4 GB,
-    and the keys of `flatten()` hold 400 MB. A compact `dump()` without indentation stays the size of the value. If you
-    pretty-print, log, or flatten untrusted input, limit its nesting depth while parsing, for example with a
-    [parser callback](parser_callbacks.md#recipe-max-nesting-depth-via-a-callback): with a limit of `d`, every line has
-    at most `n * d` indentation characters, and every flattened key has at most `d` reference tokens.
-    [`JSON_RECURSION_DEPTH_LIMIT`](../../api/macros/json_recursion_depth_limit.md) does not help here: it changes only
-    how much of the call stack a walk uses, never its result, and it does not limit the depth that `parse()` accepts.
-
 ## Input size
 
 The library does not limit the overall size of a JSON text; a value nested or wide enough will use memory
@@ -108,10 +94,22 @@ For the binary formats, an announced size is never trusted outright:
   (reported as a parse error) instead of triggering an oversized allocation.
 - When an array announces its number of elements and the array container supports `reserve()` (as `#!cpp std::vector`,
   the default, does), the library reserves storage for at most 16384 of them upfront, regardless of how large the
-  announced count is; further elements still grow the container normally as they are read.
+  announced count is; further elements still grow the container normally as they are read. A truncated input whose
+  count is larger than its remaining bytes therefore reserves at most that much before the parse error is reported.
+  Every array element takes at least one byte of input, except in the [UBJSON](../binary_formats/ubjson.md) arrays of
+  type `Z`, `T` or `F` described below.
 - An announced array or object size that exceeds what the target container could ever hold (its `max_size()`) is
   rejected immediately as [`out_of_range.408`](../../home/exceptions.md#jsonexceptionout_of_range408), without
   attempting to allocate anything.
+- A UBJSON array of the no-op type (`[$N#<n>`) is empty whatever count it declares: a SAX handler is told
+  `start_array(0)` and the DOM parser reserves nothing.
+- A UBJSON array of type `Z`, `T` or `F` has no payload, so its announced count is not bounded by the input. One array
+  may announce at most 1,048,576 elements, and all such arrays of one input together may contain at most 1,048,576
+  elements plus 4 for every byte read so far (checked when each array's header has been read); beyond that the input is
+  rejected as [`out_of_range.408`](../../home/exceptions.md#jsonexceptionout_of_range408). For a DOM that bounds the memory these
+  arrays can need to about 80 bytes per byte of input plus about 33 MB for the first 1,048,576 elements; a SAX handler
+  receives at most that many events. Every other optimized array element, and every UBJSON object member (it has a
+  key), takes at least one byte of input.
 
 ## Strings
 
@@ -154,20 +152,6 @@ values for a repeated key ends up in the parsed object. If your application must
 silently resolving them one way or another, see the
 [parser callback recipe for rejecting duplicate keys](parser_callbacks.md#recipe-rejecting-duplicate-object-keys).
 
-## Objects with many keys in `ordered_json`
-
-[`ordered_json`](../../api/ordered_json.md) keeps the members of an object in a vector in insertion order
-([`ordered_map`](../../api/ordered_map.md)) and finds a key by searching all members. Parsing JSON text into an
-`ordered_json` avoids that search for objects with many keys and takes linear time. The binary readers
-([`from_cbor`](../../api/basic_json/from_cbor.md), [`from_msgpack`](../../api/basic_json/from_msgpack.md),
-[`from_ubjson`](../../api/basic_json/from_ubjson.md), [`from_bjdata`](../../api/basic_json/from_bjdata.md),
-[`from_bson`](../../api/basic_json/from_bson.md), and [`from_bon8`](../../api/basic_json/from_bon8.md)) do not yet:
-reading an object of `n` keys into an `ordered_json` costs time proportional to `n²`. A flat object of 40,000 keys
-(about 300 KB of CBOR) takes about 1.3 s, against a few milliseconds as JSON text or into a `json`
-([#6104](https://github.com/nlohmann/json/issues/6104)). `json` is not affected. If you read untrusted binary input
-into `ordered_json`, bound the input size more tightly, read it into a `json`, or reject objects with many keys in a
-[SAX handler](sax_interface.md).
-
 ## Numbers
 
 A number whose value cannot be represented — for instance `1E1000`, which overflows `double` — is rejected while
@@ -199,11 +183,9 @@ indices (or the pointer as a whole) before using them, or use [`at`](../../api/b
 
 - Wrap parsing in a `#!cpp try`/`#!cpp catch` block, or use `allow_exceptions=false`/`accept()` if your environment
   cannot use exceptions; never let `JSON_NOEXCEPTION`'s `abort()` be the first time you think about error handling.
-- If the input's nesting depth matters to you (for example because you pretty-print, log, or flatten it), enforce your
-  own limit with a [parser callback](parser_callbacks.md#recipe-max-nesting-depth-via-a-callback) or a
+- If the input's nesting depth matters to you, enforce your own limit with a
+  [parser callback](parser_callbacks.md#recipe-max-nesting-depth-via-a-callback) or a
   [SAX handler](sax_interface.md); the library bounds the call stack but not memory use.
-- If you read untrusted binary input into `ordered_json`, mind that objects with many keys are
-  [slow to read](#objects-with-many-keys-in-ordered_json).
 - Bound the size of the input itself before parsing, independent of the library's own bounded allocations for binary
   format lengths.
 - Decide up front how you want a string with invalid UTF-8 — from any source, not only parsing — to be serialized

docs/mkdocs/docs/home/exceptions.md

@@ -1050,11 +1050,17 @@ The size of an array or object in a [binary format](../features/binary_formats/i
 the size following `#` for [UBJSON](../features/binary_formats/ubjson.md)/[BJData](../features/binary_formats/bjdata.md),
 or the encoded length for [CBOR](../features/binary_formats/cbor.md).
 
-The exception is also thrown for a [UBJSON](../features/binary_formats/ubjson.md) array of a type that is encoded by its
-marker alone (`Z`, `T` or `F`) whose declared count exceeds 1,048,576 (`1 << 20`). Such an array has no payload, so its
-count alone decides how much memory is allocated, and a handful of bytes would otherwise describe billions of values.
-[`to_ubjson`](../api/basic_json/to_ubjson.md) writes longer arrays of these types without the `$` type annotation (the `#`
-count and one marker per element are kept), so any value it produces can still be read back.
+The exception is also thrown for [UBJSON](../features/binary_formats/ubjson.md) arrays of a type that is encoded by
+its marker alone (`Z`, `T` or `F`). Such an array has no payload, so its count alone decides how much memory is
+allocated, and a handful of bytes would otherwise describe billions of values. It is thrown if the declared count of one
+array exceeds 1,048,576 (`1 << 20`), and if the counts of all such arrays read so far in one input add up to more than
+1,048,576 plus 4 for every byte of the input read when the header of the last array has been read.
+[`to_ubjson`](../api/basic_json/to_ubjson.md) writes an array of these types without the `$` type annotation (the `#`
+count and one marker per element are kept) when it is longer, or when it would bring the total of the typed arrays
+written so far above 1,048,576, so everything it produces can be read back. Only input written by another encoder, or
+by version 3.12.0 and earlier of this library, can exceed the limit: for example, 1,100 rows of 1,000 `null` values or
+210,000 rows of 25, each with a `$Z` annotation. Version 3.12.0 writes such a row in 5 bytes (6 above 255 elements),
+so rows of 20 or fewer such values are never rejected.
 
 !!! failure "Example messages"
 
@@ -1070,6 +1076,9 @@ count and one marker per element are kept), so any value it produces can still b
     ```
     [json.exception.out_of_range.408] syntax error while parsing UBJSON size: excessive array size
     ```
+    ```
+    [json.exception.out_of_range.408] syntax error while parsing UBJSON size: excessive number of elements without payload
+    ```
 
 ### json.exception.out_of_range.409
 

include/nlohmann/detail/input/binary_reader.hpp

@@ -73,13 +73,46 @@ the input.
 
 This is a sanity bound rather than a security boundary, and it is far above
 any container met in practice. @ref binary_writer falls back to the
-unoptimized encoding for longer containers, so that a value serialized by
-this library can always be read back.
+unoptimized encoding for longer containers, and for containers that together
+with the ones before them would exceed @ref valueless_elements_base, so that a
+value serialized by this library can always be read back.
 
 @sa https://github.com/nlohmann/json/issues/2793
 */
 JSON_INLINE_VARIABLE constexpr std::size_t max_valueless_container_size = 1 << 20;
 
+/*!
+@brief number of elements of valueless UBJSON containers accepted on top of
+       @ref valueless_elements_per_byte
+
+The limit @ref max_valueless_container_size applies to each container on its
+own, so on its own it lets 9 bytes announce 1048576 values, and every further
+9 bytes another 1048576. The elements of all containers of a valueless type
+in one input are therefore added up, and the sum must stay below this base
+plus @ref valueless_elements_per_byte for every byte read so far. The base is
+the per-container limit, so a container that was accepted on its own before
+the sum was introduced is still accepted at the start of an input.
+*/
+JSON_INLINE_VARIABLE constexpr std::size_t valueless_elements_base = max_valueless_container_size;
+
+/*!
+@brief number of elements of valueless UBJSON containers accepted per byte of
+       input, in addition to @ref valueless_elements_base
+
+An element of a value that has a payload costs at least one byte of input.
+Valueless elements are accepted at a rate of 4 per byte. A value takes 16
+bytes of memory in a DOM, so these arrays add about 80 bytes of memory per
+byte of input (measured: 77; one `Z` per byte needs about 32), beyond what the
+same bytes cost anyway, plus the base. This bounds the arrays of valueless
+type, not the memory of a whole document. Containers written by
+@ref binary_writer never need more than the base, because it writes an array
+in the unoptimized form once the typed arrays written so far and this one add
+up to more than that. The rate matters for input written by other encoders and
+by versions before 3.13.0 only; in the data this was measured on, no input has
+more than 0.17 elements per byte.
+*/
+JSON_INLINE_VARIABLE constexpr std::size_t valueless_elements_per_byte = 4;
+
 ///////////////////
 // binary reader //
 ///////////////////
@@ -3700,6 +3733,37 @@ class binary_reader
         return emit_unsigned(number);
     }
 
+    /*!
+    @brief add the elements of a container of a valueless type to the total of
+           the input and check the total against the input read so far
+
+    A container of type 'Z', 'T' or 'F' has no payload, so its elements are the
+    only ones for which the input does not bound the count. The check is made
+    once per container, not per element.
+
+    @param[in] count  number of elements the container declares; at most
+                      @ref max_valueless_container_size
+    @return false if the total is more than @ref valueless_elements_base plus
+            @ref valueless_elements_per_byte for every byte read so far; the
+            total is not changed then
+    */
+    bool charge_valueless_elements(const std::size_t count) noexcept
+    {
+        // the limit saturates instead of overflowing when size_t is 32 bits
+        // wide and a very large input has been read; valueless_elements
+        // never exceeds the limit, so the difference cannot underflow
+        constexpr std::size_t max_size = (std::numeric_limits<std::size_t>::max)();
+        const std::size_t limit = chars_read <= (max_size - valueless_elements_base) / valueless_elements_per_byte
+                                  ? valueless_elements_base + (valueless_elements_per_byte * chars_read)
+                                  : max_size;
+        if (JSON_HEDLEY_UNLIKELY(count > limit - valueless_elements))
+        {
+            return false;
+        }
+        valueless_elements += count;
+        return true;
+    }
+
     /*!
     @return whether array creation completed
     */
@@ -3785,24 +3849,27 @@ class binary_reader
             // declared count alone decides how much is allocated; the check is
             // made before the start event so that no container is opened that
             // is then abandoned. See @ref max_valueless_container_size.
-            if (JSON_HEDLEY_UNLIKELY((size_and_type.second == 'Z' || size_and_type.second == 'T' || size_and_type.second == 'F')
-                                     && size_and_type.first > max_valueless_container_size))
+            if (JSON_HEDLEY_UNLIKELY(size_and_type.second == 'Z' || size_and_type.second == 'T' || size_and_type.second == 'F'))
             {
-                return out_of_range_error(exception_id::container_too_large, "excessive array size", "size");
+                if (JSON_HEDLEY_UNLIKELY(size_and_type.first > max_valueless_container_size))
+                {
+                    return out_of_range_error(exception_id::container_too_large, "excessive array size", "size");
+                }
+                if (JSON_HEDLEY_UNLIKELY(!charge_valueless_elements(size_and_type.first)))
+                {
+                    return out_of_range_error(exception_id::container_too_large, "excessive number of elements without payload", "size");
+                }
             }
 
-            if (JSON_HEDLEY_UNLIKELY(!enter_array(size_and_type.first, size_and_type.second)))
+            // a no-op is not a value, so an array of them is empty whatever
+            // count it declares: the SAX parser is told so, instead of being
+            // made to reserve storage for a count that no input backs
+            const std::size_t announced = size_and_type.second == 'N' ? 0 : size_and_type.first;
+            if (JSON_HEDLEY_UNLIKELY(!enter_array(announced, size_and_type.second)))
             {
                 return false;
             }
 
-            if (size_and_type.second == 'N')
-            {
-                // a no-op is not a value, so a container of them holds none;
-                // the declared size has already been passed to the SAX parser
-                container_stack.back().remaining = 0;
-            }
-
             return size_and_type.second == 0
                    || parse_ubjson_typed_run(container_stack.back().remaining, size_and_type.second, std::integral_constant<bool, bulk_scan> {});
         }
@@ -5114,6 +5181,10 @@ class binary_reader
     /// the number of characters read
     std::size_t chars_read = 0;
 
+    /// UBJSON: the sum of the element counts of the containers of a valueless
+    /// type read so far; see @ref charge_valueless_elements
+    std::size_t valueless_elements = 0;
+
     /// whether we can assume little endianness
     const bool is_little_endian = little_endianness();
 

include/nlohmann/detail/output/binary_writer.hpp

@@ -1169,6 +1169,14 @@ class binary_writer
         write_container_iterative<msgpack_walk>(root);
     }
 
+    // a function, not an inline condition: for an object the type is never
+    // valueless and MSVC would warn about a constant expression (C4127)
+    static constexpr bool valueless_array_fits(const bool valueless_type, const std::size_t size,
+            const std::size_t written) noexcept
+    {
+        return !valueless_type || size <= detail::valueless_elements_base - written;
+    }
+
     /*!
     @brief write the start of an array (@a IsArray) or object: its opening
            marker, the optimized type, and the count
@@ -1204,20 +1212,27 @@ class binary_writer
 
             // an optimized array of a valueless type carries no payload, so a
             // reader has nothing but the declared count to bound the allocation
-            // by and refuses an excessive one. Write the unoptimized form for
-            // those, at one byte per element, so the result can be read back.
-            // Objects are not affected: every element is preceded by its key.
-            const bool valueless_type = (first_prefix == 'Z' || first_prefix == 'T' || first_prefix == 'F');
-            const bool excessive_valueless = IsArray
-                                             && valueless_type
-                                             && size > detail::max_valueless_container_size;
-
-            if (same_prefix && !excessive_valueless
-                    && !(use_bjdata && is_bjdata_excluded_type_marker(first_prefix)))
+            // by. It refuses one array of more than max_valueless_container_size
+            // elements, and all such arrays of one input together when they add
+            // up to more than valueless_elements_base plus a number per byte
+            // read. Write the unoptimized form, at one byte per element, for
+            // an array that would exceed the base with the arrays written so
+            // far, so that the result can be read back: the limit of a reader
+            // is never below the base. Objects are not affected: every element
+            // is preceded by its key.
+            if (same_prefix && !(use_bjdata && is_bjdata_excluded_type_marker(first_prefix)))
             {
-                prefix_required = false;
-                oa.write_character(to_char_type('$'));
-                oa.write_character(first_prefix);
+                const bool valueless_type = IsArray && (first_prefix == 'Z' || first_prefix == 'T' || first_prefix == 'F');
+                if (valueless_array_fits(valueless_type, size, valueless_elements_written))
+                {
+                    prefix_required = false;
+                    oa.write_character(to_char_type('$'));
+                    oa.write_character(first_prefix);
+                    if (valueless_type)
+                    {
+                        valueless_elements_written += size;
+                    }
+                }
             }
         }
 
@@ -5441,6 +5456,10 @@ class binary_writer
     /// how to treat a string value or object key that is not valid UTF-8
     /// (CBOR, MessagePack, UBJSON, BJData, and BSON; not BON8)
     const error_handler_t error_handler = binary_writer_default_error_handler();
+
+    /// UBJSON: the number of elements written so far in optimized arrays of a
+    /// valueless type, which a reader adds up and bounds
+    std::size_t valueless_elements_written = 0;
 };
 
 }  // namespace detail

single_include/nlohmann/json.hpp

@@ -18464,13 +18464,46 @@ the input.
 
 This is a sanity bound rather than a security boundary, and it is far above
 any container met in practice. @ref binary_writer falls back to the
-unoptimized encoding for longer containers, so that a value serialized by
-this library can always be read back.
+unoptimized encoding for longer containers, and for containers that together
+with the ones before them would exceed @ref valueless_elements_base, so that a
+value serialized by this library can always be read back.
 
 @sa https://github.com/nlohmann/json/issues/2793
 */
 JSON_INLINE_VARIABLE constexpr std::size_t max_valueless_container_size = 1 << 20;
 
+/*!
+@brief number of elements of valueless UBJSON containers accepted on top of
+       @ref valueless_elements_per_byte
+
+The limit @ref max_valueless_container_size applies to each container on its
+own, so on its own it lets 9 bytes announce 1048576 values, and every further
+9 bytes another 1048576. The elements of all containers of a valueless type
+in one input are therefore added up, and the sum must stay below this base
+plus @ref valueless_elements_per_byte for every byte read so far. The base is
+the per-container limit, so a container that was accepted on its own before
+the sum was introduced is still accepted at the start of an input.
+*/
+JSON_INLINE_VARIABLE constexpr std::size_t valueless_elements_base = max_valueless_container_size;
+
+/*!
+@brief number of elements of valueless UBJSON containers accepted per byte of
+       input, in addition to @ref valueless_elements_base
+
+An element of a value that has a payload costs at least one byte of input.
+Valueless elements are accepted at a rate of 4 per byte. A value takes 16
+bytes of memory in a DOM, so these arrays add about 80 bytes of memory per
+byte of input (measured: 77; one `Z` per byte needs about 32), beyond what the
+same bytes cost anyway, plus the base. This bounds the arrays of valueless
+type, not the memory of a whole document. Containers written by
+@ref binary_writer never need more than the base, because it writes an array
+in the unoptimized form once the typed arrays written so far and this one add
+up to more than that. The rate matters for input written by other encoders and
+by versions before 3.13.0 only; in the data this was measured on, no input has
+more than 0.17 elements per byte.
+*/
+JSON_INLINE_VARIABLE constexpr std::size_t valueless_elements_per_byte = 4;
+
 ///////////////////
 // binary reader //
 ///////////////////
@@ -22091,6 +22124,37 @@ class binary_reader
         return emit_unsigned(number);
     }
 
+    /*!
+    @brief add the elements of a container of a valueless type to the total of
+           the input and check the total against the input read so far
+
+    A container of type 'Z', 'T' or 'F' has no payload, so its elements are the
+    only ones for which the input does not bound the count. The check is made
+    once per container, not per element.
+
+    @param[in] count  number of elements the container declares; at most
+                      @ref max_valueless_container_size
+    @return false if the total is more than @ref valueless_elements_base plus
+            @ref valueless_elements_per_byte for every byte read so far; the
+            total is not changed then
+    */
+    bool charge_valueless_elements(const std::size_t count) noexcept
+    {
+        // the limit saturates instead of overflowing when size_t is 32 bits
+        // wide and a very large input has been read; valueless_elements
+        // never exceeds the limit, so the difference cannot underflow
+        constexpr std::size_t max_size = (std::numeric_limits<std::size_t>::max)();
+        const std::size_t limit = chars_read <= (max_size - valueless_elements_base) / valueless_elements_per_byte
+                                  ? valueless_elements_base + (valueless_elements_per_byte * chars_read)
+                                  : max_size;
+        if (JSON_HEDLEY_UNLIKELY(count > limit - valueless_elements))
+        {
+            return false;
+        }
+        valueless_elements += count;
+        return true;
+    }
+
     /*!
     @return whether array creation completed
     */
@@ -22176,24 +22240,27 @@ class binary_reader
             // declared count alone decides how much is allocated; the check is
             // made before the start event so that no container is opened that
             // is then abandoned. See @ref max_valueless_container_size.
-            if (JSON_HEDLEY_UNLIKELY((size_and_type.second == 'Z' || size_and_type.second == 'T' || size_and_type.second == 'F')
-                                     && size_and_type.first > max_valueless_container_size))
+            if (JSON_HEDLEY_UNLIKELY(size_and_type.second == 'Z' || size_and_type.second == 'T' || size_and_type.second == 'F'))
             {
-                return out_of_range_error(exception_id::container_too_large, "excessive array size", "size");
+                if (JSON_HEDLEY_UNLIKELY(size_and_type.first > max_valueless_container_size))
+                {
+                    return out_of_range_error(exception_id::container_too_large, "excessive array size", "size");
+                }
+                if (JSON_HEDLEY_UNLIKELY(!charge_valueless_elements(size_and_type.first)))
+                {
+                    return out_of_range_error(exception_id::container_too_large, "excessive number of elements without payload", "size");
+                }
             }
 
-            if (JSON_HEDLEY_UNLIKELY(!enter_array(size_and_type.first, size_and_type.second)))
+            // a no-op is not a value, so an array of them is empty whatever
+            // count it declares: the SAX parser is told so, instead of being
+            // made to reserve storage for a count that no input backs
+            const std::size_t announced = size_and_type.second == 'N' ? 0 : size_and_type.first;
+            if (JSON_HEDLEY_UNLIKELY(!enter_array(announced, size_and_type.second)))
             {
                 return false;
             }
 
-            if (size_and_type.second == 'N')
-            {
-                // a no-op is not a value, so a container of them holds none;
-                // the declared size has already been passed to the SAX parser
-                container_stack.back().remaining = 0;
-            }
-
             return size_and_type.second == 0
                    || parse_ubjson_typed_run(container_stack.back().remaining, size_and_type.second, std::integral_constant<bool, bulk_scan> {});
         }
@@ -23505,6 +23572,10 @@ class binary_reader
     /// the number of characters read
     std::size_t chars_read = 0;
 
+    /// UBJSON: the sum of the element counts of the containers of a valueless
+    /// type read so far; see @ref charge_valueless_elements
+    std::size_t valueless_elements = 0;
+
     /// whether we can assume little endianness
     const bool is_little_endian = little_endianness();
 
@@ -30656,6 +30727,14 @@ class binary_writer
         write_container_iterative<msgpack_walk>(root);
     }
 
+    // a function, not an inline condition: for an object the type is never
+    // valueless and MSVC would warn about a constant expression (C4127)
+    static constexpr bool valueless_array_fits(const bool valueless_type, const std::size_t size,
+            const std::size_t written) noexcept
+    {
+        return !valueless_type || size <= detail::valueless_elements_base - written;
+    }
+
     /*!
     @brief write the start of an array (@a IsArray) or object: its opening
            marker, the optimized type, and the count
@@ -30691,20 +30770,27 @@ class binary_writer
 
             // an optimized array of a valueless type carries no payload, so a
             // reader has nothing but the declared count to bound the allocation
-            // by and refuses an excessive one. Write the unoptimized form for
-            // those, at one byte per element, so the result can be read back.
-            // Objects are not affected: every element is preceded by its key.
-            const bool valueless_type = (first_prefix == 'Z' || first_prefix == 'T' || first_prefix == 'F');
-            const bool excessive_valueless = IsArray
-                                             && valueless_type
-                                             && size > detail::max_valueless_container_size;
-
-            if (same_prefix && !excessive_valueless
-                    && !(use_bjdata && is_bjdata_excluded_type_marker(first_prefix)))
+            // by. It refuses one array of more than max_valueless_container_size
+            // elements, and all such arrays of one input together when they add
+            // up to more than valueless_elements_base plus a number per byte
+            // read. Write the unoptimized form, at one byte per element, for
+            // an array that would exceed the base with the arrays written so
+            // far, so that the result can be read back: the limit of a reader
+            // is never below the base. Objects are not affected: every element
+            // is preceded by its key.
+            if (same_prefix && !(use_bjdata && is_bjdata_excluded_type_marker(first_prefix)))
             {
-                prefix_required = false;
-                oa.write_character(to_char_type('$'));
-                oa.write_character(first_prefix);
+                const bool valueless_type = IsArray && (first_prefix == 'Z' || first_prefix == 'T' || first_prefix == 'F');
+                if (valueless_array_fits(valueless_type, size, valueless_elements_written))
+                {
+                    prefix_required = false;
+                    oa.write_character(to_char_type('$'));
+                    oa.write_character(first_prefix);
+                    if (valueless_type)
+                    {
+                        valueless_elements_written += size;
+                    }
+                }
             }
         }
 
@@ -34928,6 +35014,10 @@ class binary_writer
     /// how to treat a string value or object key that is not valid UTF-8
     /// (CBOR, MessagePack, UBJSON, BJData, and BSON; not BON8)
     const error_handler_t error_handler = binary_writer_default_error_handler();
+
+    /// UBJSON: the number of elements written so far in optimized arrays of a
+    /// valueless type, which a reader adds up and bounds
+    std::size_t valueless_elements_written = 0;
 };
 
 }  // namespace detail

tests/src/unit-ubjson.cpp

@@ -19,8 +19,12 @@
 #include <nlohmann/json.hpp>
 using nlohmann::json;
 
+#include <cstdio>
 #include <iostream>
+#include <sstream>
+#include <utility>
 #include <fstream>
+#include <limits>
 #include <list>
 #include <set>
 #include "make_test_data_available.hpp"
@@ -2436,6 +2440,341 @@ TEST_CASE("UBJSON optimized arrays of a valueless type are bounded")
     }
 }
 
+namespace
+{
+/// SAX consumer that counts the values and keeps the id of a parse error
+struct ValuelessEventCounter : public nlohmann::json_sax<json>
+{
+    bool null() override
+    {
+        return ++values < stop_after;
+    }
+    bool boolean(bool /*val*/) override
+    {
+        return ++values < stop_after;
+    }
+    bool number_integer(json::number_integer_t /*val*/) override
+    {
+        ++values;
+        return true;
+    }
+    bool number_unsigned(json::number_unsigned_t /*val*/) override
+    {
+        ++values;
+        return true;
+    }
+    bool number_float(json::number_float_t /*val*/, const std::string& /*s*/) override
+    {
+        ++values;
+        return true;
+    }
+    bool string(std::string& /*val*/) override
+    {
+        ++values;
+        return true;
+    }
+    bool binary(json::binary_t& /*val*/) override
+    {
+        ++values;
+        return true;
+    }
+    bool start_object(std::size_t /*elements*/) override
+    {
+        return true;
+    }
+    bool key(std::string& /*val*/) override
+    {
+        return true;
+    }
+    bool end_object() override
+    {
+        return true;
+    }
+    bool start_array(std::size_t /*elements*/) override
+    {
+        return true;
+    }
+    bool end_array() override
+    {
+        return true;
+    }
+    bool parse_error(std::size_t /*position*/, const std::string& /*last_token*/, const json::exception& ex) override
+    {
+        error_id = ex.id;
+        return false;
+    }
+
+    /// the handler refuses the value that makes this many
+    std::size_t stop_after = (std::numeric_limits<std::size_t>::max)();
+    std::size_t values = 0;
+    int error_id = 0;
+};
+
+/// appends `[$<marker>#l<count>` (nine bytes) to @a v
+void append_valueless_array(std::vector<std::uint8_t>& v, const char marker, const std::uint32_t count)
+{
+    v.push_back('[');
+    v.push_back('$');
+    v.push_back(static_cast<std::uint8_t>(marker));
+    v.push_back('#');
+    v.push_back('l');
+    v.push_back(static_cast<std::uint8_t>(count >> 24));
+    v.push_back(static_cast<std::uint8_t>(count >> 16));
+    v.push_back(static_cast<std::uint8_t>(count >> 8));
+    v.push_back(static_cast<std::uint8_t>(count));
+}
+
+/// `[` followed by one `[$<marker>#l<count>` per entry of @a counts and `]`
+std::vector<std::uint8_t> valueless_arrays(const char marker, const std::vector<std::uint32_t>& counts)
+{
+    std::vector<std::uint8_t> v = {'['};
+    for (const auto count : counts)
+    {
+        append_valueless_array(v, marker, count);
+    }
+    v.push_back(']');
+    return v;
+}
+} // namespace
+
+TEST_CASE("UBJSON the valueless elements of a whole input are bounded")
+{
+    // The per-container limit alone let nine bytes (`[$Z#l` and a count of
+    // 1048576) announce a million values, and every further nine bytes another
+    // million. The elements of all containers of type 'Z', 'T' or 'F' in one
+    // input are now added up: they may not exceed 1048576 plus 4 for every
+    // byte read when the container's header has been read.
+    //
+    // The tests count events with a SAX handler where they can: building and
+    // comparing values of a million elements is what makes them slow under
+    // Valgrind, so there is one rejection and one round trip with a DOM.
+    json _;
+    const char* const budget_message = "[json.exception.out_of_range.408] syntax error while parsing UBJSON size: excessive number of elements without payload";
+
+    // two arrays that exceed the limit: the second one, with its header read
+    // at byte 19, may hold 4 * 19 = 76 values on top of the base of 1048576
+    const std::uint32_t over_first = 524288;
+    const std::uint32_t over_second = 1048576 + 76 - over_first + 1;
+
+    SECTION("the input of the original report is rejected")
+    {
+        // 362 bytes (40 arrays of 1048576 values) used to allocate 661 MB; the
+        // second array is the one that is refused
+        const auto input = valueless_arrays('Z', {over_first, over_second});
+        CHECK_THROWS_WITH_AS(_ = json::from_ubjson(input), budget_message, json::out_of_range&);
+    }
+
+    SECTION("every input adapter applies the budget")
+    {
+        const auto input = valueless_arrays('Z', {over_first, over_second});
+
+        // stream; not strict, which does not matter
+        std::stringstream stream;
+        stream.write(reinterpret_cast<const char*>(input.data()), static_cast<std::streamsize>(input.size()));
+        ValuelessEventCounter from_stream;
+        CHECK(!json::sax_parse(stream, &from_stream, json::input_format_t::ubjson, false));
+        CHECK(from_stream.error_id == 408);
+
+        // a container that is not contiguous; the counts of arrays of
+        // different types are added up
+        std::vector<std::uint8_t> mixed = {'['};
+        append_valueless_array(mixed, 'T', over_first);
+        append_valueless_array(mixed, 'F', over_second);
+        mixed.push_back(']');
+        const std::list<std::uint8_t> as_list(mixed.begin(), mixed.end());
+        ValuelessEventCounter from_list;
+        CHECK(!json::sax_parse(as_list.begin(), as_list.end(), &from_list, json::input_format_t::ubjson));
+        CHECK(from_list.error_id == 408);
+        CHECK(from_list.values == over_first);
+
+        // FILE*
+        FILE* const file = std::tmpfile(); // NOLINT(cppcoreguidelines-owning-memory)
+        REQUIRE(file != nullptr);
+        CHECK(std::fwrite(input.data(), 1, input.size(), file) == input.size());
+        CHECK(std::fseek(file, 0, SEEK_SET) == 0);
+        ValuelessEventCounter from_file;
+        CHECK(!json::sax_parse(file, &from_file, json::input_format_t::ubjson));
+        CHECK(from_file.error_id == 408);
+        std::fclose(file); // NOLINT(cert-err33-c,cppcoreguidelines-owning-memory)
+    }
+
+    SECTION("the limit is exact and the bytes read raise it")
+    {
+        // [ [$Z#l 524288] [$U#l 4096 + 4096 bytes] [$Z#l count] ]: chars_read
+        // is 1 + 9 + (9 + 4096) + 9 = 4124 when the last header has been read,
+        // so the last array may hold 4 * 4124 = 16496 values on top of the
+        // base of 1048576 less the 524288 of the first array
+        const std::uint32_t last_allowed = 1048576 + 16496 - 524288;
+        for (const std::uint32_t count :
+                {
+                    last_allowed, last_allowed + 1
+                })
+        {
+            CAPTURE(count)
+            std::vector<std::uint8_t> input = {'['};
+            append_valueless_array(input, 'Z', 524288);
+            input.insert(input.end(), {'[', '$', 'U', '#', 'l', 0x00, 0x00, 0x10, 0x00});
+            input.insert(input.end(), 4096, 0x01);
+            append_valueless_array(input, 'Z', count);
+            input.push_back(']');
+            ValuelessEventCounter sax;
+            const bool accepted = count == last_allowed;
+            CHECK(json::sax_parse(input, &sax, json::input_format_t::ubjson) == accepted);
+            CHECK(sax.error_id == (accepted ? 0 : 408));
+            CHECK(sax.values == 524288u + 4096u + (accepted ? count : 0u));
+        }
+
+        // an array of 1048576 values on its own is accepted as before
+        const std::vector<std::uint8_t> single = {'[', '$', 'Z', '#', 'l', 0x00, 0x10, 0x00, 0x00};
+        // (the handler ends the parse at the first value, so that the million
+        // values are not read)
+        ValuelessEventCounter sax;
+        sax.stop_after = 1;
+        CHECK(!json::sax_parse(single, &sax, json::input_format_t::ubjson));
+        CHECK(sax.error_id == 0);
+        CHECK(sax.values == 1);
+    }
+
+    SECTION("to_ubjson writes the typed form up to the base only")
+    {
+        // 1048 rows of 1000 values fit under the base of 1048576, the 1049th
+        // would not: its elements are written one by one. Written all typed,
+        // 1100 rows of 6 bytes would be refused by a reader (1075 rows are the
+        // most it accepts). The rows hold null, true or false, and the
+        // fallback does not depend on which
+        std::vector<std::uint8_t> encoded;
+        {
+            json table = json::array();
+            for (int row = 0; row < 1100; ++row)
+            {
+                const json cell = row % 3 == 0 ? json(nullptr) : json(row % 3 == 1);
+                table.push_back(json(1000, cell));
+            }
+            encoded = json::to_ubjson(table, true, true);
+        }
+        // outer array: `[$[#I` and the count (7 bytes), then 1048 typed rows of 6
+        // bytes (`$Z#I` and two bytes) and 52 rows of `#I`, two bytes, and 1000 `Z`
+        CHECK(encoded.size() == 7 + (1048 * 6) + (52 * (4 + 1000)));
+
+        // the output is read back
+        ValuelessEventCounter sax;
+        CHECK(json::sax_parse(encoded, &sax, json::input_format_t::ubjson));
+        CHECK(sax.error_id == 0);
+        CHECK(sax.values == 1100u * 1000u);
+    }
+
+    SECTION("BJData is not affected")
+    {
+        // BJData has no optimized arrays of these types, and the writer does
+        // not charge them
+        json table = json::array();
+        for (int row = 0; row < 3; ++row)
+        {
+            table.push_back(json(10, nullptr));
+        }
+        const auto encoded = json::to_bjdata(table, true, true);
+        CHECK(json::from_bjdata(encoded) == table);
+
+        const std::vector<std::uint8_t> input = {'[', '$', 'Z', '#', 'l', 0x00, 0x10, 0x00, 0x00};
+        CHECK_THROWS_AS(_ = json::from_bjdata(input), json::parse_error&);
+    }
+
+    SECTION("an object of a valueless type needs a key for every value")
+    {
+        // an object member costs a key, so the count is bounded by the input
+        // and is not charged to the budget
+        const std::vector<std::uint8_t> input = {'{', '$', 'Z', '#', 'l', 0x00, 0x10, 0x00, 0x00};
+        ValuelessEventCounter sax;
+        CHECK(!json::sax_parse(input, &sax, json::input_format_t::ubjson));
+        CHECK(sax.error_id == 110);
+        CHECK(sax.values == 0);
+    }
+}
+
+TEST_CASE("UBJSON an optimized array of no-ops is empty whatever its count")
+{
+    // A no-op ('N') is not a value, so `[$N#<n>` is an array without elements
+    // for every n. The count used to be passed on to the SAX parser, which
+    // told it to expect n elements, and the DOM parser reserved storage for
+    // them: 16384 values (256 KB) for a count of 2^31 in 9 bytes of input.
+    SECTION("the SAX parser is told that the array is empty")
+    {
+        for (const std::vector<std::uint8_t>& input :
+                {
+                    std::vector<std::uint8_t> {'[', '$', 'N', '#', 'i', 2},
+                    std::vector<std::uint8_t> {'[', '$', 'N', '#', 'I', 0x7F, 0xFF},
+                    std::vector<std::uint8_t> {'[', '$', 'N', '#', 'l', 0x7F, 0xFF, 0xFF, 0xFF},
+                    std::vector<std::uint8_t> {'[', '$', 'N', '#', 'i', 0}
+                })
+        {
+            SaxEventLogger logger;
+            CHECK(json::sax_parse(input, &logger, json::input_format_t::ubjson));
+            CHECK(logger.events == std::vector<std::string>({"start_array(0)", "end_array()"}));
+        }
+
+        // between other values, the next value is read as usual
+        const std::vector<std::uint8_t> nested = {'[', '[', '$', 'N', '#', 'l', 0x7F, 0xFF, 0xFF, 0xFF, 'T', '[', '$', 'N', '#', 'i', 3, ']'};
+        SaxEventLogger logger;
+        CHECK(json::sax_parse(nested, &logger, json::input_format_t::ubjson));
+        CHECK(logger.events == std::vector<std::string>({"start_array()", "start_array(0)", "end_array()", "boolean(true)", "start_array(0)", "end_array()", "end_array()"}));
+    }
+
+    SECTION("the DOM parser reserves nothing")
+    {
+        // 2000 arrays of 9 bytes that announce 2^31 elements each: with a
+        // reservation of 16384 values per array this took about 500 MB
+        std::vector<std::uint8_t> input = {'['};
+        for (int i = 0; i < 2000; ++i)
+        {
+            input.insert(input.end(), {'[', '$', 'N', '#', 'l', 0x7F, 0xFF, 0xFF, 0xFF});
+        }
+        input.push_back(']');
+
+        const json j = json::from_ubjson(input);
+        REQUIRE(j.size() == 2000);
+        std::size_t capacity = 0;
+        for (const auto& element : j)
+        {
+            CHECK(element.is_array());
+            CHECK(element.empty());
+            capacity += element.get_ref<const json::array_t&>().capacity();
+        }
+        CHECK(capacity == 0);
+
+        // the 7-byte form inside an optimized array of arrays
+        std::vector<std::uint8_t> typed = {'[', '$', '[', '#', 'I', 0x07, 0xD0};
+        for (int i = 0; i < 2000; ++i)
+        {
+            typed.insert(typed.end(), {'$', 'N', '#', 'I', 0x7F, 0xFF});
+        }
+        const json k = json::from_ubjson(typed);
+        REQUIRE(k.size() == 2000);
+        capacity = 0;
+        for (const auto& element : k)
+        {
+            CHECK(element.empty());
+            capacity += element.get_ref<const json::array_t&>().capacity();
+        }
+        CHECK(capacity == 0);
+    }
+
+    SECTION("a count that no container could hold is no longer an error")
+    {
+        // the array is empty, so nothing has to fit: 0x7FFFFFFF used to be
+        // reported as out_of_range.408 where it exceeds the maximum size of
+        // an array, and is accepted now
+        const std::vector<std::uint8_t> input = {'[', '$', 'N', '#', 'l', 0x7F, 0xFF, 0xFF, 0xFF};
+        CHECK(json::from_ubjson(input) == json::array());
+
+        // a count that does not fit into std::size_t still is
+#if SIZE_MAX == 0xffffffff
+        const std::vector<std::uint8_t> too_large = {'[', '$', 'N', '#', 'L', 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00};
+        json _;
+        CHECK_THROWS_AS(_ = json::from_ubjson(too_large), json::out_of_range&);
+#endif
+    }
+}
+
 TEST_CASE("issue #5405 - array reserve for definite-length UBJSON arrays")
 {
 #if !defined(JSON_NOEXCEPTION)