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1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errorsTo convert JSON into a FlatBuffer binary, provide flatc with a matching FlatBuffers schema and the JSON document:
flatc --binary schema.fbs data.jsonThis is schema-based serialization, not an automatic conversion of arbitrary JSON. The schema defines tables, types, vectors, enums, defaults and the root object; flatc validates the JSON against that contract and writes a binary buffer.
What the conversion does
JSON is human-readable text, but applications normally parse it and construct objects before using the data. FlatBuffers is a cross-platform, schema-defined binary format designed for direct access to serialized data. It can reduce parsing and allocation work, but whether it is faster or smaller depends on data shape, language runtime, access pattern, compression and whether full object materialization is required. The project describes these design goals in its official repository.
JSON document + schema.fbs + flatc compiler
↓
FlatBuffer binaryJSON-to-binary conversion is especially useful for build-time assets, test fixtures, catalogs, configuration packaging and legacy-data imports. For high-throughput runtime ingestion, repeatedly invoking a compiler is usually the wrong architecture; applications commonly build buffers with generated APIs instead.
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Minimal working example
1. Define the schema
namespace Example;
enum WeaponType : byte {
Sword,
Axe
}
table Weapon {
name:string;
damage:short;
}
table Monster {
pos:[float];
mana:short = 150;
hp:short = 100;
name:string;
inventory:[ubyte];
weapons:[Weapon];
equipped:WeaponType = Sword;
}
root_type Monster;
file_identifier "MONS";namespacecontrols generated-language namespacing.- A
tableis the usual flexible object type. [ubyte]is a byte vector and[Weapon]is a vector of nested tables.- The enum restricts
equippedto declared symbolic values. root_typeidentifies the top-level object.- The four-character
file_identifierhelps identify the intended schema during inspection.
The schema language also supports structs, unions, included schemas, deprecated fields, explicit IDs and attributes; see the schema-writing guide for those features.
2. Supply matching JSON
{
"pos": [1.0, 2.0, 3.0],
"mana": 120,
"hp": 80,
"name": "Orc",
"inventory": [1, 2, 3, 4],
"weapons": [
{"name": "Sword", "damage": 35},
{"name": "Axe", "damage": 50}
],
"equipped": "Sword"
}Names must match exactly, including case. Numbers must fit their declared FlatBuffers types, and enum values are normally written symbolically. Omitted fields use schema defaults or remain absent according to the type and schema rules; omission does not necessarily mean an explicitly stored value.
3. Compile the binary
flatc --binary monster.fbs monster.jsonThe compiler normally creates a file such as monster_wire.bin. The exact name can vary with options and schema attributes, so inspect the output directory rather than hard-coding a filename.
4. Generate application bindings
flatc --cpp monster.fbs
flatc --rust monster.fbs
flatc --cpp --rust monster.fbsCurrent compiler documentation lists generators including C++, Java, Kotlin, C#, Go, Python, JavaScript, TypeScript, PHP, Dart, Lua, Rust, Swift and Nim. Generator features and runtime packaging differ by language; consult the compiler documentation.
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5. Inspect the result
flatc --json monster.fbs -- monster_wire.bin
flatc --json --strict-json monster.fbs -- monster_wire.binThe -- separator marks following files as binary inputs. --strict-json quotes field names and disallows trailing commas, which is preferable for standard JSON tooling. Round-tripped text is not guaranteed to match the source byte-for-byte: defaults, field order, enum formatting and numeric formatting may differ.
Installing and pinning flatc
Verify the executable independently:
flatc --versionInstalling a language runtime library does not necessarily install the flatc compiler. Use a system package, package-manager executable or prebuilt release, or build from source. The repository documents a typical Unix-like CMake build:
cmake -G "Unix Makefiles"
make -jPin the compiler version, runtime-library version and schemas in CI. The official releases page showed v25.12.19 and a later commit-tagged release on August 18, 2026; release status changes, so verify the version you install.
Useful flatc commands
| Purpose | Command |
|---|---|
| Write binary | flatc --binary schema.fbs data.json |
| Choose output directory | flatc --binary -o build/generated schema.fbs data.json |
| Generate bindings and binary | flatc --cpp --binary -o build/generated schema.fbs data.json |
| Import schemas | flatc --binary -I schemas schemas/root.fbs data.json |
| Binary to JSON | flatc --json schema.fbs -- data.bin |
| Read size-prefixed data | flatc --json --size-prefixed schema.fbs -- data.bin |
| Emit default-valued fields in JSON | flatc --binary --defaults-json schema.fbs data.json |
| Require explicit field IDs | flatc --require-explicit-ids --cpp schema.fbs |
| Check schema conformance | flatc --conform old_schema.fbs new_schema.fbs |
--defaults-json matters when producing JSON and wanting values equal to schema defaults shown explicitly. Explicit and omitted defaults can be semantically equivalent to readers while remaining different in source and debugging output.
JSON and schema compatibility rules
Names, types and ranges
FlatBuffers distinguishes byte, ubyte, short, ushort, int, uint, long, ulong, float and double, whereas JSON has one general number syntax. Validate ranges before compilation. Large integers can lose precision before flatc if processed through JavaScript Number; use a typed pipeline when exact 64-bit values matter.
Strings and bytes
FlatBuffers strings are UTF-8-oriented. Options such as --allow-non-utf8 and --natural-utf8 are specialized interoperability controls, not a repair for malformed text. A byte vector is represented textually, for example {"payload":[0,1,2,255]} for payload:[ubyte]; large payloads should be preprocessed rather than expanded into huge JSON arrays.
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--json-nested-bytes can interpret nested FlatBuffer data as bytes, but the documentation warns that the result must be verified before use.
Enums, unions and structures
Enum JSON should use declared names such as "Ready". Unknown names fail; renaming a member changes JSON compatibility even if its numeric value remains. A union generally requires both a discriminator and a value, so test every branch. A struct has fixed inline layout and stricter evolution constraints; independently evolving application objects normally belong in tables.
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Business-required fields still need validation in preprocessing, generated APIs or application code. A schema declaration alone is not a complete business validation policy.
Validation and round-trip testing
- Parse and lint the input with a normal JSON parser.
- Compile representative fixtures with
flatc; treat diagnostics as contract failures. - Read each binary with the target runtime and use its verifier for untrusted data.
- Convert a fixture back with
--json --strict-json. - Compare normalized semantic data, not textual identity.
A practical CI sequence is:
flatc --binary schema.fbs input.json
flatc --json --strict-json schema.fbs -- input_wire.binInclude generated-code compilation and runtime reads in the same job so compiler and runtime versions cannot silently diverge.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Schema evolution
FlatBuffers supports evolution when its rules are followed. New table fields are normally appended; existing fields should be deprecated rather than removed. Renaming breaks generated accessors and JSON names. Explicit field IDs can relax append-order requirements but do not make incompatible type or semantic changes safe. Older readers generally ignore unknown fields, while newer readers can apply defaults when older buffers lack newer fields. Run --conform in CI and review semantic changes manually. See the official evolution documentation.
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Troubleshooting
flatc: command not found
Install or build the compiler, put it on PATH, and confirm with flatc --version. A runtime-only package is not enough.
Unknown field or type mismatch
Check spelling, case and nesting. Ensure objects, vectors and scalars match the schema and that integers are in range. Correct the data or deliberately revise the contract; avoid untested coercion.
Invalid enum or missing root type
Use a declared enum member, or add one intentionally. A schema used as a root must declare, for example, root_type MyTable;.
Binary will not read back
Check the schema, identifier and prefix format:
flatc --json schema.fbs -- data.bin
flatc --json --raw-binary schema.fbs -- data.bin
flatc --json --size-prefixed schema.fbs -- data.binUse --raw-binary only when the buffer genuinely lacks an identifier; it bypasses a safety check and a mismatched schema can crash or misinterpret data. Use --size-prefixed only when the producer used that format.
JSON parser rejects familiar-looking input
Unquoted keys and trailing commas are not standard JSON. Normalize the file with a JSON parser, and use --strict-json for interoperable output.
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Investigate root type, defaults, enum or union discriminators, schema-version mismatch, missing verification and lossy preprocessing. Round-trip fixtures usually expose the discrepancy.
When FlatBuffers is the right choice
| Need | Likely choice |
|---|---|
| Frequent reads, infrequent mutation, direct or low-copy access, controlled schema and multiple languages | FlatBuffers |
| Human editing, small infrequently parsed data or maximum ecosystem interoperability | JSON |
| Compact messages with mature RPC and service tooling | Protocol Buffers |
| Dynamic, schema-less data while staying in the FlatBuffers family | FlexBuffers via --flexbuffers |
| Compact dynamic maps and heterogeneous values with existing ecosystem support | MessagePack, CBOR, BSON or similar |
FlatBuffers does not guarantee zero application copies: strings, unpacked objects, mutations and language-runtime boundaries can allocate or copy. Benchmark the actual dataset, language, versions, compression, hardware and access methodology before claiming a win.
Quick Recap
Deployment checklist
- Pin
flatcand runtime versions. - Confirm the schema has the intended
root_typeand identifier. - Match JSON names, nesting, enum values and numeric ranges.
- Test unions, defaults, byte vectors and large integers.
- Generate bindings from the same schema used for conversion.
- Verify untrusted buffers before reading them.
- Run schema-conformance and round-trip checks in CI.
- Document whether binaries are identifier-bearing or size-prefixed.
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