csonpath

That's not my path, that's not your path, but csonpath.

csonpath is a partial JSONPath implementation in C. It is backend-agnostic: the same core engine can query, update, and remove data from any value representation that supports array, object, and scalar semantics—not just JSON.

Out of the box it ships with C backends for json-c and yyjson, plus language bindings for Python and Rust.

🚀 Features

JSONPath Syntax

Operations

• Find First — retrieve the first match.
• Find All — retrieve all matches (returns an array).
• Update or Create — modify existing values or create missing ones.
• Remove — delete matching elements.
• Callback — execute a custom callback on each match.
• Update or Create Callback — traverse the path, creating missing intermediate objects and arrays, then invoke the callback on each leaf node.

🌐 Links

• Project website: https://github.com/outscale/csonpath
• Join our community on Discord

📄 Table of Contents

• Features
• Installation
• Usage
• C API Reference
• Python API Reference
• CLI
• Using Multiple Backends
• Custom Backends
• Running Tests
• Directory Structure
• Contributing
• License

📦 Installation

csonpath is a C library at its core. Pick the instructions for the language you want to use it from.

C (json-c)

Prerequisites: C compiler (gcc, clang, or tcc), json-c.

Just include the header in your project. There is no separate install step required:

#include "csonpath_json-c.h"

Make sure to link against json-c when compiling:

gcc myapp.c -o myapp $(pkg-config --cflags --libs json-c)

C (yyjson)

Prerequisites: C compiler (gcc, clang, or tcc), yyjson.

#include "csonpath_yyjson.h"

Link against yyjson when compiling:

gcc myapp.c -o myapp $(pkg-config --cflags --libs yyjson)

Python

Prerequisites: Python 3.x.

Install from PyPI:

pip install csonpath

To install from source (development):

pip install .
# or
make pip-dev

Rust

Prerequisites: Rust toolchain.

The Rust crate is located in rust/. Build and test it with:

cd rust
cargo build
cargo test

To depend on it from another Rust project:

[dependencies]
csonpath = { path = "rust" }

🛠️ Usage

C (json-c)

#include "csonpath_json-c.h"

static void my_cb(json_object *parent, struct csonpath_child_info *info,
                  json_object *current, void *ud)
{
    json_object_set_string(current, "modified");
}

int main(void)
{
    struct json_object *jobj = json_tokener_parse(json_str);
    struct csonpath *p = csonpath_new("$.a");

    /* Find First: return the first match, or NULL */
    struct json_object *ret = csonpath_find_first(p, jobj);

    /* Find All: return a NEW json_object array. Caller must free it. */
    ret = csonpath_find_all(p, jobj);
    json_object_put(ret);

    /* Remove: delete matching keys (or set array slots to null). Returns count. */
    int removed = csonpath_remove(p, jobj);

    /* Update or Create: replace matches, or create the full path if missing. */
    csonpath_update_or_create(p, jobj, json_object_new_string("new_value"));

    /* Callback: call a user function for every match. */
    csonpath_callback(p, jobj, my_cb, NULL);

    /* Update or Create Callback: like callback, but creates missing parents first,
       then invokes the callback on every leaf (existing or newly created). */
    csonpath_update_or_create_callback(p, jobj, my_cb, NULL);

    csonpath_destroy(p);
    json_object_put(jobj);
    return 0;
}

Python

import csonpath

data = {"a": "value", "array": [1, 2, 3]}
p = csonpath.CsonPath("$.a")

# Find First / Find All
p.find_first(data)   # -> "value"
p.find_all(data)     # -> ["value"]

# Remove: returns number of removed items
p.set_path("$.array[*]")
p.remove(data)

# Update or Create: builds missing objects/arrays automatically
p.set_path("$.x.y.z")
p.update_or_create(data, [])
# data is now {"a": "value", "array": [1, 2, 3], "x": {"y": {"z": []}}}

# Callback
p.set_path("$.a")
p.callback(data, lambda parent, idx, cur, _: parent.__setitem__(idx, cur.upper()))

# Update or Create Callback: creates parents, then calls cb on each leaf
p.set_path("$[*].a")
p.update_or_create_callback(dst, my_sync_fn, userdata)

📘 C API Reference

The reference below uses the json-c backend as an example (struct json_object *). All backends expose the same functions with their own value type.

struct csonpath *csonpath_new(const char *path);

Create and initialize a new csonpath object.

int csonpath_set_path(struct csonpath *p, const char *path);

Change the path of an existing object.

int csonpath_compile(struct csonpath *p);

Compile the path expression. This is optional—paths are compiled automatically on first use—but explicit compilation can help catch syntax errors earlier.

void csonpath_print_instruction(struct csonpath *p);

Print the compiled bytecode instructions (useful for debugging).

struct json_object *csonpath_find_first(struct csonpath *p, struct json_object *json);

Return the first matching value, or NULL if none is found.

struct json_object *csonpath_find_all(struct csonpath *p, struct json_object *json);

Return a new json_object array containing all matches. Must be freed with json_object_put().

int csonpath_remove(struct csonpath *p, struct json_object *json);

Remove all matching elements. Returns the number of elements removed.

int csonpath_update_or_create(struct csonpath *p, struct json_object *json, struct json_object *new_val);

Replace matching values with new_val, or create the path if it does not exist.

int csonpath_callback(struct csonpath *p, struct json_object *json,
                      json_c_callback callback, void *userdata);

Invoke callback for every match.

int csonpath_update_or_create_callback(struct csonpath *p, struct json_object *json,
                                       json_c_callback callback, void *userdata);

Like callback, but traverses the path while updating/creating missing intermediate objects.

void csonpath_destroy(struct csonpath *p);

Free the csonpath object.

📗 Python API Reference

• CsonPath(path, return_empty_array=False, jq_like=False) — Create a new csonpath object. Optional flags: return_empty_array returns [] instead of None when find_all() finds nothing; jq_like allows jq-style paths without a leading $.
• set_path(path) — Change the path expression.
• find_first(json) — Return the first match, or None.
• find_all(json) — Return a list of all matches, or None (or [] if configured).
• remove(json) — Remove all matches. Returns the number of removed items.
• update_or_create(json, value) — Replace matches with value, or create the path.
• callback(json, callback, callback_data=None) — Call callback(parent, idx, current, callback_data) for every match.
• update_or_create_callback(json, callback, callback_data=None) — Same as callback, but creates missing parent objects along the path.

🖥️ CLI

A standalone C CLI is available in cli/csonpath_cli.c. It links directly against json-c and the csonpath C core, so it works without Python.

make csonpath          # build ./csonpath
make tests-cli         # run shell tests

It reads JSON from stdin, a file (-f), or a string (-s) and exposes the library operations through action flags.

# Get the first match (default)
echo '{"a": "value", "array": [1, 2, 3]}' | ./csonpath '$.a'
# => "value"

# Find all matches
echo '{"items": [{"price": 5}, {"price": 15}]}' | ./csonpath -a '$.items[?price > 10]'
# => [{"price":15}]

# One match per line
echo '{"array": [1, 2, 3]}' | ./csonpath -a -o lines '$.array[*]'
# => 1
# => 2
# => 3

# Set or create a value
echo '{"a": 1}' | ./csonpath --set '42' '$.x.y.z'
# => {"a":1,"x":{"y":{"z":42}}}

# Or with a positional value
echo '{"a": 1}' | ./csonpath '$.x.y.z' '42'

# Remove matches
echo '{"a": 1, "b": 2}' | ./csonpath -d '$.b'
# => {"a":1}

# Edit a file in place
./csonpath -f data.json -p -i --set '"2.0"' '$.version'

Options

Exit codes

🧩 Using Multiple Backends

You can include more than one csonpath backend in the same translation unit, provided you prefix all but one of them to avoid symbol collisions. Define CSONPATH_USE_PREFIX before a backend header to prefix its API with the backend name.

For example, to use both json-c and the immutable yyjson backend in the same file:

#define CSONPATH_USE_PREFIX
#include "csonpath_json-c.h"

#undef CSONPATH_USE_PREFIX
#include "csonpath_yyjson_const.h"

To use both yyjson backends (immutable and mutable) together, include the aggregator header:

#include "csonpath_yyjson.h"

When using the mutable yyjson backend, assign the document pointer to backend_ctx before mutable operations:

struct csonpath *mp = yyjson_mut_csonpath_new("$.a");
mp->backend_ctx = mdoc;
yyjson_mut_csonpath_update_or_create(mp, mroot, new_val);

🔌 Custom Backends

csonpath is designed to be backend-agnostic. The C core manipulates opaque value pointers through a small set of macros, so it can work with any data structure that supports array, object, and scalar semantics.

A backend is an adapter that implements those macros for a concrete value representation. To create a custom backend, define the required macros and types in a header file (similar to csonpath_json-c.h or csonpath_yyjson.h), then include your backend header before csonpath.h.

Existing backend adapters:

• csonpath_json-c.h — json-c values
• csonpath_yyjson.h — yyjson values
• csonpath_python.c — Python C API values (used by the Python bindings)
• rust/csonpath_rust_backend.h — Rust serde_json::Value (used by the Rust crate)

🧪 Running Tests

C Tests

make tests-c

Rust Tests

cd rust
cargo test

Python Tests

make tests-py

All Tests

make tests

📁 Directory Structure

🤝 Contributing

We welcome contributions!

Please read our Contributing Guidelines and Code of Conduct before submitting a pull request.

Feel free to open issues or pull requests!

📜 License

BSD 3-Clause. See LICENSE.