JSON to C Struct Generator

What is JSON to C Struct Generator?

C has no built-in JSON support and no garbage collector, so consuming a JSON API in C means two separate decisions: what the struct layout should look like, and who owns the memory behind every string and array. This generator handles the first decision and gives you a head start on the second. It infers a C type for each JSON value, converts keys to snake_case (the dominant C convention), emits child structs before parents so the file compiles in a single pass, and lets you choose between a static layout (fixed char buffers, no malloc) and a dynamic layout (char* with generated create/free helpers). The key thing C struct generation has to confront that managed languages do not: C strings are char* or char[N], and there is no String type that knows its own length. A JSON string of unbounded length cannot safely go into char[256] — it will truncate or overflow. So the static/dynamic choice is not cosmetic, it changes the safety profile of your code. Likewise JSON arrays have no fixed size, so each array becomes a pointer-plus-count pair (the canonical C idiom) rather than a single field. Most real C projects do not hand-roll a JSON parser; they link cJSON or jansson. This tool's output is designed to drop into either: enable library stubs and you get from_json/to_json skeletons that walk the parsed object tree.

How to Use

1. Set the root struct name (e.g. ApiResponse) and an optional prefix to namespace nested struct names, since C has no modules
2. Choose static memory mode for embedded or stack-allocated data where you control the size, or dynamic mode when string lengths are unbounded and you will malloc/free
3. Enable cJSON or jansson stubs to get from_json()/to_json() skeletons instead of writing the object-walking boilerplate by hand
4. Paste your JSON and click Generate, then copy the typedef block into a .h file and the function stubs into the matching .c file
5. In dynamic mode, call the generated free helper for every struct you create — C will not reclaim the heap memory for you

Tips & Best Practices

• Never parse JSON in C by hand for production code — link cJSON (single-file, MIT) for small projects or jansson (refcounted, more features) for larger ones. Hand-written parsers miss escape sequences and UTF-8 edge cases.
• In static mode, char[256] silently truncates longer strings. If you cannot bound the length, use dynamic mode or you will introduce data corruption that only shows up with real-world input.
• C89 has no bool — the generator maps JSON booleans to int (0/1). If you target C99 or later, change these to bool and #include <stdbool.h> for clearer code.
• For nullable nested objects, dynamic mode uses a pointer that is NULL when absent. Always NULL-check before dereferencing — a missing JSON key is the number-one cause of segfaults in JSON-consuming C code.
• cJSON_Parse returns a tree you must free with cJSON_Delete, separately from freeing your own struct. Mixing up these two ownership chains causes either leaks or double-frees.

Frequently Asked Questions

Real-world Examples

{
  "deviceId": "sensor-0427",
  "temperature": 21.4,
  "humidity": 58,
  "online": true,
  "lastError": null
}

#include <cjson/cJSON.h>
#include <stdlib.h>
#include <string.h>

typedef struct {
    char *device_id;
    double temperature;
    int humidity;
    int online;          /* 0 or 1 */
    char *last_error;    /* NULL when JSON null */
} SensorReading;

SensorReading *sensor_reading_from_json(const cJSON *root) {
    SensorReading *out = calloc(1, sizeof(SensorReading));
    const cJSON *id = cJSON_GetObjectItemCaseSensitive(root, "deviceId");
    if (cJSON_IsString(id) && id->valuestring)
        out->device_id = strdup(id->valuestring);
    out->temperature = cJSON_GetObjectItem(root, "temperature")->valuedouble;
    out->humidity = cJSON_GetObjectItem(root, "humidity")->valueint;
    out->online = cJSON_IsTrue(cJSON_GetObjectItem(root, "online"));
    const cJSON *err = cJSON_GetObjectItem(root, "lastError");
    out->last_error = cJSON_IsString(err) ? strdup(err->valuestring) : NULL;
    return out;
}

void sensor_reading_free(SensorReading *s) {
    if (!s) return;
    free(s->device_id);
    free(s->last_error);
    free(s);
}

{
  "serviceName": "edge-proxy",
  "maxConnections": 1024,
  "network": {
    "host": "0.0.0.0",
    "ports": [8080, 8443, 9090]
  }
}

#include <stddef.h>

typedef struct {
    char host[64];
    int ports[16];
    size_t ports_count;
} Network;

typedef struct {
    char service_name[64];
    int max_connections;
    Network network;     /* embedded by value, no pointer */
} Config;

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