TZ JSON - Simple C JSON Parser

TZ JSON is a simple JSON parser written in C without any dependencies on other libraries. JSON data can be extracted using query strings like common in scripting languages.

Features

• Simple and easy to use API
• No dynamic memory allocation, operates directly on input string
• No dependencies
• MIT License

Usage

Desired JSON elements are described by path strings. The API consists of several extraction functions that operate directly on the JSON input string but without modifying it:

enum tzj_type tzj_json(const char *json, char *path, const char **res, int *len);
bool tzj_str(const char *json, char *path, const char **res, int *len);
bool tzj_int(const char *json, char *path, int *value);
bool tzj_double(const char *json, char *path, double *value);
bool tzj_bool(const char *json, char *path, bool *value);

All functions return true on successful parsing and return their result by reference.

Assume the following JSON snippet referenced by const char *json

{
    "jsonrpc": "2.0",
    "method": "foo",
    "params": [
        true,
        {"key": 32}
    ],
    "id": 1
}

Extracting an Integer is as simple as that:

tzj_int(json, ".params[1].key", &intarg);

The parsing overhead can be reduced with JSON sub strings which tzj_json(...) can extract:

char *sub;
tzj_json(json, ".params", &sub, NULL);
tzj_int(sub, "[1].key", &intarg);

The string related functions tzj_json(...) and tzj_str(...) return references into the input string and thus aren't null terminated. Instead, a optionally length parameter can be passed to those functions. For further handlinf the object type enum tzj_type is returned.

char *str;
int len;
tzj_str(json, ".method", &str, &len);

To iterate over arrays, tzj_array_next(...) advances a pointer to the next array item:

enum tzj_type type;
const char *elem;

for (type = tzj_json(json, ".params[0]", &elem, NULL);
     type != TZJ_ERROR;
     type = tzj_array_next(&elem)) {

     handle_element(elem);
}

To ease the creation of JSON strings, a sprintf(...) like function is provided. In the format string all single quotes are replaced by double quotes to avoid heavy escaping. The following formats are recognized by tzj_sprintf(...):

%s:	Strings, containing double quotes will be escaped
%d:	Decimal as usual
%j:	JSON parts maybe previously extracted with tzj_json(...) This format expects the char* to the JSON and an int providing the length of the JSON

char buf[64];
const char *sub;
int len;

tzj_json(json, ".params", &sub, &len);
tzj_sprintf(buf, "{'key': %d, 'sub': %j}", 85, sub, len);

JSON RPC Example

To show all parts working together, here is an real life example. Parsing and responding to a JSON RPC request:

const char *request = "{\"jsonrpc\": \"2.0\", "
    "\"method\": \"subtract\", "
    "\"params\": [42, 23], \"id\": \"unknown type\"}";
char response[128];
const char *version;
const char *id;
int len;
int a, b;

printf("RPC Request: %s\n", request);

if (tzj_str(request, ".jsonrpc", &version, &len)
    && strncmp(version, "2.0", len) == 0
    && tzj_json(request, ".id", &id, &len)
    && tzj_int(request, ".params[0]", &a)
    && tzj_int(request, ".params[1]", &b)) {

    tzj_sprintf(response, "{'jsonrpc': '2.0', 'result': %d, 'id': %j}",
                a - b, id, len);

} else {
    fprintf(stderr, "ERROR: failed to parse request\n");
    tzj_sprintf(response, "{'jsonrpc': '2.0', "
                "'error': {'code': -32700, 'message': 'Parse error'}, "
                "'id': null}");
}

printf("RPC Response: %s\n", response);

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