cesanta/frozen

int json_scanf(const char *str, int str_len, const char *fmt, ...);
int json_vscanf(const char *str, int str_len, const char *fmt, va_list ap);

/* json_scanf's %M handler  */
typedef void (*json_scanner_t)(const char *str, int len, void *user_data);

  // str has the following JSON string (notice keys are out of order):
  // { "a": 123, "d": true, "b": [1, 2], "c": "hi" }

  int a = 0, d = 0;
  char *c = NULL;
  void *my_data = NULL;
  json_scanf(str, strlen(str), "{ a:%d, b:%M, c:%Q, d:%B }",
             &a, scan_array, my_data, &c, &d);

  // This function is called by json_scanf() call above.
  // str is "[1, 2]", user_data is my_data.
  static void scan_array(const char *str, int len, void *user_data) {
    struct json_token t;
    int i;
    printf("Parsing array: %.*s\n", len, str);
    for (i = 0; json_scanf_array_elem(str, len, "", i, &t) > 0; i++) {
      printf("Index %d, token [%.*s]\n", i, t.len, t.ptr);
    }
  }

  // str has the following JSON string - array of objects:
  // { "a": [ {"b": 123}, {"b": 345} ] }
  // This example shows how to iterate over array, and parse each object.

  int i, value, len = strlen(str);
  struct json_token t;

  for (i = 0; json_scanf_array_elem(str, len, ".a", i, &t) > 0; i++) {
    // t.type == JSON_TYPE_OBJECT
    json_scanf(t.ptr, t.len, "{b: %d}", &value);  // value is 123, then 345
  }

int json_scanf_array_elem(const char *s, int len,
                          const char *path,
                          int index,
                          struct json_token *token);

struct json_out {
  int (*printer)(struct json_out *, const char *str, size_t len);
  union {
    struct {
      char *buf;
      size_t size;
      size_t len;
    } buf;
    void *data;
    FILE *fp;
  } u;
};

struct json_out out1 = JSON_OUT_BUF(buf, len);
struct json_out out2 = JSON_OUT_FILE(fp);

typedef int (*json_printf_callback_t)(struct json_out *, va_list *ap);
int json_printf(struct json_out *, const char *fmt, ...);
int json_vprintf(struct json_out *, const char *fmt, va_list ap);

  json_printf(&out, "{%Q: %d, x: [%B, %B], y: %Q}", "foo", 123, 0, -1, "hi");
  // Result:
  // {"foo": 123, "x": [false, true], "y": "hi"}

  struct my_struct { int a, b; } mys = {1,2};
  json_printf(&out, "{foo: %M, bar: %d}", print_my_struct, &mys, 3);
  // Result:
  // {"foo": {"a": 1, "b": 2}, "bar": 3}

int print_my_struct(struct json_out *out, va_list *ap) {
  struct my_struct *p = va_arg(*ap, struct my_struct *);
  return json_printf(out, "{a: %d, b: %d}", p->a, p->b);
}

int json_printf_array(struct json_out *, va_list *ap);

/* JSON token type */
enum json_token_type {
  JSON_TYPE_INVALID = 0, /* memsetting to 0 should create INVALID value */
  JSON_TYPE_STRING,
  JSON_TYPE_NUMBER,
  JSON_TYPE_TRUE,
  JSON_TYPE_FALSE,
  JSON_TYPE_NULL,
  JSON_TYPE_OBJECT_START,
  JSON_TYPE_OBJECT_END,
  JSON_TYPE_ARRAY_START,
  JSON_TYPE_ARRAY_END,

  JSON_TYPES_CNT,
};

/*
 * Structure containing token type and value. Used in `json_walk()` and
 * `json_scanf()` with the format specifier `%T`.
 */
struct json_token {
  const char *ptr;           /* Points to the beginning of the value */
  int len;                   /* Value length */
  enum json_token_type type; /* Type of the token, possible values are above */
};

/* Callback-based API */
typedef void (*json_walk_callback_t)(void *callback_data,
                                     const char *name, size_t name_len,
                                     const char *path,
                                     const struct json_token *token);

/*
 * Parse `json_string`, invoking `callback` in a way similar to SAX parsers;
 * see `json_walk_callback_t`.
 */
int json_walk(const char *json_string, int json_string_length,
              json_walk_callback_t callback, void *callback_data);

struct frozen_args {
  json_walk_callback_t callback;
  void *callback_data;
  int limit;
};

struct frozen_args args[1];

INIT_FROZEN_ARGS(args);

args->callback = mycb;
args->callback_data = data;
args->limit = 20;

ret = json_walk_args(string, len, args);

/*
 * Same as json_printf, but prints to a file.
 * File is created if does not exist. File is truncated if already exists.
 */
int json_fprintf(const char *file_name, const char *fmt, ...);
int json_vfprintf(const char *file_name, const char *fmt, va_list ap);

/*
 * Print JSON into an allocated 0-terminated string.
 * Return allocated string, or NULL on error.
 * Example:
 *
 * ```c
 *   char *str = json_asprintf("{a:%H}", 3, "abc");
 *   printf("%s\n", str);  // Prints "616263"
 *   free(str);
 * ```
 */
char *json_asprintf(const char *fmt, ...);
char *json_vasprintf(const char *fmt, va_list ap);

/*
 * Read the whole file in memory.
 * Return malloc-ed file content, or NULL on error. The caller must free().
 */
char *json_fread(const char *file_name);

/*
 * Update given JSON string `s,len` by changing the value at given `json_path`.
 * The result is saved to `out`. If `json_fmt` == NULL, that deletes the key.
 * If path is not present, missing keys are added. Array path without an
 * index pushes a value to the end of an array.
 * Return 1 if the string was changed, 0 otherwise.
 *
 * Example:  s is a JSON string { "a": 1, "b": [ 2 ] }
 *   json_setf(s, len, out, ".a", "7");     // { "a": 7, "b": [ 2 ] }
 *   json_setf(s, len, out, ".b", "7");     // { "a": 1, "b": 7 }
 *   json_setf(s, len, out, ".b[]", "7");   // { "a": 1, "b": [ 2,7 ] }
 *   json_setf(s, len, out, ".b", NULL);    // { "a": 1 }
 */
int json_setf(const char *s, int len, struct json_out *out,
              const char *json_path, const char *json_fmt, ...);

int json_vsetf(const char *s, int len, struct json_out *out,
               const char *json_path, const char *json_fmt, va_list ap);

/*
 * Pretty-print JSON string `s,len` into `out`.
 * Return number of processed bytes in `s`.
 */
int json_prettify(const char *s, int len, struct json_out *out);

/*
 * Prettify JSON file `file_name`.
 * Return number of processed bytes, or negative number of error.
 * On error, file content is not modified.
 */
int json_prettify_file(const char *file_name);

/*
 * Iterate over an object at given JSON `path`.
 * On each iteration, fill the `key` and `val` tokens. It is OK to pass NULL
 * for `key`, or `val`, in which case they won't be populated.
 * Return an opaque value suitable for the next iteration, or NULL when done.
 *
 * Example:
 *
 * ```c
 * void *h = NULL;
 * struct json_token key, val;
 * while ((h = json_next_key(s, len, h, ".foo", &key, &val)) != NULL) {
 *   printf("[%.*s] -> [%.*s]\n", key.len, key.ptr, val.len, val.ptr);
 * }
 * ```
 */
void *json_next_key(const char *s, int len, void *handle, const char *path,
                    struct json_token *key, struct json_token *val);


/*
 * Iterate over an array at given JSON `path`.
 * Similar to `json_next_key`, but fills array index `idx` instead of `key`.
 */
void *json_next_elem(const char *s, int len, void *handle, const char *path,
                     int *idx, struct json_token *val);

json_fprintf("settings.json", "{ a: %d, b: %Q }", 123, "string_value");
json_prettify_file("settings.json"); // Optional

struct my_config { int a; char *b; } c = { .a = 0, .b = NULL };
char *content = json_fread("settings.json");
json_scanf(content, strlen(content), "{a: %d, b: %Q}", &c.a, &c.b);

const char *settings_file_name = "settings.json", *tmp_file_name = "tmp.json";
char *content = json_fread(settings_file_name);
FILE *fp = fopen(tmp_file_name, "w");
struct json_out out = JSON_OUT_FILE(fp);
json_setf(content, strlen(content), &out, ".b", "%Q", "new_string_value");
fclose(fp);
json_prettify_file(tmp_file_name);  // Optional
rename(tmp_file_name, settings_file_name);