From 7d49fc75d5a6050c852f8c5e370190958ce3358e Mon Sep 17 00:00:00 2001 From: Petri Lehtinen Date: Tue, 25 Jan 2011 21:41:52 +0200 Subject: [PATCH] Documentation for pack and unpack functions --- doc/apiref.rst | 227 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 227 insertions(+) diff --git a/doc/apiref.rst b/doc/apiref.rst index dcc0cbf..11041d4 100644 --- a/doc/apiref.rst +++ b/doc/apiref.rst @@ -782,6 +782,233 @@ affect especially the behavior of the decoder. is returned to the caller. +.. _apiref-building-values: + +Building values +=============== + +This sectinon describes functions that help to create, or *pack*, +complex JSON values, especially nested objects and arrays. Value +building is based on a *format string* that is used to tell the +functions about the expected arguments. + +For example, the format string ``"i"`` specifies a single integer +value, while the format string ``"[ssb]"`` or the equivalent ``"[s, s, +b]"`` specifies an array value with two integers and a boolean as its +items:: + + /* Create the JSON integer 42 */ + json_pack("i", 42); + + /* Create the JSON array ["foo", "bar", true] */ + json_pack("[ssb]", "foo", "bar", 1); + +Here's the full list of format characters. The type in parentheses +denotes the resulting JSON type, and the type in brackets (if any) +denotes the C type that is expected as the corresponding argument. + +``s`` (string) [const char \*] + Convert a NULL terminated UTF-8 string to a JSON string. + +``n`` (null) + Output a JSON null value. No argument is consumed. + +``b`` (boolean) [int] + Convert a C :type:`int` to JSON boolean value. Zero is converted + to ``false`` and non-zero to ``true``. + +``i`` (integer) [int] + Convert a C :type:`int` to JSON integer. + +``I`` (integer) [json_int_t] + Convert a C :type:`json_int_t` to JSON integer. + +``f`` (real) [double] + Convert a C :type:`double` to JSON real. + +``o`` (any value) [json_t \*] + Output any given JSON value as-is. If the value is added to an + array or object, the reference to the value passed to ``o`` is + stealed by the container. + +``O`` (any value) [json_t \*] + Like ``o``, but the argument's reference count is incremented. + This is useful if you pack and array an array or object and want + to keep the reference for the JSON value consumed by ``O`` to + yourself. + +``[fmt]`` (array) + Build an array with contents from the inner format string. ``fmt`` + may contain objects and arrays, i.e. recursive value building is + supported. + +``{fmt}`` (object) + Build an object with contents from the inner format string + ``fmt``. The first, third, etc. format character represent a key, + and must be ``s`` (as object keys are always strings). The second, + fourth, etc. format character represent a value. Any value may be + an object or array, i.e. recursive value building is supported. + + +.. function:: json_t *json_pack(const char *fmt, ...) + + .. refcounting:: new + + Build a new JSON value according to the format string *fmt*. For + each format character (except for ``{}[]n``), one argument is + consumed and used to build the corresponding value. Returns *NULL* + on error. + +.. function:: json_t *json_pack_ex(json_error_t *error, size_t flags, const char *fmt, ...) + json_t *json_vpack_ex(json_error_t *error, size_t flags, const char *fmt, va_list ap) + + .. refcounting:: new + + Like :func:`json_pack()`, but an in the case of an error, an error + message is written to *error*, if it's not *NULL*. The *flags* + parameter is currently unused and should be set to 0. + + As only the errors in format string (and out-of-memory errors) can + be caught by the packer, these two functions are most likely only + useful for debugging format strings. + +More examples:: + + /* Build an empty JSON object */ + json_pack("{}"); + + /* Build the JSON object {"foo": 42, "bar": 7} */ + json_pack("{sisb}", "foo", 42, "bar", 7); + + /* Like above, ':', ',' and whitespace are ignored */ + json_pack("{s:i, s:b}", "foo", 42, "bar", 7); + + /* Build the JSON array [[1, 2], {"cool": true}] */ + json_pack("[[i,i],{s:b]]", 1, 2, "cool", 1); + + +Parsing and validating values +============================= + +This sectinon describes functions that help to validate complex values +and extract, or *unpack*, data from them. Like :ref:`building values +`, this is also based on format strings. + +While a JSON value is unpacked, the type specified in the format +string is checked to match that of the JSON value. This is the +validation part of the process. By default, the unpacking functions +also check that all items of arrays and objects are unpacked. This +check be disabled with the format character ``*`` or by using the flag +``JSON_UNPACK_ONLY``. + +Here's the full list of format characters. The type in parentheses +denotes the JSON type, and the type in brackets (if any) denotes the C +type whose address should be passed. + +``s`` (string) [const char \*] + Convert a JSON string to a pointer to a NULL terminated UTF-8 + string. + +``n`` (null) + Expect a JSON null value. Nothing is extracted. + +``b`` (boolean) [int] + Convert a JSON boolean value to a C :type:`int`, so that ``true`` + is converted to 1 and ``false`` to 0. + +``i`` (integer) [int] + Convert a JSON integer to C :type:`int`. + +``I`` (integer) [json_int_t] + Convert a JSON integer to C :type:`json_int_t`. + +``f`` (real) [double] + Convert a JSON real to C :type:`double`. + +``F`` (integer or real) [double] + Convert a JSON number (integer or real) to C :type:`double`. + +``o`` (any value) [json_t \*] + Store a JSON value with no conversion to a :type:`json_t` pointer. + +``O`` (any value) [json_t \*] + Like ``O``, but the JSON value's reference count is incremented. + +``[fmt]`` (array) + Convert each item in the JSON array according to the inner format + string. ``fmt`` may contain objects and arrays, i.e. recursive + value extraction is supporetd. + +``{fmt}`` (object) + Convert each item in the JSON object according to the inner format + string ``fmt``. The first, third, etc. format character represent + a key, and must be ``s``. The corresponding argument to unpack + functions is read as the object key. The second fourth, etc. + format character represent a value and is written to the address + given as the corresponding argument. **Note** that every other + argument is read from and every other is written to. + + ``fmt`` may contain objects and arrays as values, i.e. recursive + value extraction is supporetd. + +``*`` + This special format character is used to disable the check that + all object and array items are accessed on a per-value basis. It + must appear inside an array or object as the last format character + before the closing bracket or brace. + + +.. function:: int json_unpack(json_t *root, const char *fmt, ...) + + Validate and unpack the JSON value *root* according to the format + string *fmt*. Returns 0 on success and -1 on failure. + +.. function:: int json_unpack_ex(json_t *root, json_error_t *error, size_t flags, const char *fmt, ...) + int json_vunpack_ex(json_t *root, json_error_t *error, size_t flags, const char *fmt, va_list ap) + + Validate and unpack the JSON value *root* according to the format + string *fmt*. If an error occurs and *error* is not *NULL*, write + error information to *error*. *flags* can be used to control the + behaviour of the unpacker, see below for the flags. Returns 0 on + success and -1 on failure. + +The following unpacking flags are available: + +``JSON_UNPACK_ONLY`` + Disable the validation step checking that all object and array + items are unpacked. This is equivalent to appending the format + character ``*`` to the end of every array and object in the format + string. + +``JSON_VALIDATE_ONLY`` + Don't extract any data, just validate the JSON value against the + given format string. Note that object keys must still be specified + after the format string. + +Examples:: + + /* root is the JSON integer 42 */ + int myint; + json_unpack(root, "i", &myint); + assert(myint == 42); + + /* root is the JSON object {"foo": "bar", "quux": true} */ + const char *str; + int boolean; + json_unpack(root, "{s:s, s:b}", "foo", &str, "quux", &boolean); + assert(strcmp(str, "bar") == 0 && boolean == 1); + + /* root is the JSON array [[1, 2], {"baz": null} */ + json_error_t error; + json_unpack_ex(root, &error, JSON_VALIDATE_ONLY, "[[i,i], {s:n}]", "baz"); + /* returns 0 for validation success, nothing is extracted */ + + /* root is the JSON array [1, 2, 3, 4, 5] */ + int myint1, myint2, ret; + ret = json_unpack(root, "[ii*]", &myint1, &myint2); + assert(ret == 0 && myint1 == 1 && myint2 == 2); + + Equality ======== -- 2.1.4