xref: /freebsd-11.0-release/contrib/libucl/doc/api.md

Synopsis
========

`#include <ucl.h>`

Description
===========

Libucl is a parser and `C` API to parse and generate `ucl` objects. Libucl consist of several groups of functions:

### Parser functions
Used to parse `ucl` files and provide interface to extract `ucl` object

### Emitting functions
Convert `ucl` objects to some textual or binary representation.

### Conversion functions
Help to convert `ucl` objects to C types

### Generation functions
Allow creating of `ucl` objects from C types

### Iteration functions
Iterate over `ucl` objects

### Utility functions
Provide basic utilities to manage `ucl` objects

# Parser functions

Parser functions operates with `struct ucl_parser`.

### ucl_parser_new

~~~C
struct ucl_parser* ucl_parser_new (int flags);
~~~

Creates new parser with the specified flags:

- `UCL_PARSER_KEY_LOWERCASE` - lowercase keys parsed
- `UCL_PARSER_ZEROCOPY` - try to use zero-copy mode when reading files (in zero-copy mode text chunk being parsed without copying strings so it should exist till any object parsed is used)

### ucl_parser_register_macro

~~~C
void ucl_parser_register_macro (struct ucl_parser *parser,
    const char *macro, ucl_macro_handler handler, void* ud);
~~~

Register new macro with name .`macro` parsed by handler `handler` that accepts opaque data pointer `ud`. Macro handler should be of the following type:

~~~C
bool (*ucl_macro_handler) (const unsigned char *data,
    size_t len, void* ud);`
~~~

Handler function accepts macro text `data` of length `len` and the opaque pointer `ud`. If macro is parsed successfully the handler should return `true`. `false` indicates parsing failure and the parser can be terminated.

### ucl_parser_register_variable

~~~C
void ucl_parser_register_variable (struct ucl_parser *parser,
    const char *var, const char *value);
~~~

Register new variable $`var` that should be replaced by the parser to the `value` string.

### ucl_parser_add_chunk

~~~C
bool ucl_parser_add_chunk (struct ucl_parser *parser, 
    const unsigned char *data, size_t len);
~~~

Add new text chunk with `data` of length `len` to the parser. At the moment, `libucl` parser is not a streamlined parser and chunk *must* contain the *valid* ucl object. For example, this object should be valid:

~~~json
{ "var": "value" }
~~~

while this one won't be parsed correctly:

~~~json
{ "var": 
~~~

This limitation may possible be removed in future.

### ucl_parser_add_file

~~~C
bool ucl_parser_add_file (struct ucl_parser *parser, 
    const char *filename);
~~~

Load file `filename` and parse it with the specified `parser`. This function uses `mmap` call to load file, therefore, it should not be `shrinked` during parsing. Otherwise, `libucl` can cause memory corruption and terminate the calling application. This function is also used by the internal handler of `include` macro, hence, this macro has the same limitation.

### ucl_parser_get_object

~~~C
ucl_object_t* ucl_parser_get_object (struct ucl_parser *parser);
~~~

If the `ucl` data has been parsed correctly this function returns the top object for the parser. Otherwise, this function returns the `NULL` pointer. The reference count for `ucl` object returned is increased by one, therefore, a caller should decrease reference by using `ucl_object_unref` to free object after usage.

### ucl_parser_get_error

~~~C
const char *ucl_parser_get_error(struct ucl_parser *parser);
~~~

Returns the constant error string for the parser object. If no error occurred during parsing a `NULL` object is returned. A caller should not try to free or modify this string.

### ucl_parser_free

~~~C
void ucl_parser_free (struct ucl_parser *parser);
~~~

Frees memory occupied by the parser object. The reference count for top object is decreased as well, however if the function `ucl_parser_get_object` was called previously then the top object won't be freed.

### ucl_pubkey_add

~~~C
bool ucl_pubkey_add (struct ucl_parser *parser, 
    const unsigned char *key, size_t len);
~~~

This function adds a public key from text blob `key` of length `len` to the `parser` object. This public key should be in the `PEM` format and can be used by `.includes` macro for checking signatures of files included. `Openssl` support should be enabled to make this function working. If a key cannot be added (e.g. due to format error) or `openssl` was not linked to `libucl` then this function returns `false`.

### ucl_parser_set_filevars

~~~C
bool ucl_parser_set_filevars (struct ucl_parser *parser, 
    const char *filename, bool need_expand);
~~~

Add the standard file variables to the `parser` based on the `filename` specified:

- `$FILENAME` - a filename of `ucl` input
- `$CURDIR` - a current directory of the input

For example, if a `filename` param is `../something.conf` then the variables will have the following values:

- `$FILENAME` - "../something.conf"
- `$CURDIR` - ".."

if `need_expand` parameter is `true` then all relative paths are expanded using `realpath` call. In this example if `..` is `/etc/dir` then variables will have these values:

- `$FILENAME` - "/etc/something.conf"
- `$CURDIR` - "/etc"

## Parser usage example

The following example loads, parses and extracts `ucl` object from stdin using `libucl` parser functions (the length of input is limited to 8K):

~~~C
char inbuf[8192];
struct ucl_parser *parser = NULL;
int ret = 0, r = 0;
ucl_object_t *obj = NULL;
FILE *in;

in = stdin;
parser = ucl_parser_new (0);
while (!feof (in) && r < (int)sizeof (inbuf)) {
	r += fread (inbuf + r, 1, sizeof (inbuf) - r, in);
}
ucl_parser_add_chunk (parser, inbuf, r);
fclose (in);

if (ucl_parser_get_error (parser)) {
	printf ("Error occured: %s\n", ucl_parser_get_error (parser));
	ret = 1;
}
else {
    obj = ucl_parser_get_object (parser);
}

if (parser != NULL) {
	ucl_parser_free (parser);
}
if (obj != NULL) {
	ucl_object_unref (obj);
}
return ret;
~~~

# Emitting functions

Libucl can transform UCL objects to a number of tectual formats:

- configuration (`UCL_EMIT_CONFIG`) - nginx like human readable configuration file where implicit arrays are transformed to the duplicate keys
- compact json: `UCL_EMIT_JSON_COMPACT` - single line valid json without spaces
- formatted json: `UCL_EMIT_JSON` - pretty formatted JSON with newlines and spaces
- compact yaml: `UCL_EMIT_YAML` - compact YAML output

Moreover, libucl API allows to select a custom set of emitting functions allowing 
efficent and zero-copy output of libucl objects. Libucl uses the following structure to support this feature:

~~~C
struct ucl_emitter_functions {
	/** Append a single character */
	int (*ucl_emitter_append_character) (unsigned char c, size_t nchars, void *ud);
	/** Append a string of a specified length */
	int (*ucl_emitter_append_len) (unsigned const char *str, size_t len, void *ud);
	/** Append a 64 bit integer */
	int (*ucl_emitter_append_int) (int64_t elt, void *ud);
	/** Append floating point element */
	int (*ucl_emitter_append_double) (double elt, void *ud);
	/** Opaque userdata pointer */
	void *ud;
};
~~~

This structure defines the following callbacks:

- `ucl_emitter_append_character` - a function that is called to append `nchars` characters equal to `c`
- `ucl_emitter_append_len` - used to append a string of length `len` starting from pointer `str`
- `ucl_emitter_append_int` - this function applies to integer numbers
- `ucl_emitter_append_double` - this function is intended to output floating point variable

The set of these functions could be used to output text formats of `UCL` objects to different structures or streams.

Libucl provides the following functions for emitting UCL objects:

### ucl_object_emit

~~~C
unsigned char *ucl_object_emit (ucl_object_t *obj, enum ucl_emitter emit_type);
~~~

Allocate a string that is suitable to fit the underlying UCL object `obj` and fill it with the textual representation of the object `obj` according to style `emit_type`. The caller should free the returned string after using.

### ucl_object_emit_full

~~~C
bool ucl_object_emit_full (ucl_object_t *obj, enum ucl_emitter emit_type,
		struct ucl_emitter_functions *emitter);
~~~

This function is similar to the previous with the exception that it accepts the additional argument `emitter` that defines the concrete set of output functions. This emit function could be useful for custom structures or streams emitters (including C++ ones, for example).

# Conversion functions

Conversion functions are used to convert UCL objects to primitive types, such as strings, numbers or boolean values. There are two types of conversion functions:

- safe: try to convert an ucl object to a primitive type and fail if such a conversion is not possible
- unsafe: return primitive type without additional checks, if the object cannot be converted then some reasonable default is returned (NULL for strings and 0 for numbers)

Also there is a single `ucl_object_tostring_forced` function that converts any UCL object (including compound types - arrays and objects) to a string representation. For compound and numeric types this function performs emitting to a compact json format actually.

Here is a list of all conversion functions:

- `ucl_object_toint` - returns `int64_t` of UCL object
- `ucl_object_todouble` - returns `double` of UCL object
- `ucl_object_toboolean` - returns `bool` of UCL object
- `ucl_object_tostring` - returns `const char *` of UCL object (this string is NULL terminated)
- `ucl_object_tolstring` - returns `const char *` and `size_t` len of UCL object (string can be not NULL terminated)
- `ucl_object_tostring_forced` - returns string representation of any UCL object

Strings returned by these pointers are associated with the UCL object and exist over its lifetime. A caller should not free this memory.

# Generation functions

It is possible to generate UCL objects from C primitive types. Moreover, libucl permits to create and modify complex UCL objects, such as arrays or associative objects. 

## ucl_object_new
~~~C
ucl_object_t * ucl_object_new (void)
~~~

Creates new object of type `UCL_NULL`. This object should be released by caller.

## ucl_object_typed_new
~~~C
ucl_object_t * ucl_object_typed_new (unsigned int type)
~~~

Create an object of a specified type:
- `UCL_OBJECT` - UCL object - key/value pairs
- `UCL_ARRAY` - UCL array
- `UCL_INT` - integer number
- `UCL_FLOAT` - floating point number
- `UCL_STRING` - NULL terminated string
- `UCL_BOOLEAN` - boolean value
- `UCL_TIME` - time value (floating point number of seconds)
- `UCL_USERDATA` - opaque userdata pointer (may be used in macros)
- `UCL_NULL` - null value

This object should be released by caller.

## Primitive objects generation
Libucl provides the functions similar to inverse conversion functions called with the specific C type:
- `ucl_object_fromint` - converts `int64_t` to UCL object
- `ucl_object_fromdouble` - converts `double` to UCL object
- `ucl_object_fromboolean` - converts `bool` to UCL object
- `ucl_object_fromstring` - converts `const char *` to UCL object (this string is NULL terminated)
- `ucl_object_fromlstring` - converts `const char *` and `size_t` len to UCL object (string can be not NULL terminated)

Also there is a function to generate UCL object from a string performing various parsing or conversion operations called `ucl_object_fromstring_common`.

## ucl_object_fromstring_common
~~~C
ucl_object_t * ucl_object_fromstring_common (const char *str, 
	size_t len, enum ucl_string_flags flags)
~~~

This function is used to convert a string `str` of size `len` to an UCL objects applying `flags` conversions. If `len` is equal to zero then a `str` is assumed as NULL-terminated. This function supports the following flags (a set of flags can be specified using logical `OR` operation):

- `UCL_STRING_ESCAPE` - perform JSON escape
- `UCL_STRING_TRIM` - trim leading and trailing whitespaces
- `UCL_STRING_PARSE_BOOLEAN` - parse passed string and detect boolean
- `UCL_STRING_PARSE_INT` - parse passed string and detect integer number
- `UCL_STRING_PARSE_DOUBLE` - parse passed string and detect integer or float number
- `UCL_STRING_PARSE_NUMBER` - parse passed string and detect number (both float or integer types)
- `UCL_STRING_PARSE` - parse passed string (and detect booleans and numbers)
- `UCL_STRING_PARSE_BYTES` - assume that numeric multipliers are in bytes notation, for example `10k` means `10*1024` and not `10*1000` as assumed without this flag

If parsing operations fail then the resulting UCL object will be a `UCL_STRING`. A caller should always check the type of the returned object and release it after using.

# Iteration function

Iteration are used to iterate over UCL compound types: arrays and objects. Moreover, iterations could be performed over the keys with multiple values (implicit arrays). To iterate over an object, an array or a key with multiple values there is a function `ucl_iterate_object`.

## ucl_iterate_object
~~~C
ucl_object_t* ucl_iterate_object (ucl_object_t *obj, 
	ucl_object_iter_t *iter, bool expand_values);
~~~

This function accept opaque iterator pointer `iter`. In the first call this iterator *must* be initialized to `NULL`. Iterator is changed by this function call. `ucl_iterate_object` returns the next UCL object in the compound object `obj` or `NULL` if all objects have been iterated. The reference count of the object returned is not increased, so a caller should not unref the object or modify its content (e.g. by inserting to another compound object). The object `obj` should not be changed during the iteration process as well. `expand_values` flag speicifies whether `ucl_iterate_object` should expand keys with multiple values. The general rule is that if you need to iterate throught the *object* or *explicit array*, then you always need to set this flag to `true`. However, if you get some key in the object and want to extract all its values then you should set `expand_values` to `false`. Mixing of iteration types are not permitted since the iterator is set according to the iteration type and cannot be reused. Here is an example of iteration over the objects using libucl API (assuming that `top` is `UCL_OBJECT` in this example):

~~~C
ucl_object_iter_t it = NULL, it_obj = NULL;
ucl_object_t *cur, *tmp;

/* Iterate over the object */
while ((obj = ucl_iterate_object (top, &it, true))) {
	printf ("key: \"%s\"\n", ucl_object_key (obj));
	/* Iterate over the values of a key */
	while ((cur = ucl_iterate_object (obj, &it_obj, false))) {
		printf ("value: \"%s\"\n", 
			ucl_object_tostring_forced (cur));
	}
}
~~~