freebsd-nq/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

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

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:

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

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

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:

{ "var": "value" }

while this one won't be parsed correctly:

{ "var": 

This limitation may possible be removed in future.

ucl_parser_add_file

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

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

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

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

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

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):

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:

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

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

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.