LIBMJ(3) | Library Functions Manual | LIBMJ(3) |
libmj
—
#include <mj.h>
int
mj_create
(mj_t *atom,
const char *text, ...);
int
mj_parse
(mj_t *atom,
const char *text, int *tokfrom,
int *tokto, int *toktype);
int
mj_append
(mj_t *atom,
const char *text, ...);
int
mj_append_field
(mj_t *atom,
const char *fieldname, const char
*text, ...);
int
mj_deepcopy
(mj_t *dest,
mj_t *src);
void
mj_delete
(mj_t *atom);
Access to objects and array entries is made using the following
functions:
int
mj_arraycount
(mj_t *atom);
int
mj_object_find
(mj_t *atom,
const char *name, const unsigned
startpoint, const unsigned incr);
mj_t *
mj_get_atom
(mj_t *atom,
...);
JSON object output functions:
int
mj_snprint
(char *buffer,
size_t size, mj_t *atom);
int
mj_asprint
(char **buffer,
mj_t *atom);
int
mj_string_size
(mj_t *atom);
int
mj_pretty
(mj_t *atom,
void *stream, unsigned depth,
const char *trailer);
const char *
mj_string_rep
(mj_t *atom);
libmj
is a small library interface to allow JSON text to
be created and parsed. JSON is the Java Script Object Notation, a lightweight
data-interchange format, standardised by the ECMA. The library name
libmj
is derived from a further acronym of
“minimalist JSON”.
The libmj
library can be used to create a
string in memory which contains a textual representation of a number of
objects, arbitrarily structured. The library can also be used to reconstruct
the structure. Data can thus be serialised easily and efficiently, and data
structures rebuilt to produce the original structure of the data.
JSON contains basic units called atoms, the two basic atoms being strings and numbers. Three other useful atomic values are provided: “null”, “false”, and “true”. Atoms can be grouped together as key/value pairs in an “object”, and as individual, ordered atoms, in an “array”.
To create a new object, the mj_create
()
function is used. It can be deleted using the
mj_delete
() function.
Atoms, objects and arrays can be appended to arrays and objects
using the mj_append
() function.
Objects can be printed out by using the
mj_snprint
() function. The size of a string of JSON
text can be calculated using the mj_string_size
()
function. A utility function mj_asprint
() is
provided which will allocate space dynamically, using
calloc(3), and the JSON
serialised text is copied into it. This memory can later be de-allocated
using free(3). For formatted
output to a FILE * stream, the
mj_pretty
() function is used. The calling interface
gives the ability to indent the output to a given
depth in characters and for the formatted output to be
followed by a trailer string, which is usually
NULL
for external calls, but can be any valid
string. Output is sent to the stream file stream.
The type argument given to the
mj_create
(), mj_append
(),
and mj_append_field
() functions is taken from a list
of “false” “true” “null”
“number” “integer” “string”
“array” and “object” types. An integer differs
from a number in that it cannot take on any floating point values. It is
implemented internally using a signed 64-bit integer type. This restriction
of values for an integer type may be removed at a later date.
Within a JSON object, the key values can be iterated over using an
integer index to access the individual JSON objects. The index can also be
found using the mj_object_find
() function.
The way objects arrays are implemented in
libmj
is by using varying-sized arrays internally.
Objects have the field name as the even entry in this internal array, with
the value being the odd entry. Arrays are implemented as a simple array.
Thus, to find an object in an array using
mj_object_find
(), a value of 1 should be used as the
increment value. This means that every entry in the internal array will be
examined, and the first match after the starting point will be returned. For
objects, an incremental value of 2 should be used, and an even start value
should be specified.
String values should be created and appended using two parameters
in the stdarg fields, that of the string itself, and its length in bytes
immediately after the string. A value of -1
may be
used if the string length is not known.
mj_t atom; char buf[BUFSIZ]; char *s; int cc; (void) memset(&atom, 0x0, sizeof(atom)); cc = snprintf(buf, sizeof(buf), "Hello %s\n", getenv("USER")); mj_create(&atom, "string", buf, cc); cc = mj_asprint(&s, &atom, MJ_JSON_ENCODE);
Next, the following example will take the (binary) text which has been encoded into JSON and is in the buffer buf, such as in the previous example, and re-create the original text:
int from, to, tok, cc; char *s; mj_t atom; (void) memset(&atom, 0x0, sizeof(atom)); from = to = tok = 0; mj_parse(&atom, buf, &from, &to, &tok); cc = mj_asprint(&s, &atom, MJ_HUMAN); printf("%.*s", cc, s);
The s pointer points to allocated storage with the original NUL-terminated string in it.
ECMA-262: ECMAScript Language Specification, http://www.ecma-international.org/publications/files/ecma-st/ECMA-262.pdf, Ecma International, December 2009, 5th Edition.
libmj
library first appeared in
NetBSD 6.0.
April 3, 2018 | NetBSD 9.0 |