LIBMJ(3)

NAME
libmj - minimalist JSON lightweight data interchange library

LIBRARY
libmj

SYNOPSIS
#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);

DESCRIPTION
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.

EXAMPLES
The following code fragment will make a JSON object out of the string “Hello <USERNAME>\n” in the buffer called buf where “USER” is the name of the user taken from the runtime environment. The encoded text will be in an allocated buffer called s.

    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.

SEE ALSO
calloc(3), free(3) Ecma International, December 2009, ECMA-262: ECMAScript Language Specification, http://www.ecma-international.org/publications/files/ecma-st/ECMA-262.pdf, 5th Edition.

HISTORY
The libmj library first appeared in NetBSD 6.0.

AUTHORS
Alistair Crooks <agc@NetBSD.org> wrote this implementation and manual page.