Updated: 2021/Apr/14

LIBMJ(3)                   Library Functions Manual                   LIBMJ(3)

     libmj - minimalist JSON lightweight data interchange library

     Minimalist JSON library (libmj, -lmj)

     #include <mj.h>

     mj_create(mj_t *atom, const char *text, ...);

     mj_parse(mj_t *atom, const char *text, int *tokfrom, int *tokto,
         int *toktype);

     mj_append(mj_t *atom, const char *text, ...);

     mj_append_field(mj_t *atom, const char *fieldname, const char *text,

     mj_deepcopy(mj_t *dest, mj_t *src);

     mj_delete(mj_t *atom);

     Access to objects and array entries is made using the following
     mj_arraycount(mj_t *atom);

     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:
     mj_snprint(char *buffer, size_t size, mj_t *atom);

     mj_asprint(char **buffer, mj_t *atom);

     mj_string_size(mj_t *atom);

     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

     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.

     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.

     calloc(3), free(3)

     ECMA-262: ECMAScript Language Specification,
     Ecma International, December 2009, 5th Edition.

     The libmj library first appeared in NetBSD 6.0.

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

NetBSD 9.99                      April 3, 2018                     NetBSD 9.99