Size limit of JSON data type in PostgreSQL

For jsonb fields, if you look at the jsonb.c in the source code, you'll see this function:

checkStringLen(size_t len)
{
    if (len > JENTRY_OFFLENMASK)
        ereport(ERROR,
                (errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
                 errmsg("string too long to represent as jsonb string"),
                 errdetail("Due to an implementation restriction, jsonb strings cannot exceed %d bytes.",
                           JENTRY_OFFLENMASK)));

    return len;
}

Error code says that jsonb strings cannot exceed JENTRY_OFFLENMASK bytes.

In jsonb.h, that constant is defined as:

#define JENTRY_OFFLENMASK       0x0FFFFFFF

which is 255 MB.

I checked this in the source code for PostgreSQL 9.4 up to 14.

Looking at the source for PostgreSQL 9.2.1:

Source: postgresql-9.2.1\src\backend\utils\adt\json.c:
/*
 * Input.
 */
Datum
json_in(PG_FUNCTION_ARGS)
{
    char       *text = PG_GETARG_CSTRING(0);

    json_validate_cstring(text);

    /* Internal representation is the same as text, for now */
    PG_RETURN_TEXT_P(cstring_to_text(text));
}

Update for PostgreSQL 9.3.5:

The code has changed in the json_in function, but the json internal representation is still text:

Source: postgresql-9.3.5\src\backend\utils\adt\json.c:
/*
 * Input.
 */
Datum
json_in(PG_FUNCTION_ARGS)
{
    char       *json = PG_GETARG_CSTRING(0);
    text       *result = cstring_to_text(json);
    JsonLexContext *lex;

    /* validate it */
    lex = makeJsonLexContext(result, false);
    pg_parse_json(lex, &nullSemAction);

    /* Internal representation is the same as text, for now */
    PG_RETURN_TEXT_P(result);
}

So it appears that, for now at least, json is the same as a text datatype but with JSON validation. The text datatype's maximum size is 1GB.