RFC Conformance

JSON is specified in RFC 4627, “The application/json Media Type for JavaScript Object Notation (JSON)”.

Character Encoding

Jansson only supports UTF-8 encoded JSON texts. It does not support or auto-detect any of the other encodings mentioned in the RFC, namely UTF-16LE, UTF-16BE, UTF-32LE or UTF-32BE. Pure ASCII is supported, as it’s a subset of UTF-8.


JSON strings are mapped to C-style null-terminated character arrays, and UTF-8 encoding is used internally.

All Unicode codepoints U+0000 through U+10FFFF are allowed in string values. However, U+0000 is not allowed in object keys because of API restrictions.

Unicode normalization or any other transformation is never performed on any strings (string values or object keys). When checking for equivalence of strings or object keys, the comparison is performed byte by byte between the original UTF-8 representations of the strings.


Real vs. Integer

JSON makes no distinction between real and integer numbers; Jansson does. Real numbers are mapped to the double type and integers to the json_int_t type, which is a typedef of long long or long, depending on whether long long is supported by your compiler or not.

A JSON number is considered to be a real number if its lexical representation includes one of e, E, or .; regardless if its actual numeric value is a true integer (e.g., all of 1E6, 3.0, 400E-2, and 3.14E3 are mathematical integers, but will be treated as real values). With the JSON_DECODE_INT_AS_REAL decoder flag set all numbers are interpreted as real.

All other JSON numbers are considered integers.

When encoding to JSON, real values are always represented with a fractional part; e.g., the double value 3.0 will be represented in JSON as 3.0, not 3.

Overflow, Underflow & Precision

Real numbers whose absolute values are too small to be represented in a C double will be silently estimated with 0.0. Thus, depending on platform, JSON numbers very close to zero such as 1E-999 may result in 0.0.

Real numbers whose absolute values are too large to be represented in a C double will result in an overflow error (a JSON decoding error). Thus, depending on platform, JSON numbers like 1E+999 or -1E+999 may result in a parsing error.

Likewise, integer numbers whose absolute values are too large to be represented in the json_int_t type (see above) will result in an overflow error (a JSON decoding error). Thus, depending on platform, JSON numbers like 1000000000000000 may result in parsing error.

Parsing JSON real numbers may result in a loss of precision. As long as overflow does not occur (i.e. a total loss of precision), the rounded approximate value is silently used. Thus the JSON number 1.000000000000000005 may, depending on platform, result in the double value 1.0.

Signed zeros

JSON makes no statement about what a number means; however Javascript (ECMAscript) does state that +0.0 and -0.0 must be treated as being distinct values, i.e. -0.0 ≠ 0.0. Jansson relies on the underlying floating point library in the C environment in which it is compiled. Therefore it is platform-dependent whether 0.0 and -0.0 will be distinct values. Most platforms that use the IEEE 754 floating-point standard will support signed zeros.

Note that this only applies to floating-point; neither JSON, C, or IEEE support the concept of signed integer zeros.


No support is provided in Jansson for any C numeric types other than json_int_t and double. This excludes things such as unsigned types, long double, etc. Obviously, shorter types like short, int, long (if json_int_t is long long) and float are implicitly handled via the ordinary C type coercion rules (subject to overflow semantics). Also, no support or hooks are provided for any supplemental “bignum” type add-on packages.