Text conversions

<T>_TO_STRING, TO_STRING and STRING_TO_<T> write a value as text and read it back. The standard leaves the text to the implementer; iecf writes what CODESYS, TwinCAT and Siemens write where they document it, and reads what they read (Implementer choices). Neither direction uses the C library: the digits come from iecf’s own arithmetic, the same on every target.

To STRING

• BOOL: TRUE or FALSE.
• Integers and bit strings: decimal, a - for a negative number and no +.
• REAL and LREAL: C’s %g with 7 and 15 significant digits, correctly rounded, trailing zeros dropped — inf, -inf and nan for those.
• TIME and LTIME: the literal, with the units that are not 0, in lower case.
• DATE, TOD, DT: the literal; a TOD shows milliseconds only when they are not 0.
• LDATE, LTOD, LDT: the literal, prefixed LD#, LTOD#, LDT#; the fraction of a second in groups of three digits, as many as are not 0 (.500, .000001).

From STRING

The text is read as CODESYS and TwinCAT read it: spaces before it are skipped; then comes a literal of the type — a type prefix (INT#5), a sign, a based integer (16#FF, 2#1111_0000), underscores between digits, a REAL’s fraction and exponent, a duration’s T# (any case), TRUE or FALSE in any case, or 1 or 0; what follows is ignored: '123abc' is 123. A duration or date needs its prefix. A TIME keeps whole milliseconds and a DT whole seconds, the rest dropped. A REAL is read exactly and rounded once, half to even.

No literal, or a value the type cannot hold, gives 0 (for a DATE or DT, 1970-01-01) and is flagged CONVERSION, as Siemens gives 0 and ENO = FALSE. The task goes on.