Hi everyone, from C/C++ I’m used to signed integer overflows being undefined behavior . In “the (B&R) automation world” I got the impression that it is sometimes common to rely on a certain well-defined signed integer behavior. For example in reaction programs, where timestamps are signed integers …

A signed variable follow two’s complement binary notation. [image] The carry and overflow handling are inherently managed by the binary representation. In a PLC often encoders are used, which are continue moving in one direction. The DINT value will overflow. If in software something is calulat…

Hey @Lukas_Kimme , I think I understand what you’re inquiring about. Let me share my thoughts. from C/C++ I’m used to signed integer overflows being undefined behavior while in C/C++ both b and c are undefined I think “signed integer overflow” itself, is not undefined behavior. I think spec…

Thanks @corne.geerts and @tylermatijevich . However, your answers do not answer this question for me. Regarding the question whether signed integer overflow is or is not UB in C++, cppreference states explicitly and without further restrictions: Some examples of undefined behavior are data races, …

Hi Lukas, i see the UB in your example if that would be c++. I am not sure if this is an UB in ST, too. An unabibgous code would be to check an overflow by yourself. [image] Lukas Kimme: a := 2147483647; // ==DINT_MAX b := a + 1; // ==DINT_MIN==-2147483648 (who/what guarantees this?) c := b…

Thanks Christoph. Let’s assume a first call to AsIOTimeStamp() returned a = 2147483647 = DINT_MAX and a second call returned b = -2147483648 = DINT_MIN. Now, calculating b-a or a-b in C/C++ would be UB, since the actual result overflows and is not representable in a 32bit signed integer. Another qu…

Hi Lukas, your are right. And thank you for bringing this to our attention. I assume that a nice person in the background has set the right options for the gcc: Overflow with signed integers if you add -ftrapv to the compiler options [image] you actually get a problem when you perform this op…

Hi, when AS compiles a C/C++ program, the following compiler options are set additionally to those visible in the AS project: -fvisibility=hidden -fsigned-char -fwrapv -fno-builtin -fno-diagnostics-show-caret -fwrapv defines the behavior of singed integer overflow: GNU code gen options

Great Martin and Christoph, this clarifies things quite a bit. @Martin.Schrott where did you get this information from, where is this documented? And since you mention C/C++ explicitly: is there similar information for ST? In my initial posting I also asked about reACTION, where I saw time stamps …

I don’t think this is documented. I found it by checking <project_folder>/Temp/Objects/CP1586Sim/X20CP1686X/LastBuildOptions.xml then looked for a C task and found “\AS\gnuinst\V11.3.0\6.0\lib\gcc\i686-elf\11.3.0\brelf.specs” is used for compiler options. In that file the options are listed. I c…

Thanks everyone for the discussion so far. For me it is OK to leave it at this intermediate state. Nevertheless, I don’t want to mark any answer as solution, since my initial question essentially remains unanswered. IMHO B&R should document such fundamental behavior, so developers don’t have to re…

Specification of signed integer overflow/wraparound behavior

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christoph.hilchenbac (Christoph Hilchenbach) April 29, 2025, 11:14am 12

B&R should document such fundamental behavior

I found this here:

(INT (INTEGER)

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Specification of signed integer overflow/wraparound behavior

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