0000633: left shift of negative numbers invokes undefined behaviour in C++

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0000633

OMNeT++

simulation kernel

public

2013-03-06 09:37

2017-02-01 15:36

Reporter

Harti

Assigned To

andras

Priority

normal

Severity

trivial

Reproducibility

always

Status

resolved

Resolution

fixed

Platform

OS Version

Product Version

4.2.2

Target Version

Fixed in Version

5.1pre3

Summary

0000633: left shift of negative numbers invokes undefined behaviour in C++

Description

It is undefined behavior in C++ and C to left-shift negative integers. The actual C standard has this in section 6.5.7 paragraph 4 in C++ this is in 5.8 paragraph 3. It is also in the older standards but I don't have the reference at hand.

The problem is with the definitions of FSM_Transient and FSM_Exit and can be seen in the samples/routing/nodeBurstApp.cc. FSM_Transient and FSM_Exit are

#define FSM_Transient(state) (-(state))
#define FSM_Exit(state) (((state)<<1)|1)

So the enum member declaration

SEND = FSM_Transient(1)

and the case:

case FSM_Exit(SEND)

expand to

case ((-1 << 1) | 1):

which invokes undefined behaviour.

The fix is to modify FSM_Exit to

#define FSM_Exit(state) (((state) * 2) | 1)

Given that this is mostly calculated at compile time there is no runtime difference and even the runtime calculation would probably converted to a shift by the compiler.

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Notes
(0000795) Harti (reporter) 2013-03-06 09:39	FSM_Enter should also be modified #define FSM_Enter(state) ((state) * 2)

(0001295) andras (administrator) 2017-02-01 15:36	Agreed, code will use *2 instead of <<1, to avoid undefined behavior.

Issue History
Date Modified	Username	Field	Change
2013-03-06 09:37	Harti	New Issue
2013-03-06 09:39	Harti	Note Added: 0000795
2017-02-01 15:36	andras	Note Added: 0001295
2017-02-01 15:36	andras	Status	new => resolved
2017-02-01 15:36	andras	Fixed in Version	=> 5.1pre3
2017-02-01 15:36	andras	Resolution	open => fixed
2017-02-01 15:36	andras	Assigned To	=> andras