What is the result of number when it is left shifted by -1 in C programming using the left shift operator?
e.g.:
23 << -1
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chux - Reinstate Monica
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Mithun Arunan
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2left shifting with -1 looks like a right shifting with 1 – adricadar Jul 21 '15 at 19:37
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@adricadar: Looks like (and I sometimes whish it was), but: Fundamentally wrong. – too honest for this site Jul 21 '15 at 19:40
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Note: As OP said "number", there is another restriction: "Each of the operands shall have integer type". – chux - Reinstate Monica Jul 21 '15 at 19:55
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Ummm why would anyone attempt this? If the answer is 'nobody' then why bother asking? – Martin James Jul 21 '15 at 20:31
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1@MartinJames: A negative constant shift might areise from macros. But more important are run-time yielded negative shifts, e.g. during normatization of software-floating point arithmetic. – too honest for this site Jul 21 '15 at 20:33
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1@MartinJames: If we close all questions about UB, we could very well close all C questions. The few non-UB questions would be acceptable "collateral damage". – too honest for this site Jul 21 '15 at 20:35
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Six votes for a multi-duped illegal shift operation question. Wonderful. – Martin James Jul 21 '15 at 20:39
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@MartinJames: Java is not C. It is not necessarily obvious that negative shifts are UB. Reversing the direction sounds quite reasonable to me actually (and IIRC, some Pascal or Modula implemenation did support it). – too honest for this site Jul 21 '15 at 20:39
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Yeah - I deleted it. – Martin James Jul 21 '15 at 20:40
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Oh..... thingy. I cannot find any dups now! I KNOW I've seen this before recently, but no find:( I would withdraw my dup close vote if I had made one. – Martin James Jul 21 '15 at 20:49
3 Answers
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From the C11 standard 6.5.7p3 (for former versions it is basically the same):
"If the value of the right operand is negative or is greater than or equal to the width of the promoted left operand, the behavior is undefined."
IOW: undefined behaviour. Prepare for nasal demons.
Briefly: Do not
Caution: While many programmers are aware of that and avoid negative shift counts, it is often ignored that also counts >= the bit-size of the value are also undefined. This makes something like ((unsigned int)1 << 32) - 1 actually undefined if unsigned int has 32 bits or less. For signed values things become more complicated due to the sign (thanks @chux for pointing me at that). This is a common pitfall. For some implementations, different results for constant expressions (compile-time evaluated) and run-time evaluation might occur.
too honest for this site
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1Is not `((int)1 << 31) - 1` also undefined if `int` has 32 bits or less? – chux - Reinstate Monica Jul 21 '15 at 20:04
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As Olaf said in his answer, left shifting by a negative number is undefined.
In fact, gcc will give a warning if you attempt shift in either direction by a negative number.
dbush
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3"... gcc will give a warning ...": but only if it can detect it compile time, i.e. if the shift count is a constant as in the question. – too honest for this site Jul 21 '15 at 19:53
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Bitwise Left Shift Operator in C:
- Bit Pattern of the data can be shifted by specified number of Positions to Left
- When Data is Shifted Left , trailing zero’s are filled with zero.
- Left shift Operator is Binary Operator [Bi – two]
- Binary means , Operator that require two arguments!
Syntax:
**[variable]<<[number of places]**
If Number of Places become negative, it will be undefined
Mr. Perfectionist
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