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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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    left shifting with -1 looks like a right shifting with 1 Commented Jul 21, 2015 at 19:37
  • @adricadar: Looks like (and I sometimes whish it was), but: Fundamentally wrong. Commented Jul 21, 2015 at 19:40
  • Note: As OP said "number", there is another restriction: "Each of the operands shall have integer type". Commented Jul 21, 2015 at 19:55
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    @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. Commented Jul 21, 2015 at 20:33
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    @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". Commented Jul 21, 2015 at 20:35

3 Answers 3

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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.

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2 Comments

Is not ((int)1 << 31) - 1 also undefined if int has 32 bits or less?
@chux: Updated. Thank you!
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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.

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"... 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.
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Bitwise Left Shift Operator in C:

  1. Bit Pattern of the data can be shifted by specified number of Positions to Left
  2. When Data is Shifted Left , trailing zero’s are filled with zero.
  3. Left shift Operator is Binary Operator [Bi – two]
  4. Binary means , Operator that require two arguments!

Table

Syntax:

**[variable]<<[number of places]**

If Number of Places become negative, it will be undefined

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