What Happens When You Shift Right with a Count Exceeding Type Width in C ?

Shifting Right with Count Exceeding Type Width: Undefined Behavior

In C , the right shift operator (>>) performs logical or arithmetic shift operations on integers. While the behavior of this operator is generally well-defined, there are certain conditions that can lead to undefined behavior.

One such condition is when the shift count exceeds the width of the type being shifted. The C standard explicitly states that "The behavior is undefined if the right operand is negative, or greater than or equal to the length in bits of the promoted left operand."

This implies that shifting an integer by a count greater than or equal to its bit width is undefined, regardless of whether the operand is signed or unsigned. In theory, this means that the result of such a shift is not guaranteed and may vary across different implementations.

However, in practice, some compilers may implement a specific behavior in such cases. For example, GCC issues a warning when the shift count exceeds the type width, but does not raise an error. This behavior is not explicitly defined in the C standard and may vary across different platforms.

In the code snippet provided, a right shift of an unsigned integer by 34 is performed:

<code class="cpp">unsigned int val = 0x0FFFFFFF;
unsigned int res = val >> 34;</code>

The result calculated according to the C standard should be 0, as the shift count is greater than the width of the unsigned int type (which is typically 32 bits). However, GCC gives a warning and calculates the result as 67108863 instead.

This discrepancy arises because GCC is implementing a specific behavior for this undefined case. The generated assembly code uses the SHRL instruction, which performs a logical right shift and does not sign-extend the result. As a result, the result is not zero but rather a non-zero value.

Therefore, when working with shift operations in C , it is crucial to ensure that the shift count does not exceed the width of the type being shifted. Exceeding the type width can lead to undefined behavior and unreliable results across different compilers and platforms.

The above is the detailed content of What Happens When You Shift Right with a Count Exceeding Type Width in C ?. For more information, please follow other related articles on the PHP Chinese website!