What are Trap Representations in C and C , and Do Identical-Sized Floats and Their Pointers Share the Same Binary Representation?

Trap Representation

Question:

1. What is a "trap representation" in C?
2. Does this apply to C ?
3. Do two variables, one float and one pointer to the float, have the same binary representation if they have the same size?

Answer:

1. Trap Representation:

In C, a trap representation is a bit pattern that fits the size of a type but causes undefined behavior if used as a value of that type. No type is required to have trap representations, but the only type guaranteed not to have them is unsigned char.

An example of a trap representation is a signaling NaN in a floating-point type. These patterns are undefined in C99.

2. Applicability to C :

Although the question does not mention C , it's worth noting that the concept of trap representations applies to C as well.

3. Binary Representation of Float and Pointer:

The code provided below exhibits undefined behavior:

float f = 3.5;
int *pi = (int*)&f;

However, this behavior is not related to trap representations. To obtain the integer value with the same bit pattern as the float, use the following code:

int extract_int(float f)
{
    union { int i; float f; } u;
    u.f = f;
    return u.i;
}

This code exhibits unspecified behavior in C99, meaning the standard does not define the exact integer value produced, but it guarantees that a valid integer value is obtained. Therefore, the binary representation of the float and the integer obtained using the union are not guaranteed to be identical.

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