Скачать или смотреть Understanding C Pointer Comparisons and Structure Casting: Avoiding Undefined Behavior

This guide explores pointer equality comparisons and the casting of struct members in C, focusing on how to avoid undefined behavior.
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Understanding C Pointer Comparisons and Structure Casting: Avoiding Undefined Behavior

C programming can sometimes lead developers into murky waters, especially when it comes to pointers, structures, and casting. In this post, we tackle a query that illustrates these challenges and provides clarity about equality comparisons and casting in C.

The Problem

The C code in question involves a struct and pointers, where part of the struct is "cast away." Here’s the code for reference:

[[See Video to Reveal this Text or Code Snippet]]

The questions posed are:

Is the pointer equality comparison unbounded (UB)?

Is it valid in C to cast away struct members and access the remaining members?

These questions underline common confusion regarding pointer operations in C.

Solution Breakdown

1. Pointer Equality Comparison: Is It Undefined Behavior?

According to the C standard, specifically Section 6.2.5 Paragraph 28:

All pointers to structure types must have the same representation and alignment requirements as each other. This indicates that comparisons between different types of structure pointers may yield uncertainties.

Furthermore, Section 6.3.2.3 Pointers states:

A void pointer can be converted to any object type, and vice versa, without leading to undefined behavior as long as type conversion remains aligned.

Key Takeaway:

Pointer comparisons are not universally undefined but depend on alignment. If the alignment of the types is consistent, the pointers will compare equal under the right conditions.

2. Validity of Casting Away Struct Members

When it comes to casting away struct members, the concern is whether the remaining pointer, after using a smaller struct type, retains correct alignment.

If a pointer to a larger struct type is cast to a smaller type, there’s no assurance by the C standard that the alignment remains valid. This can lead to undefined behavior, especially with "insane" compilers that might misalign types differently.

However, there is a useful guarantee provided for unions. When structures share a common initial sequence of members, it's valid to access the initial portion of these structures from a union, which can encourage safer member access practices.

Important Points:

Generally speaking, as long as the pointer to the smaller structure type is correctly aligned for its type and you avoid invoking strict aliasing rules, accessing its members should work correctly.

Accessing members across different structure types without proper setup can lead to undefined behavior.

Conclusion

Navigating pointer comparisons and casting in C requires a clear understanding of the C standards regarding structure types and alignment.

Pointer equality can be effectively managed by ensuring that both pointers maintain compatible alignment, while casting away struct members should be done with caution to avoid violations of alignment and strict aliasing rules.

By following these guidelines, developers can write more robust C code while avoiding potential pitfalls that could lead to undefined behavior.

Remember, always refer to the relevant sections of the C standard to clarify doubts about pointers and type casting!