How Do I Safely Compare Pointers in C ?

Comparing Pointers: Equality and Other Concerns

When comparing pointers to determine if they reference the same object, the intuitive solution is to use the equality operator (==). However, the C standard provides specific guidelines for pointer comparisons, expanding beyond this simple criterion.

Equality Comparison (== and !=)

According to the C 11 standard (section 5.10), pointers of the same type can be compared for equality under the following conditions:

• Both pointers are null.
• Both pointers point to the same function.
• Both pointers represent the same address.

Relational Comparisons (<, >, <=, >=)

The C 11 standard also outlines the behavior of relational operators for pointers in section 5.9. The rules are similar to those for equality comparison, but with some exceptions:

• If pointers point to different objects within an array, the results are unspecified.
• If pointers point to data members of different objects, the results are unspecified.
• If pointers point to data members of the same object, the pointer to the member declared later compares greater, assuming the members have the same access control.

Specific Cases

Keep in mind that the behavior of pointer comparisons can vary depending on the specific context.

• Arrays: Pointers to elements within the same array can be compared using relational operators (<, >, <=, >=). However, pointers that exceed the end of the array or point to different arrays will yield unspecified results.
• Union Members: Pointers to members of the same union always compare equal.

Conclusion

While a == b is generally sufficient for comparing pointers to the same object, it's essential to consider the potential caveats outlined in the C standard to ensure accurate and predictable behavior in all scenarios.

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