Comparing structures (structs) in C can be tricky. Using memcmp to directly compare the memory occupied by two structs is often unreliable due to padding bytes and complexities with pointers and floating-point numbers. Let’s explore why.
Why memcmp is Problematic for Struct Comparison
The memcmp function performs a byte-by-byte comparison of two memory blocks. While seemingly straightforward, this approach falls short when dealing with the nuanced structure of C structs.
Padding Bytes: Compilers often introduce padding bytes between struct members for alignment optimization. These padding bytes have undefined values, leading to inconsistent results when using memcmp, even if the meaningful data within the structs is identical. This introduces undefined behavior into your code.
Pointers: If a struct contains pointers, memcmp compares the memory addresses stored in those pointers, not the data they point to. Two structs might point to identical data at different memory locations, causing memcmp to incorrectly report them as unequal. Comparing pointers directly only indicates whether they refer to the same memory location, not if the data they point to is equivalent.
Floating-Point Numbers: Floating-point representations can have subtle variations for the same value (e.g., +0.0 and -0.0). memcmp might interpret these as different due to differing bit patterns, even though they represent the same numerical value. Additionally, special values like NaN (Not a Number) will never compare as equal to themselves using memcmp.
Example of a struct with padding bytes (Image source: Stack Overflow)
The Correct Way to Compare Structs: Member-wise Comparison
The most reliable way to compare two structs in C is to perform a member-wise comparison. This involves comparing each corresponding member of the two structs individually.
Example:
struct Inputs {
int input_01;
int input_02;
int input_03;
int input_04;
};
bool compare_inputs(const struct Inputs *a, const struct Inputs *b) {
return a->input_01 == b->input_01 &&
a->input_02 == b->input_02 &&
a->input_03 == b->input_03 &&
a->input_04 == b->input_04;
}Operator Overloading (C++):
In C++, you can overload the == operator to define custom comparison logic for your struct:
struct Inputs {
int input_01;
int input_02;
int input_03;
int input_04;
bool operator==(const Inputs &other) const {
return input_01 == other.input_01 &&
input_02 == other.input_02 &&
input_03 == other.input_03 &&
input_04 == other.input_04;
}
bool operator!=(const Inputs &other) const {
return !(*this == other);
}
};This allows you to directly use == to compare Inputs objects, making the code cleaner and more intuitive.
Conclusion
While memcmp might seem like a quick solution for comparing structs, it’s crucial to understand its limitations. Relying on member-wise comparison ensures accuracy and avoids potential pitfalls associated with padding bytes, pointers, and floating-point values. This approach guarantees that you’re comparing the actual data within your structs, leading to more robust and predictable code. In C++, operator overloading provides a more elegant syntax for member-wise comparison.
