MemcmpEdit
Memcmp is a foundational primitive in the C programming ecosystem, used to compare two blocks of memory byte by byte. It operates on raw data, not on human-readable strings, which makes it suitable for a wide range of binary formats, network protocols, and low-level system components. The function is part of the C programming language C standard library and is declared in the string.h header. Because it treats memory as an array of unsigned bytes, memcmp is agnostic to encoding or character sets, which can be both a strength and a caveat in practice.
In practice, memcmp is favored for its determinism and simplicity. When you supply two pointers to memory and a length n, memcmp compares the first n bytes of each block and returns an integer that indicates whether the first block is lexicographically less than, equal to, or greater than the second block. The exact return value is not standardized to a particular difference; what matters is the sign: negative, zero, or positive. This makes memcmp a portable choice for exact binary comparisons across architectures, compilers, and operating systems. See how memcmp relates to other byte-oriented operations in the C standard library and how it differs from string-focused comparisons in strcmp.
Semantics and behavior
Signature and contract: int memcmp(const void *s1, const void *s2, size_t n); It compares the first n bytes of the memory areas pointed to by s1 and s2. If n is zero, memcmp returns zero. The comparison is performed as if by unsigned characters, ensuring a consistent ordering regardless of sign or locale. The concept of "bytes" here is the fundamental unit of measurement for binary data, which is why memcmp is widely used in file format parsers, protocol handshakes, and verification routines. See the discussion of size_t and unsigned char in the context of memory operations.
Not a string comparator: memcmp does not stop at a zero byte. It considers the entire block of n bytes, which distinguishes it from string-oriented functions like strcmp that operate on nul-terminated sequences. For text processing where border conditions and locale come into play, other helpers within the C standard library are more appropriate.
Alignment and performance: In practice, implementations often leverage architecture-specific optimizations (vector instructions, loop unrolling, alignment tricks) to accelerate comparisons of large blocks. The observable effect is that memcmp can be both fast and portable, depending on the quality of the library implementation and the specific hardware. See the section on implementation and performance for more.
Implementation considerations
Early exit: A typical memcmp implementation exits as soon as a difference is found. This means timing can reveal information about where a block differs, which has implications for security-sensitive uses. When the goal is cryptographic secrecy or resistance to timing attacks, memcmp alone is not sufficient, and constant-time techniques are preferred. See constant-time comparison for alternatives in security-minded code.
Safety with binary data: Because memcmp treats data as raw bytes, it is well suited for comparing any binary blob, including serialized objects, memory-mapped files, and network packets. It is not inherently aware of structure; higher-level formats must ensure correct interpretation of the bytes being compared.
Portability: The standard specifies behavior in terms of unsigned bytes, which helps ensure consistent results across platforms. When porting code, pay attention to size_t semantics and any platform-specific differences in pointer aliasing or memory layout.
Pitfalls and best practices
Use the right tool for the job: If the goal is to compare strings, prefer a function designed for strings, such as strcmp, which accounts for terminators and character semantics. If the goal is a binary comparison, memcmp is usually appropriate.
Do not rely on memcmp for constant-time equality checks: For security-sensitive comparisons, such as verifying secrets or tokens, memcmp can reveal information through timing differences. Use dedicated constant-time comparison routines that are designed to run in time independent of the data, or adopt a well-vetted cryptographic library that provides such primitives. See constant-time comparison for guidance.
Beware of zero bytes in structured data: If the memory blocks contain embedded zero bytes but you intend to interpret them as strings, memcmp will still treat them as just bytes. Ensure your higher-level logic handles framing, length, and interpretation correctly.
Related functions and concepts
Comparison of strings is often accomplished with strcmp or related routines in the C standard library.
Memory setting or copying tasks frequently use memset or memcpy to prepare or duplicate memory regions before or after a comparison.
For byte-level reasoning, understanding unsigned char and the role of size_t helps clarify how memcmp defines its byte-wise comparison and length.
Understanding endianness and binary layout can be important when interpreting the results of memory comparisons in cross-platform code.
History and standards
Memcmp has long been part of the C language family’s standard library, with formal definitions in the early iterations of the language and carried forward in modern standards. Its behavior is codified in terms of raw memory, independent of character encoding, which reflects the low-level, systems-oriented design ethos typical of the language.
The definition and guarantees of memcmp are harmonized with other memory-handling routines in the same header, promoting a coherent approach to memory operations across platforms and compiler implementations.