strcmp与__strcmp_sse2_unaligned的区别及链接异常技术咨询

Let's break this down clearly using your debugging output and how glibc handles string comparison functions under the hood:

1. Core Role & Visibility #

• strcmp: This is the standard, publicly exposed C library function you're intended to call in your code. As your pwndbg output shows, it's marked as a gnu-indirect-function — this means it's not the actual comparison logic, but a resolver that picks the best underlying implementation based on your CPU's capabilities at runtime.
• __strcmp_sse2_unaligned: This is a private, internal implementation in glibc. It's never meant to be called directly by user code; instead, it's one of several optimized variants that strcmp can resolve to behind the scenes.

2. Implementation & Optimization #

• The __strcmp_sse2_unaligned variant is built specifically for CPUs supporting SSE2 instructions (nearly all x86_64 CPUs made since the early 2000s). It uses SSE2 vector instructions to compare up to 16 bytes of the input strings at once, which is drastically faster than a naive byte-by-byte comparison.
• The "unaligned" suffix indicates this implementation is optimized to handle string buffers that aren't aligned to 16-byte memory boundaries — a common scenario that would force slower fallback logic in aligned-only variants.
• When your program first calls strcmp, the IFUNC resolver checks your CPU's feature set, then updates the GOT entry to point to the best-fit implementation. That's exactly what you see in your got output: strcmp@GLIBC_2.2.5 -> 0x7fcc5e611570 (__strcmp_sse2_unaligned).

3. Linking & Runtime Behavior #

• The strcmp symbol at 0x7fcc5e5fbcd0 is just the IFUNC resolver stub. If you disassembled it fully, you'd see code that detects CPU features and jumps to the appropriate optimized implementation.
• Once resolved, all subsequent calls to strcmp in your program will directly use __strcmp_sse2_unaligned (or whichever variant was selected) via the GOT entry, skipping the resolver overhead entirely.

内容的提问来源于stack exchange，提问作者icepng