2nd/10_Wiki/Topics/Architecture/Link Seam (링크 접점).md

id, title, category, status, canonical_id, aliases, duplicate_of, source_trust_level, confidence_score, verification_status, tags, raw_sources, last_reinforced, github_commit, tech_stack

id	title	category	status	canonical_id	aliases	duplicate_of	source_trust_level	confidence_score	verification_status	tags	raw_sources	last_reinforced	github_commit	tech_stack
wiki-2026-0508-link-seam-링크-접점	Link Seam (링크 접점)	10_Wiki/Topics	verified	self	Link Seam Linker Seam	none	A	0.9	applied	legacy-code testing seam refactoring		2026-05-10	pending	language framework cpp none

Link Seam (링크 접점)

매 한 줄

"매 linker 의 symbol resolution 을 test 의 enabling point 로 활용". Michael Feathers, Working Effectively with Legacy Code (2004) — 매 source code 변경 없이 link-time 의 symbol substitution 으로 dependency 를 break 하는 seam. C/C++ 의 native 기법, 2026 에서는 매 dynamic library injection (LD_PRELOAD), test double linking, 또는 Bazel/CMake test target 분리 로 활용.

매 핵심

매 Feathers 의 3 seam types

1. Preprocessing seam: #define, #ifdef 의 compile-time substitution.
2. Link seam: linker 의 symbol resolution 의 test/prod binary 분리.
3. Object seam: virtual function / interface 의 polymorphic dispatch.

매 Link seam 의 enabling mechanism

• 매 production code 와 test code 가 동일 header / function signature 를 share.
• 매 production binary 는 real implementation .o link.
• 매 test binary 는 fake/stub implementation .o link.
• 매 source 파일 변경 없이 매 dependency 의 substitute.

매 응용

1. C/C++ legacy code 의 testability 도입 (most classic use case).
2. System call wrapping (LD_PRELOAD 의 fake gettimeofday).
3. Hardware abstraction layer (HAL) 의 test build.
4. Mock library injection (e.g., FFF in C, googlemock 의 link-time mock).
5. Embedded system unit testing on host.

💻 패턴

1. C link seam (canonical)

// time_provider.h
#ifndef TIME_PROVIDER_H
#define TIME_PROVIDER_H
long current_time_ms(void);
#endif

// production: time_provider_real.c
#include <time.h>
long current_time_ms(void) {
    struct timespec ts;
    clock_gettime(CLOCK_MONOTONIC, &ts);
    return ts.tv_sec * 1000 + ts.tv_nsec / 1000000;
}

// test: time_provider_fake.c
static long fake_now = 0;
long current_time_ms(void) { return fake_now; }
void set_fake_time(long t) { fake_now = t; }

// Production link
// gcc app.c time_provider_real.c -o app
// Test link
// gcc test_app.c time_provider_fake.c -o test_app

2. CMake target 분리

# CMakeLists.txt
add_library(time_real STATIC time_provider_real.c)
add_library(time_fake STATIC time_provider_fake.c)

add_executable(app main.c)
target_link_libraries(app PRIVATE time_real)

add_executable(app_test test_main.c)
target_link_libraries(app_test PRIVATE time_fake)
add_test(NAME app_test COMMAND app_test)

3. Bazel test seam

# BUILD
cc_library(
    name = "time_real",
    srcs = ["time_provider_real.c"],
    hdrs = ["time_provider.h"],
)

cc_library(
    name = "time_fake",
    srcs = ["time_provider_fake.c"],
    hdrs = ["time_provider.h"],
    testonly = True,
)

cc_binary(name = "app", srcs = ["main.c"], deps = [":time_real"])
cc_test(name = "app_test", srcs = ["test_main.c"], deps = [":time_fake"])

4. LD_PRELOAD (runtime link seam)

// fake_malloc.c
#define _GNU_SOURCE
#include <dlfcn.h>
#include <stddef.h>

static int allocation_count = 0;
int get_alloc_count(void) { return allocation_count; }

void *malloc(size_t size) {
    static void *(*real_malloc)(size_t) = NULL;
    if (!real_malloc) real_malloc = dlsym(RTLD_NEXT, "malloc");
    allocation_count++;
    return real_malloc(size);
}

// Compile: gcc -shared -fPIC fake_malloc.c -ldl -o libfakemalloc.so
// Run: LD_PRELOAD=./libfakemalloc.so ./my_program

5. Weak symbol seam (GCC)

// production_default.c
__attribute__((weak)) int sensor_read(void) {
    return read_real_sensor();
}

// test_override.c
int sensor_read(void) {
    return 42;  // strong symbol overrides weak
}

// Test build links test_override.c → uses fake.
// Prod build doesn't link test_override.c → uses weak default.

6. FFF (Fake Function Framework, C)

#include "fff.h"
DEFINE_FFF_GLOBALS;

// Header declares: void send_packet(const char* data);
FAKE_VOID_FUNC(send_packet, const char*);

void test_send(void) {
    RESET_FAKE(send_packet);
    do_thing();
    assert(send_packet_fake.call_count == 1);
}
// Link test binary with fakes instead of real send_packet.

7. Migration to object seam (preferred long-term)

// 매 link seam 은 stepping stone
// 매 long-term: interface + DI 로 evolve
class TimeProvider {
public:
  virtual long now_ms() = 0;
  virtual ~TimeProvider() = default;
};

class RealTimeProvider : public TimeProvider {
  long now_ms() override { /* real */ }
};

class FakeTimeProvider : public TimeProvider {
public:
  long fake_now = 0;
  long now_ms() override { return fake_now; }
};
// Now no link tricks needed — just inject.

매 결정 기준

상황	Approach
Legacy C code, 매 interface 도입 어려움	Link seam.
C++ codebase, 매 refactor 가능	Object seam (virtual / interface).
System call / library function 의 mocking	LD_PRELOAD link seam.
Embedded HAL의 host testing	Link seam (fake HAL implementation).
Modern code, full DI	Object seam — link seam 불필요.
Single-binary monolith 의 sub-module test	Link seam 또는 build target 분리.

기본값: 새 C/C++ 코드는 매 object seam 으로 시작. Link seam 은 매 legacy 의 testability rescue tool.

🔗 Graph

• 부모: Seam (Legacy Code) · Working Effectively with Legacy Code
• 변형: Preprocessing Seam · Object Seam
• 응용: Test Doubles · Hardware Abstraction Layer · LD_PRELOAD
• Adjacent: Dependency Injection · Mock Object · Weak Symbols

🤖 LLM 활용

언제: legacy C/C++ codebase 의 unit test 도입, system call mocking, embedded HAL host test. 언제 X: 매 modern dynamic language (Python/JS/Ruby — monkey patch 충분), 매 fresh codebase (object seam 이 cleaner).

❌ 안티패턴

• Production binary 에 test fake 누설: 매 build target separation 실패 시 매 prod 가 fake link. 매 strict CMake/Bazel target 분리.
• Symbol collision: 매 multiple translation unit 에서 동일 symbol — undefined behavior.
• LD_PRELOAD 의 prod 사용: 매 debugging 도구로만, 매 production deploy 의 anti-pattern.
• Header/source mismatch: 매 fake 가 매 signature 변경 의 stale — 매 ABI mismatch crash.
• Link seam 의 영구화: 매 stepping stone, 매 object seam 으로 evolve 의 missing.

🧪 검증 / 중복

• Verified (Feathers, Working Effectively with Legacy Code 2004, Ch.4 "The Seam Model").
• 신뢰도 A.

🕓 Changelog

날짜	변경
2026-05-08	Phase 1
2026-05-10	Manual cleanup — Feathers seam taxonomy + CMake/Bazel/LD_PRELOAD modern application