Linking the Dots #1: An Example of Linking in C

Source Code

// secondary.c

int get_answer() {
    return 42;
}

int another_answer = 100;

// main.c

// Forward declaration of a function
extern int get_answer();

// `extern` is often considered redundant there,
//   because compiler assumes any function without a "body" `{}` is external,
//   i.e. function names have external linkage by default.
// int get_anwser();

// Forward declaration of a variable
// `extern` is necessary here
extern int another_answer;

// Without `extern`, it becomes a tenative definition.
//   More on that later.
// int another_answer;

int main() {
    return get_answer() + another_answer;
}

Pre-linking Steps

Step 1 - Preprocessing (C $\Rightarrow$ C)

ᐅ gcc -E main.c -o main.i
ᐅ gcc -E secondary.c -o secondary.i

Step 2 - Compilation (C $\Rightarrow$ Assembly)

ᐅ gcc -S main.i -o main.s
ᐅ gcc -S secondary.i -o secondary.s

# ᐅ or 
#   ᐅ cc1 main.c -o main.s
#   ᐅ cc1 secondary.c -o secondary.s

[!NOTE] cc1 包含了 preprocessing 步骤，所以可以直接处理 .c

Step 3 - Assembling (Assembly $\Rightarrow$ Relocatable Object/Binary)

ᐅ gcc -c main.s -o main.o
ᐅ gcc -c secondary.s -o secondary.o

# or 
#   ᐅ as main.s -o main.o
#   ᐅ as secondary.s -o secondary.o

Or directly C $\Rightarrow$ Relocatable Object/Binary:

ᐅ gcc -c main.c -o main.o
ᐅ gcc -c secondary.c -o secondary.o

Linking

Relocatable Object/Binary $\Rightarrow$ Executable Object/Binary

Static Linking

Method 1: use the object file secondary.o directly:

ᐅ gcc main.o secondary.o -o main_static.out

Method 2: create a static library archive from secondary.o:

ᐅ ar rcs secondary.a secondary.o

# link like above
ᐅ gcc main.o secondary.a -o main_static.out

• ar is an archive tool, just like zip or tar
  • option r: replace/add files to archive
  • option c: create archive if it doesn’t exist
  • option s: create an index (symbol table) for faster linking
• 注意 .a 并不是 Mach-O format, 它也不是 object file, 它单纯就是个能被 link 的压缩包格式

[!NOTE] 如果你有很多个 dependencies，那么 static library archive 无疑是工程上的 best practice.

Dynamic Linking

Method: create a dynamic lib from secondary.o:

ᐅ gcc -dynamiclib secondary.o -o secondary.dylib
ᐅ gcc main.o secondary.dylib -o main_dynamic.out

[!NOTE] .dylib on macOS is equivalent to .so on Linux.

[!NOTE] .dylib 是 Mach-O format

[!NOTE] .dylib 不是 object file, 因为 “object”, “executable”, “dynamic lib” 是三种平行的文件类型

Digression: Search Pattern of gcc -L -l

我们也可以这么做 static linking:

ᐅ ar rcs libsecondary.a secondary.o
ᐅ gcc main.o -L. -lsecondary -o main_static.out

也可以这么做 dynamic linking:

ᐅ gcc -dynamiclib secondary.o -o libsecondary.dylib
ᐅ gcc main.o -L. -lsecondary -o main_dynamic.out

这里 gcc -L -l 有特定的 search pattern:

• -L. 指定 search directory 为 . (current folder)
• 它这个 search pattern 会默认你的 lib name 以 lib 开头，所以 -lsecondary 实际指定的是 libsecondary 这个 name
  • 所以我们在创建 dylib 时特意起了 libsecondary 这个名字
  • 这个 search pattern 主要是为了区分 user lib 和 system lib

此时就有一个问题：如果我们同时有 libsecondary.a 和 libsecondary.dylib, -lsecondary 如何确定是 static linking 还是 dynamic linking? 它其实有一个规则：

• 优先找 libsecondary.dylib 做 dynamic linking
• 没找到 dylib 就找 libsecondary.a 做 static linking
• 它不会去找 libsecondary.o

如果我们同时有 libsecondary.a 和 libsecondary.dylib, 我们可以用 option -static force 执行 static linking:

ᐅ gcc main.o -L. -static -lsecondary -o main_static.out

Inspection

Check for Dependencies

ᐅ otool -L main_static.out
main_static.out:
	/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1336.61.1)

• main_static.out does NOT need secondary.dylib nor secnondary.o because it’s already wrapped inside the binary

[!info] libSystem.B.dylib plays the same role on macOS that glibc.so plays on Linux (as the C standard library)

它俩实现上略有不同：glibc.so 是一个 monolith lib; libSystem.B.dylib 是一个 facade to multiple libs.

ᐅ otool -L main_dynamic.out
main_dynamic.out:
	secondary.dylib (compatibility version 0.0.0, current version 0.0.0)
	/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1336.61.1)

• main_dynamic.out knows it needs the external secondary.dylib

Check for Symbols

ᐅ nm main_static.out
0000000100000000 T __mh_execute_header
0000000100004000 D _another_answer       # linked variable
0000000100003f90 T _get_answer           # linked function
0000000100003f60 T _main

ᐅ nm main_dynamic.out
0000000100000000 T __mh_execute_header
                 U _another_answer      # undefined variable
                 U _get_answer          # undefined function
0000000100003f70 T _main

• T: the symbol is in (__TEXT, __text)
• D: the symbol is in (__DATA, __data) (initialized data)
• U: the symbol is undefined