My code works, I don’t know why.

國王的耳朵是驢耳朵

GNU Ld初探

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以前一直以為ld單純就是把.a, .o轉成binary，簡單測試一下發現完全不是這樣。

測試的檔案除了Makefile以外，其他的和這邊一樣

將原本的Makefile部份

原本的Makefile準備更動的部份
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$(OUT_DIR)/$(TARGET): $(OUT_OBJS)
    $(CC) -o $@ $^

改成

$(CC)改成ld
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$(OUT_DIR)/$(TARGET): $(OUT_OBJS)
    ld -o $@ $^

一跑make就出現錯誤

make log
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$ make
mkdir -p build/libs/
cc -g -MMD -I ./include -c libs/liba.c -o build/libs/liba.o
mkdir -p build/libs/
cc -g -MMD -I ./include -c libs/libb.c -o build/libs/libb.o
mkdir -p build/
cc -g -MMD -I ./include -c test.c -o build/test.o
ld -o build/test build/libs/liba.o build/libs/libb.o build/test.o
ld: warning: cannot find entry symbol _start; defaulting to 00000000004000b0
build/libs/liba.o: In function `from_liba':
./libs/liba.c:11: undefined reference to `puts'
build/libs/libb.o: In function `from_libb':
./libs/libb.c:11: undefined reference to `puts'
build/test.o: In function `main':
./test.c:10: undefined reference to `malloc'
make: *** [build/test] Error 1

ld 這邊再加上-lc又有其他的錯誤，看來的確是有東西隱藏在背面。因此需要有對照組，這時候gcc -v就可以出場了

gcc -v log節錄
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$ gcc -v -o build/test build/libs/liba.o build/libs/libb.o build/test.o
Using built-in specs.
...
 /usr/lib/gcc/x86_64-linux-gnu/4.6/collect2 --sysroot=/ --build-id --no-add-needed --as-needed --eh-frame-hdr -m elf_x86_64 --hash-style=gnu -dynamic-linker /lib64/ld-linux-x86-64.so.2 -z relro -o build/test /usr/lib/gcc/x86_64-linux-gnu/4.6/../../../x86_64-linux-gnu/crt1.o /usr/lib/gcc/x86_64-linux-gnu/4.6/../../../x86_64-linux-gnu/crti.o /usr/lib/gcc/x86_64-linux-gnu/4.6/crtbegin.o -L/usr/lib/gcc/x86_64-linux-gnu/4.6 -L/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../x86_64-linux-gnu -L/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../../lib -L/lib/x86_64-linux-gnu -L/lib/../lib -L/usr/lib/x86_64-linux-gnu -L/usr/lib/../lib -L/usr/lib/gcc/x86_64-linux-gnu/4.6/../../.. build/libs/liba.o build/libs/libb.o build/test.o -lgcc --as-needed -lgcc_s --no-as-needed -lc -lgcc --as-needed -lgcc_s --no-as-needed /usr/lib/gcc/x86_64-linux-gnu/4.6/crtend.o /usr/lib/gcc/x86_64-linux-gnu/4.6/../../../x86_64-linux-gnu/crtn.o

可以看到gcc呼叫collect2，而collect2會呼叫ld

strace collect2結果節錄
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$ strace -e execve -f gcc -o build/test build/libs/liba.o build/libs/libb.o build/test.o
execve("/usr/bin/gcc", ["gcc", "-o", "build/test",...
...
execve("/usr/lib/gcc/x86_64-linux-gnu/4.6/collect2",..., "--sysroot=/",...
...
execve("/usr/bin/ld", ["/usr/bin/ld", "--sysroot=/", ...

最後的Makefile版本變成

ld最後參數
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# LD_FLAGS
LD_FLAGS= \
  --sysroot=/ \
  --build-id \
  --no-add-needed --as-needed --eh-frame-hdr \
  -m elf_x86_64 --hash-style=gnu \
  -dynamic-linker /lib64/ld-linux-x86-64.so.2 \
  -z relro /usr/lib/gcc/x86_64-linux-gnu/4.6/../../../x86_64-linux-gnu/crt1.o\
  /usr/lib/gcc/x86_64-linux-gnu/4.6/../../../x86_64-linux-gnu/crti.o \
  /usr/lib/gcc/x86_64-linux-gnu/4.6/crtbegin.o \
  -L/usr/lib/gcc/x86_64-linux-gnu/4.6 \
  -L/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../x86_64-linux-gnu \
  -L/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../../lib \
  -L/lib/x86_64-linux-gnu \
  -L/lib/../lib -L/usr/lib/x86_64-linux-gnu \
  -L/usr/lib/../lib -L/usr/lib/gcc/x86_64-linux-gnu/4.6/../../..\
  -lgcc --as-needed -lgcc_s --no-as-needed -lc -lgcc --as-needed -lgcc_s \
  --no-as-needed \
  /usr/lib/gcc/x86_64-linux-gnu/4.6/crtend.o \
  /usr/lib/gcc/x86_64-linux-gnu/4.6/../../../x86_64-linux-gnu/crtn.o

# Build Rules
TARGET=test

$(OUT_DIR)/$(TARGET): $(OUT_OBJS)
  ld -o $@ $^ $(LD_FLAGS)

結論如下

  1. gcc在build code默默處理掉很多細節
  2. gcc -v可以協助顯示更多編譯細節

參考資料:

from Source to Binary: How GNU Toolchain Works

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