dianqk
2022-04-12
根据 7.6.1 How Linkers Resolve Duplicate Symbol Names 一个章节的描，未初始化的全局变量会成为 weak symbol。
Functions and initialized global variables get strong symbols. Uninitialized global variables get weak symbols.
我按照书中的例子实践了一下（对应的文件为 foo3.c 和 bar3.c）。
// bar3.c
int x;
void f()
{
x = 15212;
}
// foo3.c
#include <stdio.h>
int x = 15213;
int main()
{
f();
printf("x = %d\n", x);
return 0;
}
$ gcc -o foobar3 foo3.c bar3.c
/usr/bin/ld: /tmp/ccynZE9A.o:(.bss+0x0): multiple definition of `x'; /tmp/ccK9RGkg.o:(.data+0x0): first defined here
collect2: error: ld returned 1 exit status
$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/11.2.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /build/gcc/src/gcc/configure --enable-languages=c,c++,ada,fortran,go,lto,objc,obj-c++,d --enable-bootstrap --prefix=/usr --libdir=/usr/lib --libexecdir=/usr/lib --mandir=/usr/share/man --infodir=/usr/share/info --with-bugurl=https://bugs.archlinux.org/ --with-linker-hash-style=gnu --with-system-zlib --enable-__cxa_atexit --enable-cet=auto --enable-checking=release --enable-clocale=gnu --enable-default-pie --enable-default-ssp --enable-gnu-indirect-function --enable-gnu-unique-object --enable-linker-build-id --enable-lto --enable-multilib --enable-plugin --enable-shared --enable-threads=posix --disable-libssp --disable-libstdcxx-pch --disable-werror --with-build-config=bootstrap-lto --enable-link-serialization=1 gdc_include_dir=/usr/include/dlang/gdc
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 11.2.0 (GCC)
bar3.c 中的文件应当属于 weak symbol，在合并重复符号时应当会被抛弃，但这却给出了一个重复的报错。
我掏出一个 docker 起一个旧版本的 ubuntu：
root@62c178e49658:~# gcc -o foobar3 foo3.c bar3.c
root@62c178e49658:~# ./foobar3
x = 15212
root@62c178e49658:~# gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.8/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 4.8.4-2ubuntu1~14.04.4' --with-bugurl=file:///usr/share/doc/gcc-4.8/README.Bugs --enable-languages=c,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-4.8 --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.8 --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-gnu-unique-object --disable-libmudflap --enable-plugin --with-system-zlib --disable-browser-plugin --enable-java-awt=gtk --enable-gtk-cairo --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-4.8-amd64/jre --enable-java-home --with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-4.8-amd64 --with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-4.8-amd64 --with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar --enable-objc-gc --enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.8.4 (Ubuntu 4.8.4-2ubuntu1~14.04.4)
这个倒是符合 CSAPP 章节内容。
我用 nm 来看了以下两个 .o 的区别：
ubuntu 14.04:
root@62c178e49658:~# gcc -o bar3.o -c bar3.c
root@62c178e49658:~# nm bar3.o
0000000000000000 T f
0000000000000004 C x
archlinux:
$ gcc -o bar3.o -c bar3.c
$ nm bar3.o
0000000000000000 T f
0000000000000000 B x