"Multiple definition of" "first defined here" 适用于 GCC 10.2.1 但不适用于 GCC 8.3.0
"Multiple definition of" "first defined here" on GCC 10.2.1 but not GCC 8.3.0
我浏览了一下 Whosebug 和更广泛的互联网，发现导致此错误的最常见原因是混淆了 声明 (int var = 1; ) 和 定义 (int var;)，并包括来自 .h 个文件的 .c 个文件。
我刚从一个文件拆分成几个文件的小项目没有做任何这些事情。我非常困惑。
我复制了项目并删除了副本中的所有代码（这很有趣）直到我到达这里：
main.c
#include "a.h"
int main() {
}
a.h
#ifndef A_H
#define A_H
int test;
#endif
a.c
#include "a.h"
已经很规范了，对吧？然而...
$ rm *.o; gcc -g -c a.c; gcc -g -c main.c; gcc -o main main.o a.o
/usr/bin/ld: a.o:/.../a.h:5: multiple definition of `test'; main.o:/.../a.h:5: first defined here
collect2: error: ld returned 1 exit status
瓦特 :(
大约在这个时候，我开始质疑我的配置并尝试了另一台机器，通过 NFS 访问完全相同的文件。
$ rm *.o; gcc -g -c a.c; gcc -g -c main.c; gcc -o main main.o a.o
$ ./main
$ echo $?
0
哦不。我在第一台机器上坏了什么...
希望我的问题是在 GCC 8.3.0（工作） 和 GCC 10.2.1（失败） 之间悄悄发生了一些变化？也许较新版本的 GCC 默认为较新的 C 标准或其他东西，它在这方面指定了不同的行为？
或者它可能与配置有关（这都是 Debian 的股票）。这是两个版本的 gcc -dumpspecs 的输出：
8.3.0:
$ gcc --version
gcc (Debian 8.3.0-6) 8.3.0
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
$ gcc -dumpspecs
*asm:
%{m16|m32:--32} %{m16|m32|mx32:;:--64} %{mx32:--x32} %{msse2avx:%{!mavx:-msse2avx}}
*asm_debug:
%{%:debug-level-gt(0):%{gstabs*:--gstabs}%{!gstabs*:%{g*:--gdwarf2}}} %{fdebug-prefix-map=*:--debug-prefix-map %*}
*asm_final:
%{gsplit-dwarf:
objcopy --extract-dwo %{c:%{o*:%*}%{!o*:%b%O}}%{!c:%U%O} %{c:%{o*:%:replace-extension(%{o*:%*} .dwo)}%{!o*:%b.dwo}}%{!c:%b.dwo}
objcopy --strip-dwo %{c:%{o*:%*}%{!o*:%b%O}}%{!c:%U%O} }
*asm_options:
%{-target-help:%:print-asm-header()} %{v} %{w:-W} %{I*} %{gz|gz=zlib:--compress-debug-sections=zlib} %{gz=none:--compress-debug-sections=none} %{gz=zlib-gnu:--compress-debug-sections=zlib-gnu} %a %Y %{c:%W{o*}%{!o*:-o %w%b%O}}%{!c:-o %d%w%u%O}
*invoke_as:
%{!fwpa*: %{fcompare-debug=*|fdump-final-insns=*:%:compare-debug-dump-opt()} %{!S:-o %|.s |
as %(asm_options) %m.s %A } }
*cpp:
%{posix:-D_POSIX_SOURCE} %{pthread:-D_REENTRANT}
*cpp_options:
%(cpp_unique_options) %1 %{m*} %{std*&ansi&trigraphs} %{W*&pedantic*} %{w} %{f*} %{g*:%{%:debug-level-gt(0):%{g*} %{!fno-working-directory:-fworking-directory}}} %{O*} %{undef} %{save-temps*:-fpch-preprocess}
*cpp_debug_options:
%{d*}
*cpp_unique_options:
%{!Q:-quiet} %{nostdinc*} %{C} %{CC} %{v} %{I*&F*} %{P} %I %{MD:-MD %{!o:%b.d}%{o*:%.d%*}} %{MMD:-MMD %{!o:%b.d}%{o*:%.d%*}} %{M} %{MM} %{MF*} %{MG} %{MP} %{MQ*} %{MT*} %{!E:%{!M:%{!MM:%{!MT:%{!MQ:%{MD|MMD:%{o*:-MQ %*}}}}}}} %{remap} %{g3|ggdb3|gstabs3|gxcoff3|gvms3:-dD} %{!iplugindir*:%{fplugin*:%:find-plugindir()}} %{H} %C %{D*&U*&A*} %{i*} %Z %i %{E|M|MM:%W{o*}}
*trad_capable_cpp:
cc1 -E %{traditional|traditional-cpp:-traditional-cpp}
*cc1:
%{!mandroid|tno-android-cc:%(cc1_cpu) %{profile:-p};:%(cc1_cpu) %{profile:-p} %{!mglibc:%{!muclibc:%{!mbionic: -mbionic}}} %{!fno-pic:%{!fno-PIC:%{!fpic:%{!fPIC: -fPIC}}}}}
*cc1_options:
%{pg:%{fomit-frame-pointer:%e-pg and -fomit-frame-pointer are incompatible}} %{!iplugindir*:%{fplugin*:%:find-plugindir()}} %1 %{!Q:-quiet} %{!dumpbase:-dumpbase %B} %{d*} %{m*} %{aux-info*} %{fcompare-debug-second:%:compare-debug-auxbase-opt(%b)} %{!fcompare-debug-second:%{c|S:%{o*:-auxbase-strip %*}%{!o*:-auxbase %b}}}%{!c:%{!S:-auxbase %b}} %{g*} %{O*} %{W*&pedantic*} %{w} %{std*&ansi&trigraphs} %{v:-version} %{pg:-p} %{p} %{f*} %{undef} %{Qn:-fno-ident} %{Qy:} %{-help:--help} %{-target-help:--target-help} %{-version:--version} %{-help=*:--help=%*} %{!fsyntax-only:%{S:%W{o*}%{!o*:-o %b.s}}} %{fsyntax-only:-o %j} %{-param*} %{coverage:-fprofile-arcs -ftest-coverage} %{fprofile-arcs|fprofile-generate*|coverage: %{!fprofile-update=single: %{pthread:-fprofile-update=prefer-atomic}}}
*cc1plus:
*link_gcc_c_sequence:
%{static|static-pie:--start-group} %G %L %{static|static-pie:--end-group}%{!static:%{!static-pie:%G}}
*link_ssp:
%{fstack-protector|fstack-protector-all|fstack-protector-strong|fstack-protector-explicit:}
*endfile:
%{!mandroid|tno-android-ld:%{Ofast|ffast-math|funsafe-math-optimizations:crtfastmath.o%s} %{mpc32:crtprec32.o%s} %{mpc64:crtprec64.o%s} %{mpc80:crtprec80.o%s} %{fvtable-verify=none:%s; fvtable-verify=preinit:vtv_end_preinit.o%s; fvtable-verify=std:vtv_end.o%s} %{static:crtend.o%s; shared|static-pie|!no-pie:crtendS.o%s; :crtend.o%s} crtn.o%s %{fopenacc|fopenmp:crtoffloadend%O%s};:%{Ofast|ffast-math|funsafe-math-optimizations:crtfastmath.o%s} %{mpc32:crtprec32.o%s} %{mpc64:crtprec64.o%s} %{mpc80:crtprec80.o%s} %{shared: crtend_so%O%s;: crtend_android%O%s}}
*link:
%{!r:--build-id} %{!static|static-pie:--eh-frame-hdr} %{!mandroid|tno-android-ld:%{m16|m32|mx32:;:-m elf_x86_64} %{m16|m32:-m elf_i386} %{mx32:-m elf32_x86_64} --hash-style=gnu %{shared:-shared} %{!shared: %{!static: %{!static-pie: %{rdynamic:-export-dynamic} %{m16|m32:-dynamic-linker %{muclibc:/lib/ld-uClibc.so.0;:%{mbionic:/system/bin/linker;:%{mmusl:/lib/ld-musl-i386.so.1;:/lib/ld-linux.so.2}}}} %{m16|m32|mx32:;:-dynamic-linker %{muclibc:/lib/ld64-uClibc.so.0;:%{mbionic:/system/bin/linker64;:%{mmusl:/lib/ld-musl-x86_64.so.1;:/lib64/ld-linux-x86-64.so.2}}}} %{mx32:-dynamic-linker %{muclibc:/lib/ldx32-uClibc.so.0;:%{mbionic:/system/bin/linkerx32;:%{mmusl:/lib/ld-musl-x32.so.1;:/libx32/ld-linux-x32.so.2}}}}}} %{static:-static} %{static-pie:-static -pie --no-dynamic-linker -z text}};:%{m16|m32|mx32:;:-m elf_x86_64} %{m16|m32:-m elf_i386} %{mx32:-m elf32_x86_64} --hash-style=gnu %{shared:-shared} %{!shared: %{!static: %{!static-pie: %{rdynamic:-export-dynamic} %{m16|m32:-dynamic-linker %{muclibc:/lib/ld-uClibc.so.0;:%{mbionic:/system/bin/linker;:%{mmusl:/lib/ld-musl-i386.so.1;:/lib/ld-linux.so.2}}}} %{m16|m32|mx32:;:-dynamic-linker %{muclibc:/lib/ld64-uClibc.so.0;:%{mbionic:/system/bin/linker64;:%{mmusl:/lib/ld-musl-x86_64.so.1;:/lib64/ld-linux-x86-64.so.2}}}} %{mx32:-dynamic-linker %{muclibc:/lib/ldx32-uClibc.so.0;:%{mbionic:/system/bin/linkerx32;:%{mmusl:/lib/ld-musl-x32.so.1;:/libx32/ld-linux-x32.so.2}}}}}} %{static:-static} %{static-pie:-static -pie --no-dynamic-linker -z text}} %{shared: -Bsymbolic}}
*lib:
%{!mandroid|tno-android-ld:%{pthread:-lpthread} %{shared:-lc} %{!shared:%{profile:-lc_p}%{!profile:-lc}};:%{shared:-lc} %{!shared:%{profile:-lc_p}%{!profile:-lc}} %{!static: -ldl}}
*link_gomp:
*libgcc:
%{static|static-libgcc|static-pie:-lgcc -lgcc_eh}%{!static:%{!static-libgcc:%{!static-pie:%{!shared-libgcc:-lgcc --push-state --as-needed -lgcc_s --pop-state}%{shared-libgcc:-lgcc_s%{!shared: -lgcc}}}}}
*startfile:
%{!mandroid|tno-android-ld:%{shared:; pg|p|profile:%{static-pie:grcrt1.o%s;:gcrt1.o%s}; static:crt1.o%s; static-pie:rcrt1.o%s; !no-pie:Scrt1.o%s; :crt1.o%s} crti.o%s %{static:crtbeginT.o%s; shared|static-pie|!no-pie:crtbeginS.o%s; :crtbegin.o%s} %{fvtable-verify=none:%s; fvtable-verify=preinit:vtv_start_preinit.o%s; fvtable-verify=std:vtv_start.o%s} %{fopenacc|fopenmp:crtoffloadbegin%O%s};:%{shared: crtbegin_so%O%s;: %{static: crtbegin_static%O%s;: crtbegin_dynamic%O%s}}}
*cross_compile:
0
*version:
8.3.0
*multilib:
. !m32 !m64 !mx32;32:../lib32:i386-linux-gnu m32 !m64 !mx32;64:../lib:x86_64-linux-gnu !m32 m64 !mx32;x32:../libx32:x86_64-linux-gnux32 !m32 !m64 mx32;
*multilib_defaults:
m64
*multilib_extra:
*multilib_matches:
m32 m32;m64 m64;mx32 mx32;
*multilib_exclusions:
*multilib_options:
m32/m64/mx32
*multilib_reuse:
*linker:
collect2
*linker_plugin_file:
*lto_wrapper:
*lto_gcc:
*post_link:
*link_libgcc:
%D
*md_exec_prefix:
*md_startfile_prefix:
*md_startfile_prefix_1:
*startfile_prefix_spec:
*sysroot_spec:
--sysroot=%R
*sysroot_suffix_spec:
*sysroot_hdrs_suffix_spec:
*self_spec:
*cc1_cpu:
%{march=native:%>march=native %:local_cpu_detect(arch) %{!mtune=*:%>mtune=native %:local_cpu_detect(tune)}} %{mtune=native:%>mtune=native %:local_cpu_detect(tune)}
*link_command:
%{!fsyntax-only:%{!c:%{!M:%{!MM:%{!E:%{!S: %(linker) %{!fno-use-linker-plugin:%{!fno-lto: -plugin %(linker_plugin_file) -plugin-opt=%(lto_wrapper) -plugin-opt=-fresolution=%u.res %{!nostdlib:%{!nodefaultlibs:%:pass-through-libs(%(link_gcc_c_sequence))}} }}%{flto|flto=*:%<fcompare-debug*} %{flto} %{fno-lto} %{flto=*} %l %{static|shared|r:;!no-pie:-pie} %{fuse-ld=*:-fuse-ld=%*} %{gz|gz=zlib:--compress-debug-sections=zlib} %{gz=none:--compress-debug-sections=none} %{gz=zlib-gnu:--compress-debug-sections=zlib-gnu} %X %{o*} %{e*} %{N} %{n} %{r} %{s} %{t} %{u*} %{z} %{Z} %{!nostdlib:%{!nostartfiles:%S}} %{static|no-pie|static-pie:} %{L*} %(mfwrap) %(link_libgcc) %{fvtable-verify=none:} %{fvtable-verify=std: %e-fvtable-verify=std is not supported in this configuration} %{fvtable-verify=preinit: %e-fvtable-verify=preinit is not supported in this configuration} %{!nostdlib:%{!nodefaultlibs:%{%:sanitize(address):%{!shared:libasan_preinit%O%s} %{static-libasan:%{!shared:-Bstatic --whole-archive -lasan --no-whole-archive -Bdynamic}}%{!static-libasan:-lasan}} %{%:sanitize(thread):%{!shared:libtsan_preinit%O%s} %{static-libtsan:%{!shared:-Bstatic --whole-archive -ltsan --no-whole-archive -Bdynamic}}%{!static-libtsan:-ltsan}} %{%:sanitize(leak):%{!shared:liblsan_preinit%O%s} %{static-liblsan:%{!shared:-Bstatic --whole-archive -llsan --no-whole-archive -Bdynamic}}%{!static-liblsan:-llsan}}}} %o %{!nostdlib:%{!nodefaultlibs:%{mmpx:%{fcheck-pointer-bounds: %{static:--whole-archive -lmpx --no-whole-archive %:include(libmpx.spec)%(link_libmpx)} %{!static:%{static-libmpx:-Bstatic --whole-archive} %{!static-libmpx:--push-state --no-as-needed} -lmpx %{!static-libmpx:--pop-state} %{static-libmpx:--no-whole-archive -Bdynamic %:include(libmpx.spec)%(link_libmpx)}}}}%{mmpx:%{fcheck-pointer-bounds:%{!fno-chkp-use-wrappers: %{static:-lmpxwrappers} %{!static:%{static-libmpxwrappers:-Bstatic} -lmpxwrappers %{static-libmpxwrappers: -Bdynamic}}}}}}} %{mmpx:%{fcheck-pointer-bounds:%{!static:%{m16|m32|mx32:;:-z bndplt }}}} %{fopenacc|fopenmp|%:gt(%{ftree-parallelize-loops=*:%*} 1): %:include(libgomp.spec)%(link_gomp)} %{fgnu-tm:%:include(libitm.spec)%(link_itm)} %(mflib) %{fsplit-stack: --wrap=pthread_create} %{fprofile-arcs|fprofile-generate*|coverage:-lgcov} %{!nostdlib:%{!nodefaultlibs:%{%:sanitize(address): %{static-libasan|static:%:include(libsanitizer.spec)%(link_libasan)} %{static:%ecannot specify -static with -fsanitize=address}} %{%:sanitize(thread): %{static-libtsan|static:%:include(libsanitizer.spec)%(link_libtsan)} %{static:%ecannot specify -static with -fsanitize=thread}} %{%:sanitize(undefined):%{static-libubsan:-Bstatic} -lubsan %{static-libubsan:-Bdynamic} %{static-libubsan|static:%:include(libsanitizer.spec)%(link_libubsan)}} %{%:sanitize(leak): %{static-liblsan|static:%:include(libsanitizer.spec)%(link_liblsan)}}}} %{!nostdlib:%{!nodefaultlibs:%(link_ssp) %(link_gcc_c_sequence)}} %{!nostdlib:%{!nostartfiles:%E}} %{T*}
%(post_link) }}}}}}
10.2.1:
$ gcc --version
gcc (Debian 10.2.1-6) 10.2.1 20210110
Copyright (C) 2020 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
$ gcc -dumpspecs
*asm:
%{m16|m32:--32} %{m16|m32|mx32:;:--64} %{mx32:--x32} %{msse2avx:%{!mavx:-msse2avx}}
*asm_debug:
%{%:debug-level-gt(0):%{gstabs*:--gstabs}%{!gstabs*:%{g*:--gdwarf2}}} %{fdebug-prefix-map=*:--debug-prefix-map %*}
*asm_final:
%{gsplit-dwarf:
objcopy --extract-dwo %{c:%{o*:%*}%{!o*:%b%O}}%{!c:%U%O} %{c:%{o*:%:replace-extension(%{o*:%*} .dwo)}%{!o*:%b.dwo}}%{!c:%b.dwo}
objcopy --strip-dwo %{c:%{o*:%*}%{!o*:%b%O}}%{!c:%U%O} }
*asm_options:
%{-target-help:%:print-asm-header()} %{v} %{w:-W} %{I*} %{gz|gz=zlib:--compress-debug-sections=zlib} %{gz=none:--compress-debug-sections=none} %{gz=zlib-gnu:--compress-debug-sections=zlib-gnu} %a %Y %{c:%W{o*}%{!o*:-o %w%b%O}}%{!c:-o %d%w%u%O}
*invoke_as:
%{!fwpa*: %{fcompare-debug=*|fdump-final-insns=*:%:compare-debug-dump-opt()} %{!S:-o %|.s |
as %(asm_options) %m.s %A } }
*cpp:
%{posix:-D_POSIX_SOURCE} %{pthread:-D_REENTRANT}
*cpp_options:
%(cpp_unique_options) %1 %{m*} %{std*&ansi&trigraphs} %{W*&pedantic*} %{w} %{f*} %{g*:%{%:debug-level-gt(0):%{g*} %{!fno-working-directory:-fworking-directory}}} %{O*} %{undef} %{save-temps*:-fpch-preprocess} %(distro_defaults)
*cpp_debug_options:
%{d*}
*cpp_unique_options:
%{!Q:-quiet} %{nostdinc*} %{C} %{CC} %{v} %@{I*&F*} %{P} %I %{MD:-MD %{!o:%b.d}%{o*:%.d%*}} %{MMD:-MMD %{!o:%b.d}%{o*:%.d%*}} %{M} %{MM} %{MF*} %{MG} %{MP} %{MQ*} %{MT*} %{!E:%{!M:%{!MM:%{!MT:%{!MQ:%{MD|MMD:%{o*:-MQ %*}}}}}}} %{remap} %{g3|ggdb3|gstabs3|gxcoff3|gvms3:-dD} %{!iplugindir*:%{fplugin*:%:find-plugindir()}} %{H} %C %{D*&U*&A*} %{i*} %Z %i %{E|M|MM:%W{o*}}
*trad_capable_cpp:
cc1 -E %{traditional|traditional-cpp:-traditional-cpp}
*cc1:
%{!mandroid|tno-android-cc:%(cc1_cpu) %{profile:-p};:%(cc1_cpu) %{profile:-p} %{!mglibc:%{!muclibc:%{!mbionic: -mbionic}}} %{!fno-pic:%{!fno-PIC:%{!fpic:%{!fPIC: -fPIC}}}}}
*cc1_options:
%{pg:%{fomit-frame-pointer:%e-pg and -fomit-frame-pointer are incompatible}} %{!iplugindir*:%{fplugin*:%:find-plugindir()}} %1 %{!Q:-quiet} %{!dumpbase:-dumpbase %B} %{d*} %{m*} %{aux-info*} %{fcompare-debug-second:%:compare-debug-auxbase-opt(%b)} %{!fcompare-debug-second:%{c|S:%{o*:-auxbase-strip %*}%{!o*:-auxbase %b}}}%{!c:%{!S:-auxbase %b}} %{g*} %{O*} %{W*&pedantic*} %{w} %{std*&ansi&trigraphs} %{v:-version} %{pg:-p} %{p} %{f*} %{undef} %{Qn:-fno-ident} %{Qy:} %{-help:--help} %{-target-help:--target-help} %{-version:--version} %{-help=*:--help=%*} %{!fsyntax-only:%{S:%W{o*}%{!o*:-o %b.s}}} %{fsyntax-only:-o %j} %{-param*} %{coverage:-fprofile-arcs -ftest-coverage} %{fprofile-arcs|fprofile-generate*|coverage: %{!fprofile-update=single: %{pthread:-fprofile-update=prefer-atomic}}}
*cc1plus:
*link_gcc_c_sequence:
%{static|static-pie:--start-group} %G %{!nolibc:%L} %{static|static-pie:--end-group}%{!static:%{!static-pie:%G}}
*distro_defaults:
%{!fno-asynchronous-unwind-tables:-fasynchronous-unwind-tables}
*link_ssp:
%{fstack-protector|fstack-protector-all|fstack-protector-strong|fstack-protector-explicit:}
*endfile:
%{!mandroid|tno-android-ld:%{Ofast|ffast-math|funsafe-math-optimizations:crtfastmath.o%s} %{mpc32:crtprec32.o%s} %{mpc64:crtprec64.o%s} %{mpc80:crtprec80.o%s} %{!static:%{fvtable-verify=none:%s; fvtable-verify=preinit:vtv_end_preinit.o%s; fvtable-verify=std:vtv_end.o%s}} %{static:crtend.o%s; shared|static-pie|!no-pie:crtendS.o%s; :crtend.o%s} crtn.o%s %{fopenacc|fopenmp:crtoffloadend%O%s};:%{Ofast|ffast-math|funsafe-math-optimizations:crtfastmath.o%s} %{mpc32:crtprec32.o%s} %{mpc64:crtprec64.o%s} %{mpc80:crtprec80.o%s} %{shared: crtend_so%O%s;: crtend_android%O%s}}
*link:
%{!r:--build-id} %{!static|static-pie:--eh-frame-hdr} %{!mandroid|tno-android-ld:%{m16|m32|mx32:;:-m elf_x86_64} %{m16|m32:-m elf_i386} %{mx32:-m elf32_x86_64} --hash-style=gnu --as-needed %{shared:-shared} %{!shared: %{!static: %{!static-pie: %{rdynamic:-export-dynamic} %{m16|m32:-dynamic-linker %{muclibc:/lib/ld-uClibc.so.0;:%{mbionic:/system/bin/linker;:%{mmusl:/lib/ld-musl-i386.so.1;:/lib/ld-linux.so.2}}}} %{m16|m32|mx32:;:-dynamic-linker %{muclibc:/lib/ld64-uClibc.so.0;:%{mbionic:/system/bin/linker64;:%{mmusl:/lib/ld-musl-x86_64.so.1;:/lib64/ld-linux-x86-64.so.2}}}} %{mx32:-dynamic-linker %{muclibc:/lib/ldx32-uClibc.so.0;:%{mbionic:/system/bin/linkerx32;:%{mmusl:/lib/ld-musl-x32.so.1;:/libx32/ld-linux-x32.so.2}}}}}} %{static:-static} %{static-pie:-static -pie --no-dynamic-linker -z text}};:%{m16|m32|mx32:;:-m elf_x86_64} %{m16|m32:-m elf_i386} %{mx32:-m elf32_x86_64} --hash-style=gnu --as-needed %{shared:-shared} %{!shared: %{!static: %{!static-pie: %{rdynamic:-export-dynamic} %{m16|m32:-dynamic-linker %{muclibc:/lib/ld-uClibc.so.0;:%{mbionic:/system/bin/linker;:%{mmusl:/lib/ld-musl-i386.so.1;:/lib/ld-linux.so.2}}}} %{m16|m32|mx32:;:-dynamic-linker %{muclibc:/lib/ld64-uClibc.so.0;:%{mbionic:/system/bin/linker64;:%{mmusl:/lib/ld-musl-x86_64.so.1;:/lib64/ld-linux-x86-64.so.2}}}} %{mx32:-dynamic-linker %{muclibc:/lib/ldx32-uClibc.so.0;:%{mbionic:/system/bin/linkerx32;:%{mmusl:/lib/ld-musl-x32.so.1;:/libx32/ld-linux-x32.so.2}}}}}} %{static:-static} %{static-pie:-static -pie --no-dynamic-linker -z text}} %{shared: -Bsymbolic}}
*lib:
%{!mandroid|tno-android-ld:%{pthread:-lpthread} %{shared:-lc} %{!shared:%{profile:-lc_p}%{!profile:-lc}};:%{shared:-lc} %{!shared:%{profile:-lc_p}%{!profile:-lc}} %{!static: -ldl}}
*link_gomp:
*libgcc:
%{static|static-libgcc|static-pie:-lgcc -lgcc_eh}%{!static:%{!static-libgcc:%{!static-pie:%{!shared-libgcc:-lgcc --push-state --as-needed -lgcc_s --pop-state}%{shared-libgcc:-lgcc_s%{!shared: -lgcc}}}}}
*startfile:
%{!mandroid|tno-android-ld:%{shared:; pg|p|profile:%{static-pie:grcrt1.o%s;:gcrt1.o%s}; static:crt1.o%s; static-pie:rcrt1.o%s; !no-pie:Scrt1.o%s; :crt1.o%s} crti.o%s %{static:crtbeginT.o%s; shared|static-pie|!no-pie:crtbeginS.o%s; :crtbegin.o%s} %{fvtable-verify=none:%s; fvtable-verify=preinit:vtv_start_preinit.o%s; fvtable-verify=std:vtv_start.o%s} %{fopenacc|fopenmp:crtoffloadbegin%O%s};:%{shared: crtbegin_so%O%s;: %{static: crtbegin_static%O%s;: crtbegin_dynamic%O%s}}}
*cross_compile:
0
*version:
10.2.1
*multilib:
. !m32 !m64 !mx32;32:../lib32:i386-linux-gnu m32 !m64 !mx32;64:../lib:x86_64-linux-gnu !m32 m64 !mx32;x32:../libx32:x86_64-linux-gnux32 !m32 !m64 mx32;
*multilib_defaults:
m64
*multilib_extra:
*multilib_matches:
m32 m32;m64 m64;mx32 mx32;
*multilib_exclusions:
*multilib_options:
m32/m64/mx32
*multilib_reuse:
*linker:
collect2
*linker_plugin_file:
*lto_wrapper:
*lto_gcc:
*post_link:
*link_libgcc:
%D
*md_exec_prefix:
*md_startfile_prefix:
*md_startfile_prefix_1:
*startfile_prefix_spec:
*sysroot_spec:
--sysroot=%R
*sysroot_suffix_spec:
*sysroot_hdrs_suffix_spec:
*self_spec:
*cc1_cpu:
%{march=native:%>march=native %:local_cpu_detect(arch) %{!mtune=*:%>mtune=native %:local_cpu_detect(tune)}} %{mtune=native:%>mtune=native %:local_cpu_detect(tune)}
*link_command:
%{!fsyntax-only:%{!c:%{!M:%{!MM:%{!E:%{!S: %(linker) %{!fno-use-linker-plugin:%{!fno-lto: -plugin %(linker_plugin_file) -plugin-opt=%(lto_wrapper) -plugin-opt=-fresolution=%u.res %{flinker-output=*:-plugin-opt=-linker-output-known} %{!nostdlib:%{!nodefaultlibs:%:pass-through-libs(%(link_gcc_c_sequence))}} }}%{flto|flto=*:%<fcompare-debug*} %{flto} %{fno-lto} %{flto=*} %l %{static|shared|r:;!no-pie:-pie} %{fuse-ld=*:-fuse-ld=%*} %{gz|gz=zlib:--compress-debug-sections=zlib} %{gz=none:--compress-debug-sections=none} %{gz=zlib-gnu:--compress-debug-sections=zlib-gnu} %X %{o*} %{e*} %{N} %{n} %{r} %{s} %{t} %{u*} %{z} %{Z} %{!nostdlib:%{!r:%{!nostartfiles:%S}}} %{static|no-pie|static-pie:} %@{L*} %(mfwrap) %(link_libgcc) %{fvtable-verify=none:} %{fvtable-verify=std: %e-fvtable-verify=std is not supported in this configuration} %{fvtable-verify=preinit: %e-fvtable-verify=preinit is not supported in this configuration} %{!nostdlib:%{!r:%{!nodefaultlibs:%{%:sanitize(address):%{!shared:libasan_preinit%O%s} %{static-libasan:%{!shared:-Bstatic --whole-archive -lasan --no-whole-archive -Bdynamic}}%{!static-libasan:%{!fuse-ld=gold:--push-state} --no-as-needed -lasan %{fuse-ld=gold:--as-needed;:--pop-state}}} %{%:sanitize(thread):%{!shared:libtsan_preinit%O%s} %{static-libtsan:%{!shared:-Bstatic --whole-archive -ltsan --no-whole-archive -Bdynamic}}%{!static-libtsan:%{!fuse-ld=gold:--push-state} --no-as-needed -ltsan %{fuse-ld=gold:--as-needed;:--pop-state}}} %{%:sanitize(leak):%{!shared:liblsan_preinit%O%s} %{static-liblsan:%{!shared:-Bstatic --whole-archive -llsan --no-whole-archive -Bdynamic}}%{!static-liblsan:%{!fuse-ld=gold:--push-state} --no-as-needed -llsan %{fuse-ld=gold:--as-needed;:--pop-state}}}}}} %o %{fopenacc|fopenmp|%:gt(%{ftree-parallelize-loops=*:%*} 1): %:include(libgomp.spec)%(link_gomp)} %{fgnu-tm:%:include(libitm.spec)%(link_itm)} %(mflib) %{fsplit-stack: --wrap=pthread_create} %{fprofile-arcs|fprofile-generate*|coverage:-lgcov} %{!nostdlib:%{!r:%{!nodefaultlibs:%{%:sanitize(address): %{static-libasan|static:%:include(libsanitizer.spec)%(link_libasan)} %{static:%ecannot specify -static with -fsanitize=address}} %{%:sanitize(thread): %{static-libtsan|static:%:include(libsanitizer.spec)%(link_libtsan)} %{static:%ecannot specify -static with -fsanitize=thread}} %{%:sanitize(undefined):%{static-libubsan:-Bstatic} %{!static-libubsan:--push-state --no-as-needed} -lubsan %{static-libubsan:-Bdynamic} %{!static-libubsan:--pop-state} %{static-libubsan|static:%:include(libsanitizer.spec)%(link_libubsan)}} %{%:sanitize(leak): %{static-liblsan|static:%:include(libsanitizer.spec)%(link_liblsan)}}}}} %{!nostdlib:%{!r:%{!nodefaultlibs:%(link_ssp) %(link_gcc_c_sequence)}}} %{!nostdlib:%{!r:%{!nostartfiles:%E}}} %{T*}
%(post_link) }}}}}}
是的，行为发生了变化。
在 C 中，您应该只在一个翻译单元中定义一个全局变量，其他要访问该变量的翻译单元应将其声明为“extern”。
在您的代码中，a.h 包含在 a.c 和 main.c 中，因此该变量定义了两次。要解决此问题，您应该将 a.h 中的“int test”更改为“extern int test”，然后将“int test”添加到 a.c 以仅定义一次变量。
在 C 中，不初始化变量的全局变量的定义被认为是“暂定的”。您可以在同一个编译单元中对一个变量进行多个暂定定义。标准 C 不允许在不同的编译单元中进行多个临时定义，但 unix 系统上的 C 编译器历来允许。
旧版本的 gcc 默认允许在不同的编译单元中对一个全局变量进行多个临时定义（但不允许多个非临时定义）。 gcc-10 没有。您可以使用命令行选项“-fcommon”恢复旧行为，但不鼓励这样做。
我浏览了一下 Whosebug 和更广泛的互联网，发现导致此错误的最常见原因是混淆了 声明 (int var = 1; ) 和 定义 (int var;)，并包括来自 .h 个文件的 .c 个文件。
我刚从一个文件拆分成几个文件的小项目没有做任何这些事情。我非常困惑。
我复制了项目并删除了副本中的所有代码（这很有趣）直到我到达这里：
main.c
#include "a.h"
int main() {
}
a.h
#ifndef A_H
#define A_H
int test;
#endif
a.c
#include "a.h"
已经很规范了，对吧？然而...
$ rm *.o; gcc -g -c a.c; gcc -g -c main.c; gcc -o main main.o a.o
/usr/bin/ld: a.o:/.../a.h:5: multiple definition of `test'; main.o:/.../a.h:5: first defined here
collect2: error: ld returned 1 exit status
瓦特 :(
大约在这个时候，我开始质疑我的配置并尝试了另一台机器，通过 NFS 访问完全相同的文件。
$ rm *.o; gcc -g -c a.c; gcc -g -c main.c; gcc -o main main.o a.o
$ ./main
$ echo $?
0
哦不。我在第一台机器上坏了什么...
希望我的问题是在 GCC 8.3.0（工作） 和 GCC 10.2.1（失败） 之间悄悄发生了一些变化？也许较新版本的 GCC 默认为较新的 C 标准或其他东西，它在这方面指定了不同的行为？
或者它可能与配置有关（这都是 Debian 的股票）。这是两个版本的 gcc -dumpspecs 的输出：
8.3.0:
$ gcc --version
gcc (Debian 8.3.0-6) 8.3.0
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
$ gcc -dumpspecs
*asm:
%{m16|m32:--32} %{m16|m32|mx32:;:--64} %{mx32:--x32} %{msse2avx:%{!mavx:-msse2avx}}
*asm_debug:
%{%:debug-level-gt(0):%{gstabs*:--gstabs}%{!gstabs*:%{g*:--gdwarf2}}} %{fdebug-prefix-map=*:--debug-prefix-map %*}
*asm_final:
%{gsplit-dwarf:
objcopy --extract-dwo %{c:%{o*:%*}%{!o*:%b%O}}%{!c:%U%O} %{c:%{o*:%:replace-extension(%{o*:%*} .dwo)}%{!o*:%b.dwo}}%{!c:%b.dwo}
objcopy --strip-dwo %{c:%{o*:%*}%{!o*:%b%O}}%{!c:%U%O} }
*asm_options:
%{-target-help:%:print-asm-header()} %{v} %{w:-W} %{I*} %{gz|gz=zlib:--compress-debug-sections=zlib} %{gz=none:--compress-debug-sections=none} %{gz=zlib-gnu:--compress-debug-sections=zlib-gnu} %a %Y %{c:%W{o*}%{!o*:-o %w%b%O}}%{!c:-o %d%w%u%O}
*invoke_as:
%{!fwpa*: %{fcompare-debug=*|fdump-final-insns=*:%:compare-debug-dump-opt()} %{!S:-o %|.s |
as %(asm_options) %m.s %A } }
*cpp:
%{posix:-D_POSIX_SOURCE} %{pthread:-D_REENTRANT}
*cpp_options:
%(cpp_unique_options) %1 %{m*} %{std*&ansi&trigraphs} %{W*&pedantic*} %{w} %{f*} %{g*:%{%:debug-level-gt(0):%{g*} %{!fno-working-directory:-fworking-directory}}} %{O*} %{undef} %{save-temps*:-fpch-preprocess}
*cpp_debug_options:
%{d*}
*cpp_unique_options:
%{!Q:-quiet} %{nostdinc*} %{C} %{CC} %{v} %{I*&F*} %{P} %I %{MD:-MD %{!o:%b.d}%{o*:%.d%*}} %{MMD:-MMD %{!o:%b.d}%{o*:%.d%*}} %{M} %{MM} %{MF*} %{MG} %{MP} %{MQ*} %{MT*} %{!E:%{!M:%{!MM:%{!MT:%{!MQ:%{MD|MMD:%{o*:-MQ %*}}}}}}} %{remap} %{g3|ggdb3|gstabs3|gxcoff3|gvms3:-dD} %{!iplugindir*:%{fplugin*:%:find-plugindir()}} %{H} %C %{D*&U*&A*} %{i*} %Z %i %{E|M|MM:%W{o*}}
*trad_capable_cpp:
cc1 -E %{traditional|traditional-cpp:-traditional-cpp}
*cc1:
%{!mandroid|tno-android-cc:%(cc1_cpu) %{profile:-p};:%(cc1_cpu) %{profile:-p} %{!mglibc:%{!muclibc:%{!mbionic: -mbionic}}} %{!fno-pic:%{!fno-PIC:%{!fpic:%{!fPIC: -fPIC}}}}}
*cc1_options:
%{pg:%{fomit-frame-pointer:%e-pg and -fomit-frame-pointer are incompatible}} %{!iplugindir*:%{fplugin*:%:find-plugindir()}} %1 %{!Q:-quiet} %{!dumpbase:-dumpbase %B} %{d*} %{m*} %{aux-info*} %{fcompare-debug-second:%:compare-debug-auxbase-opt(%b)} %{!fcompare-debug-second:%{c|S:%{o*:-auxbase-strip %*}%{!o*:-auxbase %b}}}%{!c:%{!S:-auxbase %b}} %{g*} %{O*} %{W*&pedantic*} %{w} %{std*&ansi&trigraphs} %{v:-version} %{pg:-p} %{p} %{f*} %{undef} %{Qn:-fno-ident} %{Qy:} %{-help:--help} %{-target-help:--target-help} %{-version:--version} %{-help=*:--help=%*} %{!fsyntax-only:%{S:%W{o*}%{!o*:-o %b.s}}} %{fsyntax-only:-o %j} %{-param*} %{coverage:-fprofile-arcs -ftest-coverage} %{fprofile-arcs|fprofile-generate*|coverage: %{!fprofile-update=single: %{pthread:-fprofile-update=prefer-atomic}}}
*cc1plus:
*link_gcc_c_sequence:
%{static|static-pie:--start-group} %G %L %{static|static-pie:--end-group}%{!static:%{!static-pie:%G}}
*link_ssp:
%{fstack-protector|fstack-protector-all|fstack-protector-strong|fstack-protector-explicit:}
*endfile:
%{!mandroid|tno-android-ld:%{Ofast|ffast-math|funsafe-math-optimizations:crtfastmath.o%s} %{mpc32:crtprec32.o%s} %{mpc64:crtprec64.o%s} %{mpc80:crtprec80.o%s} %{fvtable-verify=none:%s; fvtable-verify=preinit:vtv_end_preinit.o%s; fvtable-verify=std:vtv_end.o%s} %{static:crtend.o%s; shared|static-pie|!no-pie:crtendS.o%s; :crtend.o%s} crtn.o%s %{fopenacc|fopenmp:crtoffloadend%O%s};:%{Ofast|ffast-math|funsafe-math-optimizations:crtfastmath.o%s} %{mpc32:crtprec32.o%s} %{mpc64:crtprec64.o%s} %{mpc80:crtprec80.o%s} %{shared: crtend_so%O%s;: crtend_android%O%s}}
*link:
%{!r:--build-id} %{!static|static-pie:--eh-frame-hdr} %{!mandroid|tno-android-ld:%{m16|m32|mx32:;:-m elf_x86_64} %{m16|m32:-m elf_i386} %{mx32:-m elf32_x86_64} --hash-style=gnu %{shared:-shared} %{!shared: %{!static: %{!static-pie: %{rdynamic:-export-dynamic} %{m16|m32:-dynamic-linker %{muclibc:/lib/ld-uClibc.so.0;:%{mbionic:/system/bin/linker;:%{mmusl:/lib/ld-musl-i386.so.1;:/lib/ld-linux.so.2}}}} %{m16|m32|mx32:;:-dynamic-linker %{muclibc:/lib/ld64-uClibc.so.0;:%{mbionic:/system/bin/linker64;:%{mmusl:/lib/ld-musl-x86_64.so.1;:/lib64/ld-linux-x86-64.so.2}}}} %{mx32:-dynamic-linker %{muclibc:/lib/ldx32-uClibc.so.0;:%{mbionic:/system/bin/linkerx32;:%{mmusl:/lib/ld-musl-x32.so.1;:/libx32/ld-linux-x32.so.2}}}}}} %{static:-static} %{static-pie:-static -pie --no-dynamic-linker -z text}};:%{m16|m32|mx32:;:-m elf_x86_64} %{m16|m32:-m elf_i386} %{mx32:-m elf32_x86_64} --hash-style=gnu %{shared:-shared} %{!shared: %{!static: %{!static-pie: %{rdynamic:-export-dynamic} %{m16|m32:-dynamic-linker %{muclibc:/lib/ld-uClibc.so.0;:%{mbionic:/system/bin/linker;:%{mmusl:/lib/ld-musl-i386.so.1;:/lib/ld-linux.so.2}}}} %{m16|m32|mx32:;:-dynamic-linker %{muclibc:/lib/ld64-uClibc.so.0;:%{mbionic:/system/bin/linker64;:%{mmusl:/lib/ld-musl-x86_64.so.1;:/lib64/ld-linux-x86-64.so.2}}}} %{mx32:-dynamic-linker %{muclibc:/lib/ldx32-uClibc.so.0;:%{mbionic:/system/bin/linkerx32;:%{mmusl:/lib/ld-musl-x32.so.1;:/libx32/ld-linux-x32.so.2}}}}}} %{static:-static} %{static-pie:-static -pie --no-dynamic-linker -z text}} %{shared: -Bsymbolic}}
*lib:
%{!mandroid|tno-android-ld:%{pthread:-lpthread} %{shared:-lc} %{!shared:%{profile:-lc_p}%{!profile:-lc}};:%{shared:-lc} %{!shared:%{profile:-lc_p}%{!profile:-lc}} %{!static: -ldl}}
*link_gomp:
*libgcc:
%{static|static-libgcc|static-pie:-lgcc -lgcc_eh}%{!static:%{!static-libgcc:%{!static-pie:%{!shared-libgcc:-lgcc --push-state --as-needed -lgcc_s --pop-state}%{shared-libgcc:-lgcc_s%{!shared: -lgcc}}}}}
*startfile:
%{!mandroid|tno-android-ld:%{shared:; pg|p|profile:%{static-pie:grcrt1.o%s;:gcrt1.o%s}; static:crt1.o%s; static-pie:rcrt1.o%s; !no-pie:Scrt1.o%s; :crt1.o%s} crti.o%s %{static:crtbeginT.o%s; shared|static-pie|!no-pie:crtbeginS.o%s; :crtbegin.o%s} %{fvtable-verify=none:%s; fvtable-verify=preinit:vtv_start_preinit.o%s; fvtable-verify=std:vtv_start.o%s} %{fopenacc|fopenmp:crtoffloadbegin%O%s};:%{shared: crtbegin_so%O%s;: %{static: crtbegin_static%O%s;: crtbegin_dynamic%O%s}}}
*cross_compile:
0
*version:
8.3.0
*multilib:
. !m32 !m64 !mx32;32:../lib32:i386-linux-gnu m32 !m64 !mx32;64:../lib:x86_64-linux-gnu !m32 m64 !mx32;x32:../libx32:x86_64-linux-gnux32 !m32 !m64 mx32;
*multilib_defaults:
m64
*multilib_extra:
*multilib_matches:
m32 m32;m64 m64;mx32 mx32;
*multilib_exclusions:
*multilib_options:
m32/m64/mx32
*multilib_reuse:
*linker:
collect2
*linker_plugin_file:
*lto_wrapper:
*lto_gcc:
*post_link:
*link_libgcc:
%D
*md_exec_prefix:
*md_startfile_prefix:
*md_startfile_prefix_1:
*startfile_prefix_spec:
*sysroot_spec:
--sysroot=%R
*sysroot_suffix_spec:
*sysroot_hdrs_suffix_spec:
*self_spec:
*cc1_cpu:
%{march=native:%>march=native %:local_cpu_detect(arch) %{!mtune=*:%>mtune=native %:local_cpu_detect(tune)}} %{mtune=native:%>mtune=native %:local_cpu_detect(tune)}
*link_command:
%{!fsyntax-only:%{!c:%{!M:%{!MM:%{!E:%{!S: %(linker) %{!fno-use-linker-plugin:%{!fno-lto: -plugin %(linker_plugin_file) -plugin-opt=%(lto_wrapper) -plugin-opt=-fresolution=%u.res %{!nostdlib:%{!nodefaultlibs:%:pass-through-libs(%(link_gcc_c_sequence))}} }}%{flto|flto=*:%<fcompare-debug*} %{flto} %{fno-lto} %{flto=*} %l %{static|shared|r:;!no-pie:-pie} %{fuse-ld=*:-fuse-ld=%*} %{gz|gz=zlib:--compress-debug-sections=zlib} %{gz=none:--compress-debug-sections=none} %{gz=zlib-gnu:--compress-debug-sections=zlib-gnu} %X %{o*} %{e*} %{N} %{n} %{r} %{s} %{t} %{u*} %{z} %{Z} %{!nostdlib:%{!nostartfiles:%S}} %{static|no-pie|static-pie:} %{L*} %(mfwrap) %(link_libgcc) %{fvtable-verify=none:} %{fvtable-verify=std: %e-fvtable-verify=std is not supported in this configuration} %{fvtable-verify=preinit: %e-fvtable-verify=preinit is not supported in this configuration} %{!nostdlib:%{!nodefaultlibs:%{%:sanitize(address):%{!shared:libasan_preinit%O%s} %{static-libasan:%{!shared:-Bstatic --whole-archive -lasan --no-whole-archive -Bdynamic}}%{!static-libasan:-lasan}} %{%:sanitize(thread):%{!shared:libtsan_preinit%O%s} %{static-libtsan:%{!shared:-Bstatic --whole-archive -ltsan --no-whole-archive -Bdynamic}}%{!static-libtsan:-ltsan}} %{%:sanitize(leak):%{!shared:liblsan_preinit%O%s} %{static-liblsan:%{!shared:-Bstatic --whole-archive -llsan --no-whole-archive -Bdynamic}}%{!static-liblsan:-llsan}}}} %o %{!nostdlib:%{!nodefaultlibs:%{mmpx:%{fcheck-pointer-bounds: %{static:--whole-archive -lmpx --no-whole-archive %:include(libmpx.spec)%(link_libmpx)} %{!static:%{static-libmpx:-Bstatic --whole-archive} %{!static-libmpx:--push-state --no-as-needed} -lmpx %{!static-libmpx:--pop-state} %{static-libmpx:--no-whole-archive -Bdynamic %:include(libmpx.spec)%(link_libmpx)}}}}%{mmpx:%{fcheck-pointer-bounds:%{!fno-chkp-use-wrappers: %{static:-lmpxwrappers} %{!static:%{static-libmpxwrappers:-Bstatic} -lmpxwrappers %{static-libmpxwrappers: -Bdynamic}}}}}}} %{mmpx:%{fcheck-pointer-bounds:%{!static:%{m16|m32|mx32:;:-z bndplt }}}} %{fopenacc|fopenmp|%:gt(%{ftree-parallelize-loops=*:%*} 1): %:include(libgomp.spec)%(link_gomp)} %{fgnu-tm:%:include(libitm.spec)%(link_itm)} %(mflib) %{fsplit-stack: --wrap=pthread_create} %{fprofile-arcs|fprofile-generate*|coverage:-lgcov} %{!nostdlib:%{!nodefaultlibs:%{%:sanitize(address): %{static-libasan|static:%:include(libsanitizer.spec)%(link_libasan)} %{static:%ecannot specify -static with -fsanitize=address}} %{%:sanitize(thread): %{static-libtsan|static:%:include(libsanitizer.spec)%(link_libtsan)} %{static:%ecannot specify -static with -fsanitize=thread}} %{%:sanitize(undefined):%{static-libubsan:-Bstatic} -lubsan %{static-libubsan:-Bdynamic} %{static-libubsan|static:%:include(libsanitizer.spec)%(link_libubsan)}} %{%:sanitize(leak): %{static-liblsan|static:%:include(libsanitizer.spec)%(link_liblsan)}}}} %{!nostdlib:%{!nodefaultlibs:%(link_ssp) %(link_gcc_c_sequence)}} %{!nostdlib:%{!nostartfiles:%E}} %{T*}
%(post_link) }}}}}}
10.2.1:
$ gcc --version
gcc (Debian 10.2.1-6) 10.2.1 20210110
Copyright (C) 2020 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
$ gcc -dumpspecs
*asm:
%{m16|m32:--32} %{m16|m32|mx32:;:--64} %{mx32:--x32} %{msse2avx:%{!mavx:-msse2avx}}
*asm_debug:
%{%:debug-level-gt(0):%{gstabs*:--gstabs}%{!gstabs*:%{g*:--gdwarf2}}} %{fdebug-prefix-map=*:--debug-prefix-map %*}
*asm_final:
%{gsplit-dwarf:
objcopy --extract-dwo %{c:%{o*:%*}%{!o*:%b%O}}%{!c:%U%O} %{c:%{o*:%:replace-extension(%{o*:%*} .dwo)}%{!o*:%b.dwo}}%{!c:%b.dwo}
objcopy --strip-dwo %{c:%{o*:%*}%{!o*:%b%O}}%{!c:%U%O} }
*asm_options:
%{-target-help:%:print-asm-header()} %{v} %{w:-W} %{I*} %{gz|gz=zlib:--compress-debug-sections=zlib} %{gz=none:--compress-debug-sections=none} %{gz=zlib-gnu:--compress-debug-sections=zlib-gnu} %a %Y %{c:%W{o*}%{!o*:-o %w%b%O}}%{!c:-o %d%w%u%O}
*invoke_as:
%{!fwpa*: %{fcompare-debug=*|fdump-final-insns=*:%:compare-debug-dump-opt()} %{!S:-o %|.s |
as %(asm_options) %m.s %A } }
*cpp:
%{posix:-D_POSIX_SOURCE} %{pthread:-D_REENTRANT}
*cpp_options:
%(cpp_unique_options) %1 %{m*} %{std*&ansi&trigraphs} %{W*&pedantic*} %{w} %{f*} %{g*:%{%:debug-level-gt(0):%{g*} %{!fno-working-directory:-fworking-directory}}} %{O*} %{undef} %{save-temps*:-fpch-preprocess} %(distro_defaults)
*cpp_debug_options:
%{d*}
*cpp_unique_options:
%{!Q:-quiet} %{nostdinc*} %{C} %{CC} %{v} %@{I*&F*} %{P} %I %{MD:-MD %{!o:%b.d}%{o*:%.d%*}} %{MMD:-MMD %{!o:%b.d}%{o*:%.d%*}} %{M} %{MM} %{MF*} %{MG} %{MP} %{MQ*} %{MT*} %{!E:%{!M:%{!MM:%{!MT:%{!MQ:%{MD|MMD:%{o*:-MQ %*}}}}}}} %{remap} %{g3|ggdb3|gstabs3|gxcoff3|gvms3:-dD} %{!iplugindir*:%{fplugin*:%:find-plugindir()}} %{H} %C %{D*&U*&A*} %{i*} %Z %i %{E|M|MM:%W{o*}}
*trad_capable_cpp:
cc1 -E %{traditional|traditional-cpp:-traditional-cpp}
*cc1:
%{!mandroid|tno-android-cc:%(cc1_cpu) %{profile:-p};:%(cc1_cpu) %{profile:-p} %{!mglibc:%{!muclibc:%{!mbionic: -mbionic}}} %{!fno-pic:%{!fno-PIC:%{!fpic:%{!fPIC: -fPIC}}}}}
*cc1_options:
%{pg:%{fomit-frame-pointer:%e-pg and -fomit-frame-pointer are incompatible}} %{!iplugindir*:%{fplugin*:%:find-plugindir()}} %1 %{!Q:-quiet} %{!dumpbase:-dumpbase %B} %{d*} %{m*} %{aux-info*} %{fcompare-debug-second:%:compare-debug-auxbase-opt(%b)} %{!fcompare-debug-second:%{c|S:%{o*:-auxbase-strip %*}%{!o*:-auxbase %b}}}%{!c:%{!S:-auxbase %b}} %{g*} %{O*} %{W*&pedantic*} %{w} %{std*&ansi&trigraphs} %{v:-version} %{pg:-p} %{p} %{f*} %{undef} %{Qn:-fno-ident} %{Qy:} %{-help:--help} %{-target-help:--target-help} %{-version:--version} %{-help=*:--help=%*} %{!fsyntax-only:%{S:%W{o*}%{!o*:-o %b.s}}} %{fsyntax-only:-o %j} %{-param*} %{coverage:-fprofile-arcs -ftest-coverage} %{fprofile-arcs|fprofile-generate*|coverage: %{!fprofile-update=single: %{pthread:-fprofile-update=prefer-atomic}}}
*cc1plus:
*link_gcc_c_sequence:
%{static|static-pie:--start-group} %G %{!nolibc:%L} %{static|static-pie:--end-group}%{!static:%{!static-pie:%G}}
*distro_defaults:
%{!fno-asynchronous-unwind-tables:-fasynchronous-unwind-tables}
*link_ssp:
%{fstack-protector|fstack-protector-all|fstack-protector-strong|fstack-protector-explicit:}
*endfile:
%{!mandroid|tno-android-ld:%{Ofast|ffast-math|funsafe-math-optimizations:crtfastmath.o%s} %{mpc32:crtprec32.o%s} %{mpc64:crtprec64.o%s} %{mpc80:crtprec80.o%s} %{!static:%{fvtable-verify=none:%s; fvtable-verify=preinit:vtv_end_preinit.o%s; fvtable-verify=std:vtv_end.o%s}} %{static:crtend.o%s; shared|static-pie|!no-pie:crtendS.o%s; :crtend.o%s} crtn.o%s %{fopenacc|fopenmp:crtoffloadend%O%s};:%{Ofast|ffast-math|funsafe-math-optimizations:crtfastmath.o%s} %{mpc32:crtprec32.o%s} %{mpc64:crtprec64.o%s} %{mpc80:crtprec80.o%s} %{shared: crtend_so%O%s;: crtend_android%O%s}}
*link:
%{!r:--build-id} %{!static|static-pie:--eh-frame-hdr} %{!mandroid|tno-android-ld:%{m16|m32|mx32:;:-m elf_x86_64} %{m16|m32:-m elf_i386} %{mx32:-m elf32_x86_64} --hash-style=gnu --as-needed %{shared:-shared} %{!shared: %{!static: %{!static-pie: %{rdynamic:-export-dynamic} %{m16|m32:-dynamic-linker %{muclibc:/lib/ld-uClibc.so.0;:%{mbionic:/system/bin/linker;:%{mmusl:/lib/ld-musl-i386.so.1;:/lib/ld-linux.so.2}}}} %{m16|m32|mx32:;:-dynamic-linker %{muclibc:/lib/ld64-uClibc.so.0;:%{mbionic:/system/bin/linker64;:%{mmusl:/lib/ld-musl-x86_64.so.1;:/lib64/ld-linux-x86-64.so.2}}}} %{mx32:-dynamic-linker %{muclibc:/lib/ldx32-uClibc.so.0;:%{mbionic:/system/bin/linkerx32;:%{mmusl:/lib/ld-musl-x32.so.1;:/libx32/ld-linux-x32.so.2}}}}}} %{static:-static} %{static-pie:-static -pie --no-dynamic-linker -z text}};:%{m16|m32|mx32:;:-m elf_x86_64} %{m16|m32:-m elf_i386} %{mx32:-m elf32_x86_64} --hash-style=gnu --as-needed %{shared:-shared} %{!shared: %{!static: %{!static-pie: %{rdynamic:-export-dynamic} %{m16|m32:-dynamic-linker %{muclibc:/lib/ld-uClibc.so.0;:%{mbionic:/system/bin/linker;:%{mmusl:/lib/ld-musl-i386.so.1;:/lib/ld-linux.so.2}}}} %{m16|m32|mx32:;:-dynamic-linker %{muclibc:/lib/ld64-uClibc.so.0;:%{mbionic:/system/bin/linker64;:%{mmusl:/lib/ld-musl-x86_64.so.1;:/lib64/ld-linux-x86-64.so.2}}}} %{mx32:-dynamic-linker %{muclibc:/lib/ldx32-uClibc.so.0;:%{mbionic:/system/bin/linkerx32;:%{mmusl:/lib/ld-musl-x32.so.1;:/libx32/ld-linux-x32.so.2}}}}}} %{static:-static} %{static-pie:-static -pie --no-dynamic-linker -z text}} %{shared: -Bsymbolic}}
*lib:
%{!mandroid|tno-android-ld:%{pthread:-lpthread} %{shared:-lc} %{!shared:%{profile:-lc_p}%{!profile:-lc}};:%{shared:-lc} %{!shared:%{profile:-lc_p}%{!profile:-lc}} %{!static: -ldl}}
*link_gomp:
*libgcc:
%{static|static-libgcc|static-pie:-lgcc -lgcc_eh}%{!static:%{!static-libgcc:%{!static-pie:%{!shared-libgcc:-lgcc --push-state --as-needed -lgcc_s --pop-state}%{shared-libgcc:-lgcc_s%{!shared: -lgcc}}}}}
*startfile:
%{!mandroid|tno-android-ld:%{shared:; pg|p|profile:%{static-pie:grcrt1.o%s;:gcrt1.o%s}; static:crt1.o%s; static-pie:rcrt1.o%s; !no-pie:Scrt1.o%s; :crt1.o%s} crti.o%s %{static:crtbeginT.o%s; shared|static-pie|!no-pie:crtbeginS.o%s; :crtbegin.o%s} %{fvtable-verify=none:%s; fvtable-verify=preinit:vtv_start_preinit.o%s; fvtable-verify=std:vtv_start.o%s} %{fopenacc|fopenmp:crtoffloadbegin%O%s};:%{shared: crtbegin_so%O%s;: %{static: crtbegin_static%O%s;: crtbegin_dynamic%O%s}}}
*cross_compile:
0
*version:
10.2.1
*multilib:
. !m32 !m64 !mx32;32:../lib32:i386-linux-gnu m32 !m64 !mx32;64:../lib:x86_64-linux-gnu !m32 m64 !mx32;x32:../libx32:x86_64-linux-gnux32 !m32 !m64 mx32;
*multilib_defaults:
m64
*multilib_extra:
*multilib_matches:
m32 m32;m64 m64;mx32 mx32;
*multilib_exclusions:
*multilib_options:
m32/m64/mx32
*multilib_reuse:
*linker:
collect2
*linker_plugin_file:
*lto_wrapper:
*lto_gcc:
*post_link:
*link_libgcc:
%D
*md_exec_prefix:
*md_startfile_prefix:
*md_startfile_prefix_1:
*startfile_prefix_spec:
*sysroot_spec:
--sysroot=%R
*sysroot_suffix_spec:
*sysroot_hdrs_suffix_spec:
*self_spec:
*cc1_cpu:
%{march=native:%>march=native %:local_cpu_detect(arch) %{!mtune=*:%>mtune=native %:local_cpu_detect(tune)}} %{mtune=native:%>mtune=native %:local_cpu_detect(tune)}
*link_command:
%{!fsyntax-only:%{!c:%{!M:%{!MM:%{!E:%{!S: %(linker) %{!fno-use-linker-plugin:%{!fno-lto: -plugin %(linker_plugin_file) -plugin-opt=%(lto_wrapper) -plugin-opt=-fresolution=%u.res %{flinker-output=*:-plugin-opt=-linker-output-known} %{!nostdlib:%{!nodefaultlibs:%:pass-through-libs(%(link_gcc_c_sequence))}} }}%{flto|flto=*:%<fcompare-debug*} %{flto} %{fno-lto} %{flto=*} %l %{static|shared|r:;!no-pie:-pie} %{fuse-ld=*:-fuse-ld=%*} %{gz|gz=zlib:--compress-debug-sections=zlib} %{gz=none:--compress-debug-sections=none} %{gz=zlib-gnu:--compress-debug-sections=zlib-gnu} %X %{o*} %{e*} %{N} %{n} %{r} %{s} %{t} %{u*} %{z} %{Z} %{!nostdlib:%{!r:%{!nostartfiles:%S}}} %{static|no-pie|static-pie:} %@{L*} %(mfwrap) %(link_libgcc) %{fvtable-verify=none:} %{fvtable-verify=std: %e-fvtable-verify=std is not supported in this configuration} %{fvtable-verify=preinit: %e-fvtable-verify=preinit is not supported in this configuration} %{!nostdlib:%{!r:%{!nodefaultlibs:%{%:sanitize(address):%{!shared:libasan_preinit%O%s} %{static-libasan:%{!shared:-Bstatic --whole-archive -lasan --no-whole-archive -Bdynamic}}%{!static-libasan:%{!fuse-ld=gold:--push-state} --no-as-needed -lasan %{fuse-ld=gold:--as-needed;:--pop-state}}} %{%:sanitize(thread):%{!shared:libtsan_preinit%O%s} %{static-libtsan:%{!shared:-Bstatic --whole-archive -ltsan --no-whole-archive -Bdynamic}}%{!static-libtsan:%{!fuse-ld=gold:--push-state} --no-as-needed -ltsan %{fuse-ld=gold:--as-needed;:--pop-state}}} %{%:sanitize(leak):%{!shared:liblsan_preinit%O%s} %{static-liblsan:%{!shared:-Bstatic --whole-archive -llsan --no-whole-archive -Bdynamic}}%{!static-liblsan:%{!fuse-ld=gold:--push-state} --no-as-needed -llsan %{fuse-ld=gold:--as-needed;:--pop-state}}}}}} %o %{fopenacc|fopenmp|%:gt(%{ftree-parallelize-loops=*:%*} 1): %:include(libgomp.spec)%(link_gomp)} %{fgnu-tm:%:include(libitm.spec)%(link_itm)} %(mflib) %{fsplit-stack: --wrap=pthread_create} %{fprofile-arcs|fprofile-generate*|coverage:-lgcov} %{!nostdlib:%{!r:%{!nodefaultlibs:%{%:sanitize(address): %{static-libasan|static:%:include(libsanitizer.spec)%(link_libasan)} %{static:%ecannot specify -static with -fsanitize=address}} %{%:sanitize(thread): %{static-libtsan|static:%:include(libsanitizer.spec)%(link_libtsan)} %{static:%ecannot specify -static with -fsanitize=thread}} %{%:sanitize(undefined):%{static-libubsan:-Bstatic} %{!static-libubsan:--push-state --no-as-needed} -lubsan %{static-libubsan:-Bdynamic} %{!static-libubsan:--pop-state} %{static-libubsan|static:%:include(libsanitizer.spec)%(link_libubsan)}} %{%:sanitize(leak): %{static-liblsan|static:%:include(libsanitizer.spec)%(link_liblsan)}}}}} %{!nostdlib:%{!r:%{!nodefaultlibs:%(link_ssp) %(link_gcc_c_sequence)}}} %{!nostdlib:%{!r:%{!nostartfiles:%E}}} %{T*}
%(post_link) }}}}}}
是的，行为发生了变化。
在 C 中，您应该只在一个翻译单元中定义一个全局变量，其他要访问该变量的翻译单元应将其声明为“extern”。
在您的代码中，a.h 包含在 a.c 和 main.c 中，因此该变量定义了两次。要解决此问题，您应该将 a.h 中的“int test”更改为“extern int test”，然后将“int test”添加到 a.c 以仅定义一次变量。
在 C 中，不初始化变量的全局变量的定义被认为是“暂定的”。您可以在同一个编译单元中对一个变量进行多个暂定定义。标准 C 不允许在不同的编译单元中进行多个临时定义，但 unix 系统上的 C 编译器历来允许。
旧版本的 gcc 默认允许在不同的编译单元中对一个全局变量进行多个临时定义（但不允许多个非临时定义）。 gcc-10 没有。您可以使用命令行选项“-fcommon”恢复旧行为，但不鼓励这样做。