I've had a bit of a look around Stackoverflow and the wider Internet and identified that the most common causes for this error are conflation of declaration (int var = 1;) and definition (int var;), and including .c files from .h files.
My small project I just split from one file into several is not doing any of these things. I'm very confused.
I made a copy of the project and deleted all the code in the copy (which was fun) until I reached here:
main.c
#include "a.h"
int main() {
}
a.h
#ifndef A_H
#define A_H
int test;
#endif
a.c
#include "a.h"
Pretty much as canonical as it gets, right? And yet...
$ 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
WAT :(
It was around this time I started questioning my configuration and tried another machine, accessing the exact same files over NFS.
$ rm *.o; gcc -g -c a.c; gcc -g -c main.c; gcc -o main main.o a.o
$ ./main
$ echo $?
0
Oooh no. What did I break on the first machine...
Hopefully my issue is rather that Something Quietly Changed between GCC 8.3.0 (working) and GCC 10.2.1 (failing)? Perhaps the newer version of GCC is defaulting to a newer C standard or something, which specifies different behavior in this area?
Alternatively perhaps it's configuration-related (this is all stock Debian). Here's the output of gcc -dumpspecs for both versions:
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: