[llvm-dev] Cross compiling an empty program results in a segfault

[Eric Christopher via llvm-dev]

You'll want to use a sysroot to accomplish this as Eli mentioned above :)

-eric

On Fri, May 3, 2019 at 8:25 PM Ivan Medoedov via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> I managed to do it with
>
> ld.lld -v -o hi -m elf_x86_64 -dynamic-linker \
>  /lib64/ld-linux-x86-64.so.2 \
>  /usr/lib/x86_64-linux-gnu/crt1.o \
>  /usr/lib/x86_64-linux-gnu/crti.o \
>  hi_lin.o \
>  /usr/lib/x86_64-linux-gnu/libc.so \
>  /usr/lib/x86_64-linux-gnu/crtn.o
>
> I had to create  /usr/lib/x86_64-linux-gnu/ directory on my Mac and copy about 8 libc files. This felt like a hack, but it worked.
>
>
> On Sat, May 4, 2019 at 1:51 AM Ivan Medoedov <ivan.medoedov at gmail.com> wrote:
>>
>> The reason I was trying to link it myself is because clang failed:
>>
>> ld: unknown option: --eh-frame-hdr
>>
>> My host is macOS, trying to compile for Linux:
>>
>> clang -target x86_64-pc-linux-gnu -o main_linux main.c
>>
>> main.c is an empty program with 0 dependencies.
>>
>> On Sat, May 4, 2019 at 1:43 AM Ivan Medoedov <ivan.medoedov at gmail.com> wrote:
>>>
>>> Sorry for the late reply.
>>>
>>> Thanks, Eli. That was really helpful.
>>>
>>> On Fri, Apr 12, 2019 at 9:31 PM Eli Friedman <efriedma at quicinc.com> wrote:
>>>>
>>>> “--entry main” is not going to do what you want; the entry point for a Linux program is not equivalent to the C “main”.  http://dbp-consulting.com/tutorials/debugging/linuxProgramStartup.html has a description of how a C program normally starts on Linux.
>>>>
>>>>
>>>>
>>>> Generally, to build a C program, you don’t want to invoke the linker directly; instead, use clang to invoke the linker correctly.  This will pass the appropriate flags and link against the appropriate libraries.  When you’re cross compiling, to make this work, you have to actually have the appropriate libraries, so you’ll have to build or download them separately.  Then you tell clang the path to those libraries using –sysroot.  https://clang.llvm.org/docs/CrossCompilation.html has a general description of various issues with cross-compiling.
>>>>
>>>>
>>>>
>>>> -Eli
>>>>
>>>>
>>>>
>>>> From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Ivan Medoedov via llvm-dev
>>>> Sent: Wednesday, April 10, 2019 1:28 PM
>>>> To: llvm-dev at lists.llvm.org
>>>> Subject: [EXT] [llvm-dev] Cross compiling an empty program results in a segfault
>>>>
>>>>
>>>>
>>>> Hello,
>>>>
>>>>
>>>>
>>>> I'm trying to cross compile an empty program on macOS for Linux:
>>>>
>>>>
>>>>
>>>> cat test.c
>>>>
>>>>
>>>>
>>>> int main() {
>>>>
>>>>         int a = 2+3;
>>>>
>>>>         return 0;
>>>>
>>>> }
>>>>
>>>>
>>>>
>>>> clang -o -c -target x86_64-linux-gnu test.c
>>>>
>>>>
>>>>
>>>> If I copy the resulting test.o to the Linux machine and run clang test.o, it works fine.
>>>>
>>>>
>>>>
>>>> However if I try to link it on macOS, I get a segfault when I launch ./a.out
>>>>
>>>>
>>>>
>>>> ld.lld --entry main test.o
>>>>
>>>>
>>>>
>>>> [copy a.out to Linux]
>>>>
>>>>
>>>>
>>>> ./a.out
>>>>
>>>> segmentation fault 0x00000000001 in ?? ()
>>>>
>>>>
>>>>
>>>> Would really appreciate your help here!
>>>>
>>>>
>>>>
>>>> Thanks
>
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