llvm dev - Apr 2019 - [EXT] 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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“--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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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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