llvm dev - Jun 2019 - ld.lld fails with "undefined symbol: _fopen" on macOS, while working as expected on Linux

Hi,

I'm trying to cross-compile my app.

#include <stdio.h>
int main() {
  puts("test");
  fopen("test", "r"); // removing fopen() call removes the
error
  return 0;
}

clang -c -target x86_64-linux-gnu test.c

ld.lld -v -o test -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 test.o
/usr/lib/x86_64-linux-gnu/libc.so /usr/lib/x86_64-linux-gnu/crtn.o

This works fine on Linux, but when I run the same command on macOS, I get

LLD 8.0.0 (compatible with GNU linkers)
ld.lld: error: undefined symbol: _fopen
>>> referenced by fopen.c
>>>               test.o:(main)
I haven't set up --sysroot yet, I just copied all these files from the
Linux machine for now.

This only happens with fopen(). Using puts() or printf() is fine.

LLVM version is 8.0.0.

Thanks!
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Hi Ivan,

On Fri, 7 Jun 2019 at 11:19, Ivan Medoedov via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> LLD 8.0.0 (compatible with GNU linkers)
> ld.lld: error: undefined symbol: _fopen
> >>> referenced by fopen.c
> >>>               test.o:(main)
I suspect you're getting the macOS version of stdio.h, which makes
fopen some kind of alias. You really should use a sysroot when
cross-compiling, though you *might* get away with just
-I/path/to/linux/usr/include. Just what you've copied across might
well be sufficient to use with --sysroot.

Cheers.

Tim.

Thanks, Tim.

You are right, it's using macOS' stdio.h, which is a terrible idea.

Clang docs on --sysroot:

When you have extracted your cross-compiler from a zip file into a
directory, you have to use --sysroot=<path>. The path is the root
directory
where you have unpacked your file, and Clang will look for the directories
bin, lib, include in there.

I have a silly question: where can I download the mentioned zip file with
all bin, lib, and include files for x64 Linux target?

So far I've been copying files one by one (/lib64/ld-linux-x86-64.so.2
/usr/lib/x86_64-linux-gnu/crt1.o /usr/lib/x86_64-linux-gnu/crti.o, etc) and
it's been a painful process.

On Fri, Jun 7, 2019 at 9:34 PM Tim Northover <t.p.northover at gmail.com>
wrote:
> Hi Ivan,
>
> On Fri, 7 Jun 2019 at 11:19, Ivan Medoedov via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> > LLD 8.0.0 (compatible with GNU linkers)
> > ld.lld: error: undefined symbol: _fopen
> > >>> referenced by fopen.c
> > >>>               test.o:(main)
>
> I suspect you're getting the macOS version of stdio.h, which makes
> fopen some kind of alias. You really should use a sysroot when
> cross-compiling, though you *might* get away with just
> -I/path/to/linux/usr/include. Just what you've copied across might
> well be sufficient to use with --sysroot.
>
> Cheers.
>
> Tim.
>
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