[Libguestfs] [PATCH nbdkit] build: Use dlsym as sentinel function for -ldl.

[Eric Blake]

On 4/24/19 3:42 PM, Richard W.M. Jones wrote:
> When testing which “dl library” we must use for dl* symbols, autoconf
> runs a test similar to:
> 
>   $ cat conftest.c
>   char dlopen ();
>   int main () { return dlopen (); }
>   $ gcc -o conftest $CFLAGS conftest.c [try various -ldl options here]
> 
> When using ‘CFLAGS="-fsanitize=address"’ this succeeds even if no dl
> libraries are used at all, since it appears that using this option
> causes dlopen to be included in the final binary implicitly.  This
> causes configure to set DL_LIBS='', but unfortunately dlsym is not
> included implicitly and so linking fails.
> 
> Although I believe this is a bug in GCC 9, as a workaround use dlsym
> as the sentinel function instead.

Not just gcc 9; I reproduced it on gcc 8 in Fedora 29. But I agree that
it is odd that the use of -fsanitize=address is sufficient to pull in a
subset of symbols that you normally only get from the explicit library
link line.  At any rate,

> 
> The new output from ‘./configure CFLAGS="-fsanitize=address"’ is:
> 
>   checking for library containing dlsym... -ldl
>   [...]
>   checking for dladdr... yes
> 
> See also: https://bugzilla.redhat.com/show_bug.cgi?id=1702761
> ---
>  configure.ac | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)

this change looks correct. ACK.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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