[PATCH 1/2] Add the __libc_single_threaded variable

[Szabolcs Nagy]

The 05/20/2020 20:12, Florian Weimer via Libc-alpha wrote:
> The variable is placed in libc.so, and it can be true only in
> an outer libc, not libcs loaded via dlmopen or static dlopen.
> Since thread creation from inner namespaces does not work,
> pthread_create can update __libc_single_threaded directly.
> 
> Using __libc_early_init and its initial flag, implementation of this
> variable is very straightforward.  A future version may reset the flag
> during fork (but not in an inner namespace), or after joining all
> threads except one.
> ---
...
> +* The GNU C Library now provides the header file <sys/single_threaded.h>
> +  which declares the variable __libc_single_threaded.  Applications are
> +  encouraged to use this variable for single-thread optimizations,
> +  instead of weak references to symbols historically defined in
> +  libpthread.

i wonder if the new header can be included into
threads.h and pthread.h, and a feature macro added
for it, so users can avoid doing a header check.

if the name is already in implementation reserved
namespace this should be possible, i'm not sure
if such indirect include is best practice though.