/usr/include/x86_64-linux-musl/sys/poll.h is in musl-dev 0.9.15-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | #warning redirecting incorrect #include <sys/poll.h> to <poll.h>
#include <poll.h>
|