musl/src/internal/ksigaction.h

/* This is the structure used for the rt_sigaction syscall on most archs,
 * but it can be overridden by a file with the same name in the top-level
 * arch dir for a given arch, if necessary. */
struct k_sigaction {
	void (*handler)(int);
	unsigned long flags;
	void (*restorer)(void);
	unsigned mask[2];
};