[RFC PATCH v1 2/8] nolibc/sys: Implement `signal(2)` function

[Ammar Faizi <ammarfaizi2@gnuweeb.org>]

From: Ammar Faizi <ammarfaizi2@gnuweeb.org>

signal() function is the simpler version of sigaction(). Unlike
sigaction(), which fully controls the `struct sigaction`, the caller
only cares about the sa_handler when calling the signal() function.

signal() internally calls sigaction(). This implementation is
currently only available on the x86-64 arch.

Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
---
 tools/include/nolibc/sys.h | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/tools/include/nolibc/sys.h b/tools/include/nolibc/sys.h
index 91532a2fbe2c..ca348939eb50 100644
--- a/tools/include/nolibc/sys.h
+++ b/tools/include/nolibc/sys.h
@@ -1106,6 +1106,36 @@ int sigaction(int signum, const struct sigaction *act, struct sigaction *oldact)
 #endif
 }
 
+/*
+ * sighandler_t signal(int signum, sighandler_t handler);
+ */
+
+static __attribute__((unused))
+sighandler_t signal(int signum, sighandler_t handler)
+{
+#ifdef __HAVE_ARCH_RESTORE_RT
+	const struct sigaction act = {
+		.sa_handler = handler,
+		.sa_flags = SA_RESTORER,
+		.sa_restorer = __arch_restore_rt
+	};
+	sighandler_t old_sah;
+	struct sigaction old;
+	int ret;
+
+	ret = sys_sigaction(signum, &act, &old);
+	if (ret < 0) {
+		SET_ERRNO(-ret);
+		old_sah = SIG_ERR;
+	} else {
+		old_sah = old.sa_handler;
+	}
+	return old_sah;
+#else
+	SET_ERRNO(ENOSYS);
+	return SIG_ERR;
+#endif
+}
 
 /*
  * int stat(const char *path, struct stat *buf);
-- 
Ammar Faizi