posix: signal: check RTSIG limit

Use build assert to make sure that the realtime signal
constants are configured properly in the Kconfig.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>

lib/posix/Kconfig

@@ -44,6 +44,7 @@ source "lib/posix/Kconfig.fnmatch"
 source "lib/posix/Kconfig.fs"
 source "lib/posix/Kconfig.getopt"
 source "lib/posix/Kconfig.key"
+source "lib/posix/Kconfig.limits"
 source "lib/posix/Kconfig.mqueue"
 source "lib/posix/Kconfig.mutex"
 source "lib/posix/Kconfig.pthread"

lib/posix/Kconfig.limits

@@ -0,0 +1,13 @@
+# Copyright (c) 2023 Meta
+#
+# SPDX-License-Identifier: Apache-2.0
+
+if POSIX_SIGNAL
+config POSIX_LIMITS_RTSIG_MAX
+	int "_POSIX_RTSIG_MAX value in limits.h"
+	default 8
+	help
+	  Define the _POSIX_RTSIG_MAX value in limits.h.
+	  IEEE 1003.1 defines this to be 8.
+
+endif

lib/posix/signal.c

@@ -13,6 +13,9 @@
 #define SIGNO_WORD_IDX(_signo) (signo / BITS_PER_LONG)
 #define SIGNO_WORD_BIT(_signo) (signo & BIT_MASK(LOG2(BITS_PER_LONG)))
 
+BUILD_ASSERT(CONFIG_POSIX_LIMITS_RTSIG_MAX >= 0);
+BUILD_ASSERT(CONFIG_POSIX_RTSIG_MAX >= CONFIG_POSIX_LIMITS_RTSIG_MAX);
+
 static inline bool signo_valid(int signo)
 {
 	return ((signo > 0) && (signo < _NSIG));