posix: signal: include_next to get the libc signal() decl

Although the ANSI C version of signal() is not yet supported in
Zephyr, because Zephyr does not use process-wide signals, some
users rely on its declaration.

Include the libc version of signal.h when POSIX signals are
not configured.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>

include/zephyr/posix/signal.h

@@ -6,13 +6,14 @@
 #ifndef ZEPHYR_INCLUDE_POSIX_SIGNAL_H_
 #define ZEPHYR_INCLUDE_POSIX_SIGNAL_H_
 
+#ifdef CONFIG_POSIX_SIGNAL
+
 #include "posix_types.h"
 
-#ifdef __cplusplus
-extern "C" {
-#endif
+#define SIG_DFL ((sighandler_t)0)
+#define SIG_ERR ((sighandler_t)1)
+#define SIG_IGN ((sighandler_t)-1)
 
-#ifdef CONFIG_POSIX_SIGNAL
 #define SIGHUP    1  /**< Hangup */
 #define SIGINT    2  /**< Interrupt */
 #define SIGQUIT   3  /**< Quit */
@@ -49,7 +50,9 @@ extern "C" {
 #define SIGRTMAX (SIGRTMIN + CONFIG_POSIX_RTSIG_MAX)
 #define _NSIG (SIGRTMAX + 1)
 
-BUILD_ASSERT(CONFIG_POSIX_RTSIG_MAX >= 0);
+#ifdef __cplusplus
+extern "C" {
+#endif
 
 typedef struct {
 	unsigned long sig[DIV_ROUND_UP(_NSIG, BITS_PER_LONG)];
@@ -61,7 +64,6 @@ int sigfillset(sigset_t *set);
 int sigaddset(sigset_t *set, int signo);
 int sigdelset(sigset_t *set, int signo);
 int sigismember(const sigset_t *set, int signo);
-#endif /* CONFIG_POSIX_SIGNAL */
 
 #ifndef SIGEV_NONE
 #define SIGEV_NONE 1
@@ -90,8 +92,18 @@ struct sigevent {
 	pthread_attr_t *sigev_notify_attributes;
 };
 
+/* Note: signal() is intentionally unimplemented in Zephyr */
+typedef void (*sighandler_t)(int signo);
+sighandler_t signal(int signum, sighandler_t handler);
+
 #ifdef __cplusplus
 }
 #endif
 
+#else
+
+#include_next <signal.h>
+
+#endif /* CONFIG_POSIX_SIGNAL */
+
 #endif /* ZEPHYR_INCLUDE_POSIX_SIGNAL_H_ */

lib/posix/signal.c

@@ -10,6 +10,8 @@
 
 #include <zephyr/posix/signal.h>
 
+BUILD_ASSERT(CONFIG_POSIX_RTSIG_MAX >= 0);
+
 #define SIGNO_WORD_IDX(_signo) (signo / BITS_PER_LONG)
 #define SIGNO_WORD_BIT(_signo) (signo & BIT_MASK(LOG2(BITS_PER_LONG)))