posix: add stubs for signal.h functions that need process support

Since Zephyr itself does not currently support processes, but
conformant applications should still be able to link, add stubs
for the remaining POSIX functions in the POSIX_SIGNALS Option
Group.

The POSIX_SIGNALS Option Group is required for PSE51, PSE52,
PSE53, PSE54, and likely many other POSIX Subprofiles.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>

include/zephyr/posix/signal.h

@@ -77,6 +77,16 @@ typedef struct {
 #define SIG_UNBLOCK 2
 #endif
 
+#define SIG_DFL ((void *)0)
+#define SIG_IGN ((void *)1)
+#define SIG_ERR ((void *)-1)
+
+#define SI_USER 1
+#define SI_QUEUE 2
+#define SI_TIMER 3
+#define SI_ASYNCIO 4
+#define SI_MESGQ 5
+
 typedef int	sig_atomic_t;		/* Atomic entity type (ANSI) */
 
 union sigval {
@@ -92,12 +102,34 @@ struct sigevent {
 	int sigev_signo;
 };
 
+typedef struct {
+	int si_signo;
+	int si_code;
+	union sigval si_value;
+} siginfo_t;
+
+struct sigaction {
+	void (*sa_handler)(int signno);
+	sigset_t sa_mask;
+	int sa_flags;
+	void (*sa_sigaction)(int signo, siginfo_t *info, void *context);
+};
+
+unsigned int alarm(unsigned int seconds);
+int kill(pid_t pid, int sig);
+int pause(void);
+int raise(int signo);
+int sigaction(int sig, const struct sigaction *ZRESTRICT act, struct sigaction *ZRESTRICT oact);
+int sigpending(sigset_t *set);
+int sigsuspend(const sigset_t *sigmask);
+int sigwait(const sigset_t *ZRESTRICT set, int *ZRESTRICT signo);
 char *strsignal(int signum);
 int sigemptyset(sigset_t *set);
 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);
+void (*signal(int signo, void (*)(int signo)))(int signo);
 int sigprocmask(int how, const sigset_t *ZRESTRICT set, sigset_t *ZRESTRICT oset);
 
 int pthread_sigmask(int how, const sigset_t *ZRESTRICT set, sigset_t *ZRESTRICT oset);

lib/libc/Kconfig

@@ -105,6 +105,7 @@ config NEWLIB_LIBC
 	imply POSIX_FD_MGMT_ALIAS_LSEEK
 	imply POSIX_FILE_SYSTEM_ALIAS_FSTAT
 	imply POSIX_MULTI_PROCESS_ALIAS_GETPID
+	imply POSIX_SIGNALS_ALIAS_KILL
 	help
 	  Build with newlib library. The newlib library is expected to be
 	  part of the SDK in this case.

lib/posix/options/Kconfig.signal

@@ -24,6 +24,7 @@ endif # POSIX_REALTIME_SIGNALS
 config POSIX_SIGNALS
 	bool "POSIX signals [EXPERIMENTAL]"
 	select EXPERIMENTAL
+	select POSIX_MULTI_PROCESS
 	help
 	  Enable support for POSIX signals.
 
@@ -36,6 +37,12 @@ config POSIX_SIGNAL_STRING_DESC
 	  Use full description for the strsignal API.
 	  Will use 256 bytes of ROM.
 
+# These options are intended to be used for compatibility with external POSIX
+# implementations such as those in Newlib or Picolibc.
+
+config POSIX_SIGNALS_ALIAS_KILL
+	bool
+
 endif
 
 endmenu # "Signal support"

lib/posix/options/signal.c

@@ -118,3 +118,62 @@ int sigprocmask(int how, const sigset_t *ZRESTRICT set, sigset_t *ZRESTRICT oset
 	errno = ENOSYS;
 	return -1;
 }
+
+/*
+ * The functions below are provided so that conformant POSIX applications and libraries can still
+ * link.
+ */
+
+unsigned int alarm(unsigned int seconds)
+{
+	ARG_UNUSED(seconds);
+	return 0;
+}
+
+int kill(pid_t pid, int sig)
+{
+	ARG_UNUSED(pid);
+	ARG_UNUSED(sig);
+	errno = ENOSYS;
+	return -1;
+}
+#ifdef CONFIG_POSIX_SIGNALS_ALIAS_KILL
+FUNC_ALIAS(kill, _kill, int);
+#endif /* CONFIG_POSIX_SIGNALS_ALIAS_KILL */
+
+int pause(void)
+{
+	errno = ENOSYS;
+	return -1;
+}
+
+int sigaction(int sig, const struct sigaction *ZRESTRICT act, struct sigaction *ZRESTRICT oact)
+{
+	ARG_UNUSED(sig);
+	ARG_UNUSED(act);
+	ARG_UNUSED(oact);
+	errno = ENOSYS;
+	return -1;
+}
+
+int sigpending(sigset_t *set)
+{
+	ARG_UNUSED(set);
+	errno = ENOSYS;
+	return -1;
+}
+
+int sigsuspend(const sigset_t *sigmask)
+{
+	ARG_UNUSED(sigmask);
+	errno = ENOSYS;
+	return -1;
+}
+
+int sigwait(const sigset_t *ZRESTRICT set, int *ZRESTRICT sig)
+{
+	ARG_UNUSED(set);
+	ARG_UNUSED(sig);
+	errno = ENOSYS;
+	return -1;
+}