further use of _Noreturn, for non-plain-C functions

note that POSIX does not specify these functions as _Noreturn, because
POSIX is aligned with C99, not the new C11 standard. when POSIX is
eventually updated to C11, it will almost surely give these functions
the _Noreturn attribute. for now, the actual _Noreturn keyword is not
used anyway when compiling with a c99 compiler, which is what POSIX
requires; the GCC __attribute__ is used instead if it's available,
however.

in a few places, I've added infinite for loops at the end of _Noreturn
functions to silence compiler warnings. presumably
__buildin_unreachable could achieve the same thing, but it would only
work on newer GCCs and would not be portable. the loops should have
near-zero code size cost anyway.

like the previous _Noreturn commit, this one is based on patches
contributed by philomath.

src/exit/_Exit.c

@@ -4,5 +4,5 @@
 _Noreturn void _Exit(int ec)
 {
 	__syscall(SYS_exit_group, ec);
-	__syscall(SYS_exit, ec);
+	for (;;) __syscall(SYS_exit, ec);
 }

src/exit/assert.c

@@ -1,7 +1,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 
-void __assert_fail(const char *expr, const char *file, int line, const char *func)
+_Noreturn void __assert_fail(const char *expr, const char *file, int line, const char *func)
 {
 	fprintf(stderr, "Assertion failed: %s (%s: %s: %d)\n", expr, file, func, line);
 	fflush(NULL);

src/linux/err.c

@@ -15,13 +15,13 @@ void vwarnx(const char *fmt, va_list ap)
 	putc('\n', stderr);
 }
 
-void verr(int status, const char *fmt, va_list ap)
+_Noreturn void verr(int status, const char *fmt, va_list ap)
 {
 	vwarn(fmt, ap);
 	exit(status);
 }
 
-void verrx(int status, const char *fmt, va_list ap)
+_Noreturn void verrx(int status, const char *fmt, va_list ap)
 {
 	vwarnx(fmt, ap);
 	exit(status);
@@ -43,7 +43,7 @@ void warnx(const char *fmt, ...)
 	va_end(ap);
 }
 
-void err(int status, const char *fmt, ...)
+_Noreturn void err(int status, const char *fmt, ...)
 {
 	va_list ap;
 	va_start(ap, fmt);
@@ -51,7 +51,7 @@ void err(int status, const char *fmt, ...)
 	va_end(ap);
 }
 
-void errx(int status, const char *fmt, ...)
+_Noreturn void errx(int status, const char *fmt, ...)
 {
 	va_list ap;
 	va_start(ap, fmt);

src/signal/siglongjmp.c

@@ -3,7 +3,7 @@
 #include <stdlib.h>
 #include "syscall.h"
 
-void siglongjmp(sigjmp_buf buf, int ret)
+_Noreturn void siglongjmp(sigjmp_buf buf, int ret)
 {
 	if (buf->__fl) __syscall(SYS_rt_sigprocmask, SIG_SETMASK,
 		buf->__ss, 0, __SYSCALL_SSLEN);

src/thread/pthread_create.c

@@ -8,7 +8,7 @@ weak_alias(dummy_0, __synccall_lock);
 weak_alias(dummy_0, __synccall_unlock);
 weak_alias(dummy_0, __pthread_tsd_run_dtors);
 
-void pthread_exit(void *result)
+_Noreturn void pthread_exit(void *result)
 {
 	pthread_t self = pthread_self();
 	int n;
@@ -43,7 +43,7 @@ void pthread_exit(void *result)
 		__unmapself(self->map_base, self->map_size);
 	}
 
-	__syscall(SYS_exit, 0);
+	for (;;) __syscall(SYS_exit, 0);
 }
 
 void __do_cleanup_push(struct __ptcb *cb)

src/unistd/_exit.c

@@ -1,7 +1,7 @@
 #include <unistd.h>
 #include <stdlib.h>
 
-void _exit(int status)
+_Noreturn void _exit(int status)
 {
 	_Exit(status);
 }