libc: cleanup atexit and put exits() in its own compilation unit

this avoids having to pull in atexit() and its dependencies
(lock(), unlock()) into every program. (as exits() is called
by _main() from main9.s).

sys/man/2/exits

@@ -14,7 +14,6 @@ void	exits(char *msg)
 .PP
 .B
 int	atexit(void(*)(void))
-.PP
 .B
 void	atexitdont(void(*)(void))
 .fi
@@ -75,6 +74,8 @@ returns 0 if that limit has been reached.
 .I Atexitdont
 cancels a previous registration of an exit function.
 .SH SOURCE
+.B /sys/src/libc/port/exits.c
+.br
 .B /sys/src/libc/port/atexit.c
 .SH "SEE ALSO"
 .IR fork (2),

sys/src/libc/port/atexit.c

@@ -1,7 +1,7 @@
 #include <u.h>
 #include <libc.h>
 
-#define	NEXIT	33
+extern void (*_onexit)(void);
 
 typedef struct Onex Onex;
 struct Onex{
@@ -10,16 +10,31 @@ struct Onex{
 };
 
 static Lock onexlock;
-Onex onex[NEXIT];
+static Onex onex[33];
+
+static void
+onexit(void)
+{
+	int i, pid;
+	void (*f)(void);
+
+	pid = getpid();
+	for(i = nelem(onex)-1; i >= 0; i--)
+		if((f = onex[i].f) != nil && onex[i].pid == pid) {
+			onex[i].f = nil;
+			(*f)();
+		}
+}
 
 int
 atexit(void (*f)(void))
 {
 	int i;
 
+	_onexit = onexit;
 	lock(&onexlock);
-	for(i=0; i<NEXIT; i++)
-		if(onex[i].f == 0) {
+	for(i=0; i<nelem(onex); i++)
+		if(onex[i].f == nil) {
 			onex[i].pid = getpid();
 			onex[i].f = f;
 			unlock(&onexlock);
@@ -35,26 +50,7 @@ atexitdont(void (*f)(void))
 	int i, pid;
 
 	pid = getpid();
-	for(i=0; i<NEXIT; i++)
+	for(i=0; i<nelem(onex); i++)
 		if(onex[i].f == f && onex[i].pid == pid)
-			onex[i].f = 0;
+			onex[i].f = nil;
 }
-
-#pragma profile off
-
-void
-exits(char *s)
-{
-	int i, pid;
-	void (*f)(void);
-
-	pid = getpid();
-	for(i = NEXIT-1; i >= 0; i--)
-		if((f = onex[i].f) && pid == onex[i].pid) {
-			onex[i].f = 0;
-			(*f)();
-		}
-	_exits(s);
-}
-
-#pragma profile on

sys/src/libc/port/exits.c

@@ -0,0 +1,13 @@
+#include <u.h>
+#include <libc.h>
+
+void (*_onexit)(void);
+
+#pragma profile off
+
+void
+exits(char *s)
+{
+	if(_onexit != nil) (*_onexit)();
+	_exits(s);
+}

sys/src/libc/port/mkfile

@@ -22,6 +22,7 @@ CFILES=\
 	ctype.c\
 	encodefmt.c\
 	execl.c\
+	exits.c\
 	exp.c\
 	fabs.c\
 	floor.c\