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vncs: fix cmdpid fork bug

we cannot do:

cmdpid = rfork(... RFMEM);

because cmdpid is a global variable in the data segment
and hence shared between parent and child process. use a
temporary variable on the stack.

--- a/sys/src/cmd/vnc/vncs.c

+++ b/sys/src/cmd/vnc/vncs.c

@@ -87,7 +87,7 @@

 void

 main(int argc, char **argv)

 {

-	int altnet, baseport, cfd, display, exnum, fd, h, killing, w;

+	int altnet, baseport, cfd, display, exnum, fd, pid, h, killing, w;

 	char adir[NETPATHLEN], ldir[NETPATHLEN];

 	char net[NETPATHLEN], *p;

 	char *kbdfs[] = { "/bin/aux/kbdfs", "-dq", nil };

@@ -197,8 +197,8 @@

 		sysfatal("mounter: %r");

 	close(fd);

-	cmdpid = rfork(RFPROC|RFMEM|RFFDG|RFNOTEG);

-	switch(cmdpid){

+	pid = rfork(RFPROC|RFMEM|RFFDG|RFNOTEG);

+	switch(pid){

 	case -1:

 		sysfatal("rfork: %r");

 		break;

@@ -211,8 +211,8 @@

 		open("/dev/cons", OWRITE);

 		/* start and mount kbdfs */

-		cmdpid = rfork(RFPROC|RFMEM|RFFDG|RFREND);

-		switch(cmdpid){

+		pid = rfork(RFPROC|RFMEM|RFFDG|RFREND);

+		switch(pid){

 		case -1:

 			sysfatal("rfork: %r");

 			break;

@@ -221,9 +221,9 @@

 			fprint(2, "exec %s: %r\n", kbdfs[0]);

 			_exits("kbdfs");

 		}

-		if(waitpid() != cmdpid){

+		if(waitpid() != pid){

 			rendezvous(&kbdin, nil);

-			sysfatal("%s: %r", kbdfs[0]);

+			sysfatal("waitpid: %s: %r", kbdfs[0]);

 		}

 		rendezvous(&kbdin, nil);

@@ -240,6 +240,7 @@

 		fprint(2, "exec %s: %r\n", argv[0]);

 		_exits(nil);

 	}

+	cmdpid = pid;

 	/* wait for kbdfs to get mounted */

 	rendezvous(&kbdin, nil);