all: remove need for `-fcommon`

Use `extern` where appropriate so that we can remove `-fcommon`
on macOS and others.  On macOS, `-fcommon` generates a number of
linker warnings.

I've tested building on macOS, Linux, FreeBSD, NetBSD, OpenBSD
and illumos.  I am not in a position to test on AIX or other
more esoteric platforms, however.

39 files changed, 339 insertions, 193 deletions

diff --git a/bin/9c b/bin/9c
index 195fec81..656ba84d 100755
--- a/bin/9c
+++ b/bin/9c
@@ -23,7 +23,7 @@ usegcc()
 		-Wno-deprecated-declarations \
 		-fno-omit-frame-pointer \
 		-fsigned-char \
-		-fcommon \
+		-fno-common \
 		-std=gnu11 \
 	"
 	# want to put -fno-optimize-sibling-calls here but
@@ -85,7 +85,7 @@ useclang()
 		-Wno-unused-but-set-variable \
 		-fsigned-char \
 		-fno-caret-diagnostics \
-		-fcommon \
+		-fno-common \
 	"
 	cflags="$cflags -g"
 	cflags="$cflags $CC9FLAGS"
diff --git a/src/cmd/9660/dump9660.c b/src/cmd/9660/dump9660.c
index 59ee31c0..228d56c6 100644
--- a/src/cmd/9660/dump9660.c
+++ b/src/cmd/9660/dump9660.c
@@ -15,8 +15,6 @@ Conform *map;
 static void addprotofile(char *new, char *old, Dir *d, void *a);
 void usage(void);
 
-char *argv0;
-
 void
 usage(void)
 {
diff --git a/src/cmd/9pfuse/main.c b/src/cmd/9pfuse/main.c
index 4fa330a0..ca9f3872 100644
--- a/src/cmd/9pfuse/main.c
+++ b/src/cmd/9pfuse/main.c
@@ -60,7 +60,6 @@
 #endif
 
 int debug;
-char *argv0;
 char *aname = "";
 void fusedispatch(void*);
 Channel *fusechan;
diff --git a/src/cmd/9term/dat.h b/src/cmd/9term/dat.h
index a215253f..1e22a424 100644
--- a/src/cmd/9term/dat.h
+++ b/src/cmd/9term/dat.h
@@ -209,19 +209,19 @@ struct Timer
 #define Extern extern
 #endif
 
-Extern	Font		*font;
+extern	Font		*font;
 Extern	Mousectl	*mousectl;
 Extern	Mouse	*mouse;
 Extern	Keyboardctl	*keyboardctl;
-Extern	Display	*display;
+extern	Display	*display;
 Extern	Image	*view;
 Extern	Screen	*wscreen;
-Extern	Cursor	boxcursor;
-Extern	Cursor	crosscursor;
-Extern	Cursor	sightcursor;
-Extern	Cursor	whitearrow;
-Extern	Cursor	query;
-Extern	Cursor	*corners[9];
+extern	Cursor	boxcursor;
+extern	Cursor	crosscursor;
+extern	Cursor	sightcursor;
+extern	Cursor	whitearrow;
+extern	Cursor	query;
+extern	Cursor	*corners[9];
 Extern	Image	*background;
 Extern	Image	*lightgrey;
 Extern	Image	*red;
diff --git a/src/cmd/acid/acid.h b/src/cmd/acid/acid.h
index 5b1339fe..da7e314d 100644
--- a/src/cmd/acid/acid.h
+++ b/src/cmd/acid/acid.h
@@ -47,14 +47,14 @@ Extern Node*	code;
 Extern int	na;
 Extern int	wtflag;
 Extern Regs*	acidregs;
-Extern Regs*	correg;
-Extern Map*	cormap;
-Extern Map*	symmap;
+extern Regs*	correg;
+extern Map*	cormap;
+extern Map*	symmap;
 Extern Lsym*	hash[Hashsize];
 Extern long	dogc;
 Extern Rplace*	ret;
-Extern char*	symfil;
-Extern char*	corfil;
+extern char*	symfil;
+extern char*	corfil;
 Extern int	gotint;
 Extern long	flen;
 Extern Gc*	gcl;
@@ -66,8 +66,8 @@ Extern Node*	fomt;
 Extern List*	tracelist;
 Extern int	initialising;
 Extern int	quiet;
-Extern Fhdr*	corhdr;
-Extern Fhdr*	symhdr;
+extern Fhdr*	corhdr;
+extern Fhdr*	symhdr;
 
 extern void	(*expop[])(Node*, Node*);
 #define expr(n, r) (r)->store.comt=0; (*expop[(unsigned char)((n)->op)])(n, r);
diff --git a/src/cmd/acme/dat.c b/src/cmd/acme/dat.c
new file mode 100644
index 00000000..537f7d75
--- /dev/null
+++ b/src/cmd/acme/dat.c
@@ -0,0 +1,62 @@
+#include <u.h>
+#include <libc.h>
+#include <draw.h>
+#include <thread.h>
+#include <cursor.h>
+#include <mouse.h>
+#include <keyboard.h>
+#include <frame.h>
+#include <fcall.h>
+#include <plumb.h>
+#include <libsec.h>
+#include "dat.h"
+
+uint		globalincref;
+uint		seq;
+uint		maxtab;	/* size of a tab, in units of the '0' character */
+
+Mouse		*mouse;
+Mousectl		*mousectl;
+Keyboardctl	*keyboardctl;
+Reffont		reffont;
+Image		*modbutton;
+Image		*colbutton;
+Image		*button;
+Image		*but2col;
+Image		*but3col;
+Row			row;
+int			timerpid;
+Disk			*disk;
+Text			*seltext;
+Text			*argtext;
+Text			*mousetext;	/* global because Text.close needs to clear it */
+Text			*typetext;		/* global because Text.close needs to clear it */
+Text			*barttext;		/* shared between mousetask and keyboardthread */
+int			bartflag;
+Window		*activewin;
+Column		*activecol;
+Rectangle		nullrect;
+int			fsyspid;
+char			*cputype;
+char			*objtype;
+char			*acmeshell;
+//char			*fontnames[2];
+extern char		wdir[]; /* must use extern because no dimension given */
+int			globalautoindent;
+int			dodollarsigns;
+
+Channel	*cplumb;		/* chan(Plumbmsg*) */
+Channel	*cwait;		/* chan(Waitmsg) */
+Channel	*ccommand;	/* chan(Command*) */
+Channel	*ckill;		/* chan(Rune*) */
+Channel	*cxfidalloc;	/* chan(Xfid*) */
+Channel	*cxfidfree;	/* chan(Xfid*) */
+Channel	*cnewwindow;	/* chan(Channel*) */
+Channel	*mouseexit0;	/* chan(int) */
+Channel	*mouseexit1;	/* chan(int) */
+Channel	*cexit;		/* chan(int) */
+Channel	*cerr;		/* chan(char*) */
+Channel	*cedit;		/* chan(int) */
+Channel	*cwarn;		/* chan(void*)[1] (really chan(unit)[1]) */
+
+QLock	editoutlk;
diff --git a/src/cmd/acme/dat.h b/src/cmd/acme/dat.h
index 03150aa6..8f11d6a3 100644
--- a/src/cmd/acme/dat.h
+++ b/src/cmd/acme/dat.h
@@ -509,53 +509,53 @@ enum	/* editing */
 	Collecting
 };
 
-uint		globalincref;
-uint		seq;
-uint		maxtab;	/* size of a tab, in units of the '0' character */
-
-Display		*display;
-Image		*screen;
-Font			*font;
-Mouse		*mouse;
-Mousectl		*mousectl;
-Keyboardctl	*keyboardctl;
-Reffont		reffont;
-Image		*modbutton;
-Image		*colbutton;
-Image		*button;
-Image		*but2col;
-Image		*but3col;
-Cursor		boxcursor;
-Cursor2		boxcursor2;
-Row			row;
-int			timerpid;
-Disk			*disk;
-Text			*seltext;
-Text			*argtext;
-Text			*mousetext;	/* global because Text.close needs to clear it */
-Text			*typetext;		/* global because Text.close needs to clear it */
-Text			*barttext;		/* shared between mousetask and keyboardthread */
-int			bartflag;
-int			swapscrollbuttons;
-Window		*activewin;
-Column		*activecol;
-Buffer		snarfbuf;
-Rectangle		nullrect;
-int			fsyspid;
-char			*cputype;
-char			*objtype;
-char			*home;
-char			*acmeshell;
-char			*fontnames[2];
-Image		*tagcols[NCOL];
-Image		*textcols[NCOL];
+extern uint		globalincref;
+extern uint		seq;
+extern uint		maxtab;	/* size of a tab, in units of the '0' character */
+
+extern Display		*display;
+extern Image		*screen;
+extern Font			*font;
+extern Mouse		*mouse;
+extern Mousectl		*mousectl;
+extern Keyboardctl	*keyboardctl;
+extern Reffont		reffont;
+extern Image		*modbutton;
+extern Image		*colbutton;
+extern Image		*button;
+extern Image		*but2col;
+extern Image		*but3col;
+extern Cursor		boxcursor;
+extern Cursor2		boxcursor2;
+extern Row			row;
+extern int			timerpid;
+extern Disk			*disk;
+extern Text			*seltext;
+extern Text			*argtext;
+extern Text			*mousetext;	/* global because Text.close needs to clear it */
+extern Text			*typetext;		/* global because Text.close needs to clear it */
+extern Text			*barttext;		/* shared between mousetask and keyboardthread */
+extern int			bartflag;
+extern int			swapscrollbuttons;
+extern Window		*activewin;
+extern Column		*activecol;
+extern Buffer		snarfbuf;
+extern Rectangle		nullrect;
+extern int			fsyspid;
+extern char			*cputype;
+extern char			*objtype;
+extern char			*home;
+extern char			*acmeshell;
+extern char			*fontnames[2];
+extern Image		*tagcols[NCOL];
+extern Image		*textcols[NCOL];
 extern char		wdir[]; /* must use extern because no dimension given */
-int			editing;
-int			erroutfd;
-int			messagesize;		/* negotiated in 9P version setup */
-int			globalautoindent;
-int			dodollarsigns;
-char*		mtpt;
+extern int			editing;
+extern int			erroutfd;
+extern int			messagesize;		/* negotiated in 9P version setup */
+extern int			globalautoindent;
+extern int			dodollarsigns;
+extern char*		mtpt;
 
 enum
 {
@@ -563,20 +563,20 @@ enum
 	Kscrollonedown	= KF|0x21
 };
 
-Channel	*cplumb;		/* chan(Plumbmsg*) */
-Channel	*cwait;		/* chan(Waitmsg) */
-Channel	*ccommand;	/* chan(Command*) */
-Channel	*ckill;		/* chan(Rune*) */
-Channel	*cxfidalloc;	/* chan(Xfid*) */
-Channel	*cxfidfree;	/* chan(Xfid*) */
-Channel	*cnewwindow;	/* chan(Channel*) */
-Channel	*mouseexit0;	/* chan(int) */
-Channel	*mouseexit1;	/* chan(int) */
-Channel	*cexit;		/* chan(int) */
-Channel	*cerr;		/* chan(char*) */
-Channel	*cedit;		/* chan(int) */
-Channel	*cwarn;		/* chan(void*)[1] (really chan(unit)[1]) */
-
-QLock	editoutlk;
+extern Channel	*cplumb;		/* chan(Plumbmsg*) */
+extern Channel	*cwait;		/* chan(Waitmsg) */
+extern Channel	*ccommand;	/* chan(Command*) */
+extern Channel	*ckill;		/* chan(Rune*) */
+extern Channel	*cxfidalloc;	/* chan(Xfid*) */
+extern Channel	*cxfidfree;	/* chan(Xfid*) */
+extern Channel	*cnewwindow;	/* chan(Channel*) */
+extern Channel	*mouseexit0;	/* chan(int) */
+extern Channel	*mouseexit1;	/* chan(int) */
+extern Channel	*cexit;		/* chan(int) */
+extern Channel	*cerr;		/* chan(char*) */
+extern Channel	*cedit;		/* chan(int) */
+extern Channel	*cwarn;		/* chan(void*)[1] (really chan(unit)[1]) */
+
+extern QLock	editoutlk;
 
 #define	STACK	65536
diff --git a/src/cmd/acme/ecmd.c b/src/cmd/acme/ecmd.c
index f7613172..42f79960 100644
--- a/src/cmd/acme/ecmd.c
+++ b/src/cmd/acme/ecmd.c
@@ -19,7 +19,7 @@ char	Enoname[] = "no file name given";
 
 Address	addr;
 File	*menu;
-Rangeset	sel;
+extern Rangeset	sel;
 extern	Text*	curtext;
 Rune	*collection;
 int	ncollection;
diff --git a/src/cmd/acme/mail/mail.c b/src/cmd/acme/mail/mail.c
index e1047bb8..6d051821 100644
--- a/src/cmd/acme/mail/mail.c
+++ b/src/cmd/acme/mail/mail.c
@@ -18,7 +18,7 @@ char *srvname;
 Window	*wbox;
 Message	mbox;
 Message	replies;
-char		*home;
+extern char		*home;
 CFid		*plumbsendfd;
 CFid		*plumbseemailfd;
 CFid		*plumbshowmailfd;
diff --git a/src/cmd/acme/mkfile b/src/cmd/acme/mkfile
index 18bea9e0..ba5877c2 100644
--- a/src/cmd/acme/mkfile
+++ b/src/cmd/acme/mkfile
@@ -25,6 +25,7 @@ OFILES=\
 	util.$O\
 	wind.$O\
 	xfid.$O\
+	dat.$O\
 
 HFILES=dat.h\
 	edit.h\
diff --git a/src/cmd/astro/astro.h b/src/cmd/astro/astro.h
index 4648e34e..248dbd88 100644
--- a/src/cmd/astro/astro.h
+++ b/src/cmd/astro/astro.h
@@ -85,55 +85,55 @@ struct	Tim
 	char	tz[4];
 };
 
-double	converge;
-
-char	flags[128];
-int	nperiods;
-double	wlong, awlong, nlat, elev;
-double	obliq, phi, eps, tobliq;
-double	dphi, deps;
-double	day, deld, per;
-double	eday, capt, capt2, capt3, gst;
-double	pi, pipi, radian, radsec, deltat;
-double	erad, glat;
-double	xms, yms, zms;
-double	xdot, ydot, zdot;
-
-double	ecc, incl, node, argp, mrad, anom, motion;
-
-double	lambda, beta, rad, mag, semi;
-double	alpha, delta, rp, hp;
-double	ra, decl, semi2;
-double	lha, decl2, lmb2;
-double	az, el;
-
-double	meday, seday, mhp, salph, sdelt, srad;
-
-double*	cafp;
-char*	cacp;
-
-double	rah, ram, ras, dday, dmin, dsec;
-long	sao;
-double	da, dd, px, epoch;
-char	line[100];
-Obj2	osun;
-Obj2	omoon;
-Obj2	oshad;
-Obj2	omerc;
-Obj2	ovenus;
-Obj2	omars;
-Obj2	osat;
-Obj2	ouran;
-Obj2	onept;
-Obj2	oplut;
-Obj2	ojup;
-Obj2	ostar;
-Obj2	ocomet;
-Obj3	occ;
-Obj2*	eobj1;
-Obj2*	eobj2;
-
-char*	startab;
+extern	double	converge;
+
+extern	char	flags[128];
+extern	int	nperiods;
+extern	double	wlong, awlong, nlat, elev;
+extern	double	obliq, phi, eps, tobliq;
+extern	double	dphi, deps;
+extern	double	day, deld, per;
+extern	double	eday, capt, capt2, capt3, gst;
+extern	double	pi, pipi, radian, radsec, deltat;
+extern	double	erad, glat;
+extern	double	xms, yms, zms;
+extern	double	xdot, ydot, zdot;
+
+extern	double	ecc, incl, node, argp, mrad, anom, motion;
+
+extern	double	lambda, beta, rad, mag, semi;
+extern	double	alpha, delta, rp, hp;
+extern	double	ra, decl, semi2;
+extern	double	lha, decl2, lmb2;
+extern	double	az, el;
+
+extern	double	meday, seday, mhp, salph, sdelt, srad;
+
+extern	double*	cafp;
+extern	char*	cacp;
+
+extern	double	rah, ram, ras, dday, dmin, dsec;
+extern	long	sao;
+extern	double	da, dd, px, epoch;
+extern	char	line[100];
+extern	Obj2	osun;
+extern	Obj2	omoon;
+extern	Obj2	oshad;
+extern	Obj2	omerc;
+extern	Obj2	ovenus;
+extern	Obj2	omars;
+extern	Obj2	osat;
+extern	Obj2	ouran;
+extern	Obj2	onept;
+extern	Obj2	oplut;
+extern	Obj2	ojup;
+extern	Obj2	ostar;
+extern	Obj2	ocomet;
+extern	Obj3	occ;
+extern	Obj2*	eobj1;
+extern	Obj2*	eobj2;
+
+extern	char*	startab;
 
 extern	int	dmo[];
 extern	Obj2*	objlst[];
diff --git a/src/cmd/astro/main.c b/src/cmd/astro/main.c
index 43ce4cc4..c9fbf2a5 100644
--- a/src/cmd/astro/main.c
+++ b/src/cmd/astro/main.c
@@ -1,5 +1,53 @@
 #include "astro.h"
 
+double	converge;
+
+char	flags[128];
+int	nperiods;
+double	wlong, awlong, nlat, elev;
+double	obliq, phi, eps, tobliq;
+double	dphi, deps;
+double	day, deld, per;
+double	eday, capt, capt2, capt3, gst;
+double	pi, pipi, radian, radsec, deltat;
+double	erad, glat;
+double	xms, yms, zms;
+double	xdot, ydot, zdot;
+
+double	ecc, incl, node, argp, mrad, anom, motion;
+
+double	lambda, beta, rad, mag, semi;
+double	alpha, delta, rp, hp;
+double	ra, decl, semi2;
+double	lha, decl2, lmb2;
+double	az, el;
+
+double	meday, seday, mhp, salph, sdelt, srad;
+
+double*	cafp;
+char*	cacp;
+
+double	rah, ram, ras, dday, dmin, dsec;
+long	sao;
+double	da, dd, px, epoch;
+char	line[100];
+Obj2	osun;
+Obj2	omoon;
+Obj2	oshad;
+Obj2	omerc;
+Obj2	ovenus;
+Obj2	omars;
+Obj2	osat;
+Obj2	ouran;
+Obj2	onept;
+Obj2	oplut;
+Obj2	ojup;
+Obj2	ostar;
+Obj2	ocomet;
+Obj3	occ;
+Obj2*	eobj1;
+Obj2*	eobj2;
+
 char*	herefile;
 
 int
diff --git a/src/cmd/db/setup.c b/src/cmd/db/setup.c
index 15a196e1..892e2c9f 100644
--- a/src/cmd/db/setup.c
+++ b/src/cmd/db/setup.c
@@ -4,12 +4,12 @@
 #include "defs.h"
 #include "fns.h"
 
-char	*symfil;
-char	*corfil;
+extern	char	*symfil;
+extern	char	*corfil;
 
-Map	*symmap;
-Map	*cormap;
-Regs	*correg;
+extern	Map	*symmap;
+extern	Map	*cormap;
+extern	Regs	*correg;
 Map	*dotmap;
 
 void
diff --git a/src/cmd/diff/diff.h b/src/cmd/diff/diff.h
index 25e39c6c..94c7bf0d 100644
--- a/src/cmd/diff/diff.h
+++ b/src/cmd/diff/diff.h
@@ -1,10 +1,10 @@
 #define stdout bstdout
 
-char mode;			/* '\0', 'e', 'f', 'h' */
-char bflag;			/* ignore multiple and trailing blanks */
-char rflag;			/* recurse down directory trees */
-char mflag;			/* pseudo flag: doing multiple files, one dir */
-int anychange;
+extern char mode;			/* '\0', 'e', 'f', 'h' */
+extern char bflag;			/* ignore multiple and trailing blanks */
+extern char rflag;			/* recurse down directory trees */
+extern char mflag;			/* pseudo flag: doing multiple files, one dir */
+extern int anychange;
 extern Biobuf	stdout;
 extern int	binary;
 
diff --git a/src/cmd/diff/main.c b/src/cmd/diff/main.c
index 2697cb88..fe1161aa 100644
--- a/src/cmd/diff/main.c
+++ b/src/cmd/diff/main.c
@@ -8,6 +8,12 @@
 
 Biobuf	stdout;
 
+char mode;			/* '\0', 'e', 'f', 'h' */
+char bflag;			/* ignore multiple and trailing blanks */
+char rflag;			/* recurse down directory trees */
+char mflag;			/* pseudo flag: doing multiple files, one dir */
+int anychange;
+
 static char *tmp[] = {"/tmp/diff1XXXXXXXXXXX", "/tmp/diff2XXXXXXXXXXX"};
 static int whichtmp;
 static char *progname;
diff --git a/src/cmd/fontsrv/a.h b/src/cmd/fontsrv/a.h
index 2eeb404f..5dc82f35 100644
--- a/src/cmd/fontsrv/a.h
+++ b/src/cmd/fontsrv/a.h
@@ -1,6 +1,6 @@
 typedef struct XFont XFont;
-XFont *xfont;
-int nxfont;
+extern XFont *xfont;
+extern int nxfont;
 
 enum {
 	SubfontSize = 32,
diff --git a/src/cmd/fontsrv/main.c b/src/cmd/fontsrv/main.c
index fd629801..e6f7b9c5 100644
--- a/src/cmd/fontsrv/main.c
+++ b/src/cmd/fontsrv/main.c
@@ -13,6 +13,9 @@
 
 #include "a.h"
 
+XFont *xfont;
+int nxfont;
+
 void
 usage(void)
 {
diff --git a/src/cmd/fossil/9fsys.c b/src/cmd/fossil/9fsys.c
index 121e5b66..293f675f 100644
--- a/src/cmd/fossil/9fsys.c
+++ b/src/cmd/fossil/9fsys.c
@@ -22,7 +22,7 @@ struct Fsys {
 	Fsys*	next;
 };
 
-int mempcnt;			/* from fossil.c */
+extern int mempcnt;			/* from fossil.c */
 
 int	fsGetBlockSize(Fs *fs);
 
diff --git a/src/cmd/hoc/code.c b/src/cmd/hoc/code.c
index af60c194..0de502a3 100644
--- a/src/cmd/hoc/code.c
+++ b/src/cmd/hoc/code.c
@@ -15,7 +15,7 @@ Inst	*progp;		/* next free spot for code generation */
 Inst	*pc;		/* program counter during execution */
 Inst	*progbase = prog; /* start of current subprogram */
 int	returning;	/* 1 if return stmt seen */
-int	indef;	/* 1 if parsing a func or proc */
+extern	int	indef;	/* 1 if parsing a func or proc */
 
 typedef struct Frame {	/* proc/func call stack frame */
 	Symbol	*sp;	/* symbol table entry */
diff --git a/src/cmd/htmlroff/a.h b/src/cmd/htmlroff/a.h
index a4209178..1df612c2 100644
--- a/src/cmd/htmlroff/a.h
+++ b/src/cmd/htmlroff/a.h
@@ -30,7 +30,9 @@ enum
 	ArgMode = 1<<3,
 	HtmlMode = 1<<4,
 
-	MaxLine = 1024
+	MaxLine = 1024,
+
+	MAXIF = 20
 };
 
 Rune*	L(char*);
@@ -77,7 +79,7 @@ int		linefmt(Fmt*);
 void		nr(Rune*, int);
 void		_nr(Rune*, Rune*);
 void		out(Rune*);
-void		(*outcb)(Rune);
+extern void		(*outcb)(Rune);
 void		outhtml(Rune*);
 void		outrune(Rune);
 void		outtrap(void);
diff --git a/src/cmd/htmlroff/roff.c b/src/cmd/htmlroff/roff.c
index f52e0734..4ac4f980 100644
--- a/src/cmd/htmlroff/roff.c
+++ b/src/cmd/htmlroff/roff.c
@@ -6,7 +6,6 @@ enum
 	MAXRAW = 40,
 	MAXESC = 60,
 	MAXLINE = 1024,
-	MAXIF = 20,
 	MAXARG = 10
 };
 
diff --git a/src/cmd/htmlroff/t16.c b/src/cmd/htmlroff/t16.c
index 9389a59a..8da1d1ff 100644
--- a/src/cmd/htmlroff/t16.c
+++ b/src/cmd/htmlroff/t16.c
@@ -16,9 +16,8 @@
  *	.if .ie .el
  *
  */
-
-int iftrue[20];
-int niftrue;
+extern int iftrue[MAXIF];
+extern int niftrue;
 
 void
 startbody(void)
diff --git a/src/cmd/ndb/dns.c b/src/cmd/ndb/dns.c
index 723989b9..f05d4c22 100644
--- a/src/cmd/ndb/dns.c
+++ b/src/cmd/ndb/dns.c
@@ -101,7 +101,8 @@ char *udpaddr = "udp!*!domain";
 char	*logfile = "dns";
 char	*dbfile;
 char	mntpt[Maxpath];
-char	*LOG;
+
+extern	char *LOG;
 
 void
 usage(void)
diff --git a/src/cmd/ndb/dnstcp.c b/src/cmd/ndb/dnstcp.c
index 3d4d1e90..18224d49 100644
--- a/src/cmd/ndb/dnstcp.c
+++ b/src/cmd/ndb/dnstcp.c
@@ -23,11 +23,12 @@ char	mntpt[Maxpath];
 ulong	now;
 int	maxage;
 uchar	ipaddr[IPaddrlen];	/* my ip address */
-char	*LOG;
 char	*zonerefreshprogram;
 char	*tcpaddr;
 char	*udpaddr;
 
+extern	char *LOG;
+
 void
 usage(void)
 {
diff --git a/src/cmd/plumb/plumber.c b/src/cmd/plumb/plumber.c
index 5ead2e93..86bec15c 100644
--- a/src/cmd/plumb/plumber.c
+++ b/src/cmd/plumb/plumber.c
@@ -7,10 +7,11 @@
 #include "plumber.h"
 
 int debug;
+int nports;
+char **ports;
 int foreground=0;
 char	*plumbfile;
 char *user;
-char *home;
 char *progname;
 Ruleset **rules;
 int	printerrors=1;
diff --git a/src/cmd/plumb/plumber.h b/src/cmd/plumb/plumber.h
index 9ffe74d2..4eb7f131 100644
--- a/src/cmd/plumb/plumber.h
+++ b/src/cmd/plumb/plumber.h
@@ -89,11 +89,11 @@ void		makeports(Ruleset*[]);
 void		printinputstack(void);
 int		popinput(void);
 
-Ruleset	**rules;
-char		*user;
-char		*home;
-jmp_buf	parsejmp;
-char		*lasterror;
-char		**ports;
-int		nports;
-int		debug;
+extern Ruleset	**rules;
+extern char	*user;
+extern char	*home;
+extern jmp_buf	parsejmp;
+extern char	*lasterror;
+extern char	**ports;
+extern int	nports;
+extern int	debug;
diff --git a/src/cmd/rc/exec.c b/src/cmd/rc/exec.c
index 0320976a..94d3321a 100644
--- a/src/cmd/rc/exec.c
+++ b/src/cmd/rc/exec.c
@@ -8,6 +8,24 @@
  */
 char *argv0="rc";
 
+thread *runq;
+code *codebuf;				/* compiler output */
+int ntrap;				/* number of outstanding traps */
+int trap[NSIG];				/* number of outstanding traps per type */
+int eflagok;			/* kludge flag so that -e doesn't exit in startup */
+tree *cmdtree;
+char *promptstr;
+char tok[NTOK];
+var *gvar[NVAR];				/* hash for globals */
+int mypid;
+char **argp;
+char **args;
+int nerror;		/* number of errors encountered during compilation */
+int ndot;
+int lastc;
+int kidpid;
+io *err;
+
 void
 start(code *c, int pc, var *local)
 {
diff --git a/src/cmd/rc/exec.h b/src/cmd/rc/exec.h
index 06cfd64e..f9a9dfb0 100644
--- a/src/cmd/rc/exec.h
+++ b/src/cmd/rc/exec.h
@@ -56,17 +56,17 @@ struct thread{
 	tree *treenodes;		/* tree nodes created by this process */
 	thread *ret;		/* who continues when this finishes */
 };
-thread *runq;
+extern thread *runq;
 code *codecopy(code*);
-code *codebuf;				/* compiler output */
-int ntrap;				/* number of outstanding traps */
-int trap[NSIG];				/* number of outstanding traps per type */
+extern code *codebuf;				/* compiler output */
+extern int ntrap;				/* number of outstanding traps */
+extern int trap[NSIG];				/* number of outstanding traps per type */
 struct builtin{
 	char *name;
 	void (*fnc)(void);
 };
 extern struct builtin Builtin[];
-int eflagok;			/* kludge flag so that -e doesn't exit in startup */
+extern int eflagok;			/* kludge flag so that -e doesn't exit in startup */
 extern int havefork;
 
 void execcd(void), execwhatis(void), execeval(void), execexec(void);
diff --git a/src/cmd/rc/io.h b/src/cmd/rc/io.h
index 6c75cc5b..c970132f 100644
--- a/src/cmd/rc/io.h
+++ b/src/cmd/rc/io.h
@@ -10,7 +10,7 @@ struct io{
 	int fd;
 	char *bufp, *ebuf, *strp, buf[NBUF];
 };
-io *err;
+extern io *err;
 io *openfd(int), *openstr(void), *opencore(char *, int);
 int emptybuf(io*);
 void pchr(io*, int);
diff --git a/src/cmd/rc/rc.h b/src/cmd/rc/rc.h
index 2fd6758b..dd8b064c 100644
--- a/src/cmd/rc/rc.h
+++ b/src/cmd/rc/rc.h
@@ -65,7 +65,7 @@ tree *mung1(tree*, tree*), *mung2(tree*, tree*, tree*);
 tree *mung3(tree*, tree*, tree*, tree*), *epimung(tree*, tree*);
 tree *simplemung(tree*), *heredoc(tree*);
 void freetree(tree*);
-tree *cmdtree;
+extern tree *cmdtree;
 /*
  * The first word of any code vector is a reference count.
  * Always create a new reference to a code vector by calling codecopy(.).
@@ -76,9 +76,9 @@ union code{
 	int i;
 	char *s;
 };
-char *promptstr;
+extern char *promptstr;
 #define	NTOK	8192
-char tok[NTOK];
+extern char tok[NTOK];
 #define	APPEND	1
 #define	WRITE	2
 #define	READ	3
@@ -98,7 +98,7 @@ struct var{
 };
 var *vlook(char*), *gvlook(char*), *newvar(char*, var*);
 #define	NVAR	521
-var *gvar[NVAR];				/* hash for globals */
+extern var *gvar[NVAR];				/* hash for globals */
 #define	new(type)	((type *)emalloc(sizeof(type)))
 char *emalloc(long);
 void *Malloc(ulong);
@@ -109,7 +109,7 @@ struct here{
 	char *name;
 	struct here *next;
 };
-int mypid;
+extern int mypid;
 /*
  * Glob character escape in strings:
  *	In a string, GLOB must be followed by *?[ or GLOB.
@@ -128,9 +128,9 @@ int mypid;
 #define	threebyte(c)	((c&0xf0)==0xe0)
 #define	fourbyte(c)	((c&0xf8)==0xf0)
 
-char **argp;
-char **args;
-int nerror;		/* number of errors encountered during compilation */
+extern char **argp;
+extern char **args;
+extern int nerror;		/* number of errors encountered during compilation */
 extern int doprompt;		/* is it time for a prompt? */
 /*
  * Which fds are the reading/writing end of a pipe?
@@ -146,8 +146,8 @@ extern char *Rcmain(), Fdprefix[];
  * How many dot commands have we executed?
  * Used to ensure that -v flag doesn't print rcmain.
  */
-int ndot;
+extern int ndot;
 char *getstatus(void);
-int lastc;
-int lastword;
-int kidpid;
+extern int lastc;
+extern int lastword;
+extern int kidpid;
diff --git a/src/cmd/rio/winwatch.c b/src/cmd/rio/winwatch.c
index 410f7bd7..da021278 100644
--- a/src/cmd/rio/winwatch.c
+++ b/src/cmd/rio/winwatch.c
@@ -33,7 +33,7 @@ int onwin;
 int rows, cols;
 int sortlabels;
 int showwmnames;
-Font *font;
+extern Font *font;
 Image *lightblue;
 
 
diff --git a/src/cmd/sam/sam.c b/src/cmd/sam/sam.c
index a660e4e3..9b5b3e39 100644
--- a/src/cmd/sam/sam.c
+++ b/src/cmd/sam/sam.c
@@ -20,7 +20,7 @@ int	downloaded;
 int	dflag;
 int	Rflag;
 char	*machine;
-char	*home;
+extern char	*home;
 int	bpipeok;
 int	termlocked;
 char	*samterm = SAMTERM;
diff --git a/src/cmd/scat/dssread.c b/src/cmd/scat/dssread.c
index bd72132a..4ccd38c2 100644
--- a/src/cmd/scat/dssread.c
+++ b/src/cmd/scat/dssread.c
@@ -5,7 +5,6 @@
 
 static	void	dodecode(Biobuf*, Pix*, int, int, uchar*);
 static	int32	getlong(uchar*);
-int	debug;
 
 Img*
 dssread(char *file)
diff --git a/src/cmd/scat/scat.c b/src/cmd/scat/scat.c
index dff596b1..a21b8cb9 100644
--- a/src/cmd/scat/scat.c
+++ b/src/cmd/scat/scat.c
@@ -34,6 +34,11 @@ Record	*rec;
 Record	*orec;
 Record	*cur;
 
+int	nplate;
+Plate	plate[2000];		/* needs to go to 2000 when the north comes */
+int	debug;
+Gam	gam;
+
 char	*dir;
 int	saodb;
 int	ngcdb;
diff --git a/src/cmd/scat/sky.h b/src/cmd/scat/sky.h
index bc55fbfc..21b1904c 100644
--- a/src/cmd/scat/sky.h
+++ b/src/cmd/scat/sky.h
@@ -303,13 +303,14 @@ struct	Img
 #define	ARCSECONDS_PER_RADIAN	(DEG(1)*3600)
 #define	MILLIARCSEC	(1000*60*60)
 
-int	nplate;
-Plate	plate[2000];		/* needs to go to 2000 when the north comes */
-double	PI_180;
-double	TWOPI;
-double	LN2;
-int	debug;
-struct
+extern int	nplate;
+extern Plate	plate[2000];		/* needs to go to 2000 when the north comes */
+extern double	PI_180;
+extern double	TWOPI;
+extern double	LN2;
+extern int	debug;
+typedef struct Gam Gam;
+struct Gam
 {
 	float	min;
 	float	max;
@@ -318,7 +319,8 @@ struct
 	float	mult1;
 	float	mult2;
 	int	neg;
-} gam;
+};
+extern Gam gam;
 
 typedef struct Picture Picture;
 struct Picture
diff --git a/src/cmd/tcs/tcs.c b/src/cmd/tcs/tcs.c
index d796ba63..682e108f 100644
--- a/src/cmd/tcs/tcs.c
+++ b/src/cmd/tcs/tcs.c
@@ -28,7 +28,6 @@ int clean = 0;
 int verbose = 0;
 long ninput, noutput, nrunes, nerrors;
 char *file = "stdin";
-char *argv0;
 Rune runes[N];
 char obuf[UTFmax*N];	/* maximum bloat from N runes */
 long tab[NRUNE];
diff --git a/src/cmd/venti/srv/part.c b/src/cmd/venti/srv/part.c
index c1fb929f..68ffbc53 100644
--- a/src/cmd/venti/srv/part.c
+++ b/src/cmd/venti/srv/part.c
@@ -17,6 +17,7 @@
 #include "fns.h"
 
 u32int	maxblocksize;
+#pragma weak readonly
 int	readonly;
 
 int findsubpart(Part *part, char *name);
diff --git a/src/lib9/argv0.c b/src/lib9/argv0.c
index 4c61f444..b9a9b8b4 100644
--- a/src/lib9/argv0.c
+++ b/src/lib9/argv0.c
@@ -1,5 +1,6 @@
 #include <lib9.h>
 
+#pragma weak argv0
 char *argv0;
 
 /*
diff --git a/src/libthread/thread.c b/src/libthread/thread.c
index 79e0ec71..b5bfed3e 100644
--- a/src/libthread/thread.c
+++ b/src/libthread/thread.c
@@ -690,6 +690,7 @@ threadrwakeup(Rendez *r, int all, ulong pc)
 
 static int threadargc;
 static char **threadargv;
+#pragma weak mainstacksize
 int mainstacksize;
 extern int _p9usepwlibrary;	/* getgrgid etc. smash the stack - tell _p9dir just say no */
 static void