use always static for usage and keep usage definition above main

cksum.c

@@ -7,7 +7,12 @@
 #include "util.h"
 
 static void cksum(FILE *, const char *);
-static void usage(void);
+
+static void
+usage(void)
+{
+	eprintf("usage: %s [files...]\n", argv0);
+}
 
 static const unsigned long crctab[] = {         0x00000000,
 0x04c11db7, 0x09823b6e, 0x0d4326d9, 0x130476dc, 0x17c56b6b,
@@ -113,8 +118,3 @@ cksum(FILE *fp, const char *s)
 	putchar('\n');
 }
 
-void
-usage(void)
-{
-	eprintf("usage: %s [files...]\n", argv0);
-}

du.c

@@ -20,7 +20,7 @@ static bool kflag = false;
 static long du(const char *);
 static void print(long n, char *path);
 
-void
+static void
 usage(void)
 {
 	eprintf("usage: %s [-a | -s] [-k] [file...]\n", argv0);

env.c

@@ -8,7 +8,11 @@
 
 extern char **environ;
 
-static void usage(void);
+static void
+usage(void)
+{
+	eprintf("usage: env [-i] [-u name]... [name=value]... [cmd [arg...]]\n");
+}
 
 int
 main(int argc, char *argv[])
@@ -39,8 +43,3 @@ main(int argc, char *argv[])
 	return EXIT_SUCCESS;
 }
 
-void
-usage(void)
-{
-	eprintf("usage: env [-i] [-u name]... [name=value]... [cmd [arg...]]\n");
-}

grep.c

@@ -12,7 +12,6 @@ enum { Match = 0, NoMatch = 1, Error = 2 };
 
 static void addpattern(const char *);
 static bool grep(FILE *, const char *);
-static void usage(void);
 
 static bool eflag = false;
 static bool vflag = false;
@@ -25,6 +24,12 @@ static struct plist {
 	struct plist *next;
 } *phead;
 
+static void
+usage(void)
+{
+	enprintf(Error, "usage: %s [-Ecilnqv] [-e pattern] pattern [files...]\n", argv0);
+}
+
 int
 main(int argc, char *argv[])
 {
@@ -158,8 +163,3 @@ end:
 	return match;
 }
 
-void
-usage(void)
-{
-	enprintf(Error, "usage: %s [-Ecilnqv] [-e pattern] pattern [files...]\n", argv0);
-}

test.c

@@ -9,7 +9,15 @@
 
 static bool unary(const char *, const char *);
 static bool binary(const char *, const char *, const char *);
-static void usage(void);
+
+static void
+usage(void)
+{
+	const char *ket = (*argv0 == '[') ? " ]" : "";
+
+	eprintf("usage: %s string%s\n"
+		"       %s [!] [-bcdefgkhLnprSstuwxz] string%s\n", argv0, ket, argv0, ket);
+}
 
 int
 main(int argc, char *argv[])
@@ -149,11 +157,3 @@ binary(const char *arg1, const char *op, const char *arg2)
 	return false;
 }
 
-void
-usage(void)
-{
-	const char *ket = (*argv0 == '[') ? " ]" : "";
-
-	eprintf("usage: %s string%s\n"
-	        "       %s [!] [-bcdefgkhLnprSstuwxz] string%s\n", argv0, ket, argv0, ket);
-}

yes.c

@@ -3,7 +3,11 @@
 #include <stdlib.h>
 #include "util.h"
 
-static void usage(void);
+static void
+usage(void)
+{
+	eprintf("usage: %s [string]\n", argv0);
+}
 
 int
 main(int argc, char *argv[])
@@ -18,8 +22,3 @@ main(int argc, char *argv[])
 	return EXIT_FAILURE; /* should not reach */
 }
 
-void
-usage(void)
-{
-	eprintf("usage: %s [string]\n", argv0);
-}