Cleaning up little temperature converter program

based a bit on how the example qsort program was written, plus fixing a
pedantic error.

digressions/temperatures.c

@@ -1,6 +1,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <libgen.h>
 
 
 double c2f(double celsius) {
@@ -13,43 +14,44 @@ double f2c(double fahrenheit) {
 }
 
 
-int usage(const char *msg) {
-    printf("Usage: temperatures <temp>[fFcC]\n");
+void usage(const char *prog, const char *msg) {
+    fprintf(stderr, "Usage: %s <temp>[fFcC]\n", prog);
     if (msg != NULL) {
-        printf("%s\n", msg);
+        fprintf(stderr, "%s\n", msg);
     }
-    return 1;
 }
 
 
 int main(int argc, char * argv[]) {
-    if (argc < 2) {
-        usage(NULL);
-        return 2;
-    }
-
+    const char * prog = basename(argv[0]);
     int ret;
     char msg[255];
     double temp;
     char scale = '\0';
     const char *in_fmt = "%lf%c";
     const char *out_fmt = "%.2lf%c\n";
+    const char *temp_string;
+
+    if (argc < 2) {
+        usage(prog, NULL);
+        exit(EXIT_FAILURE);
+    }
 
     if (((int)strlen(argv[1]) == 1) && (argv[1][0] == '-')) {
         ret = fscanf(stdin, in_fmt, &temp, &scale);
     } else {
-        const char * temp_string = argv[1];
+        temp_string = argv[1];
         ret = sscanf(temp_string, in_fmt, &temp, &scale);
     }
 
     if ((ret == 0) || (ret == EOF)) {
-        usage("No temperature provided!");
-        return 1;
+        usage(prog, "No temperature provided!");
+        exit(EXIT_FAILURE);
     }
 
     if (ret == 1) {
-        usage("No scale provided!");
-        return 1;
+        usage(prog, "No scale provided!");
+        exit(EXIT_FAILURE);
     }
 
     switch (scale) {
@@ -63,9 +65,9 @@ int main(int argc, char * argv[]) {
             break;
         default:
             sprintf(msg, "Unknown scale: '%c'", scale);
-            usage(msg);
+            usage(prog, msg);
             break;
     }
 
-    return 0;
+    exit(EXIT_SUCCESS);
 }