More goofing around with temperature conversion

for make near-mindless C practice.

digressions/temperatures.c

@@ -1,5 +1,6 @@
 #include <stdio.h>
 #include <stdlib.h>
+#include <string.h>
 
 
 double c2f(double celsius) {
@@ -12,36 +13,57 @@ double f2c(double fahrenheit) {
 }
 
 
-int die_usage(const char *msg) {
+int usage(const char *msg) {
     printf("Usage: temperatures <temp>[fFcC]\n");
+    if (msg != NULL) {
         printf("%s\n", msg);
-    exit(1);
+    }
+    return 1;
 }
 
 
 int main(int argc, char * argv[]) {
     if (argc < 2) {
-        die_usage("");
+        usage(NULL);
+        return 2;
     }
 
+    int ret;
     char msg[255];
     double temp;
     char scale = '\0';
-    const char * temp_string = argv[1];
+    const char *in_fmt = "%lf%c";
+    const char *out_fmt = "%.2lf%c\n";
+
+    if (((int)strlen(argv[1]) == 1) && (argv[1][0] == '-')) {
+        ret = fscanf(stdin, in_fmt, &temp, &scale);
+    } else {
+        const char * temp_string = argv[1];
+        ret = sscanf(temp_string, in_fmt, &temp, &scale);
+    }
+
+    if ((ret == 0) || (ret == EOF)) {
+        usage("No temperature provided!");
+        return 1;
+    }
+
+    if (ret == 1) {
+        usage("No scale provided!");
+        return 1;
+    }
 
-    sscanf(temp_string, "%lf%c", &temp, &scale);
     switch (scale) {
         case 'c':
         case 'C':
-            printf("%.2lf\n", c2f(temp));
+            printf(out_fmt, c2f(temp), 'F');
             break;
         case 'f':
         case 'F':
-            printf("%.2lf\n", f2c(temp));
+            printf(out_fmt, f2c(temp), 'C');
             break;
         default:
-            sprintf(msg, "Unknown scale '%c'", scale);
+            sprintf(msg, "Unknown scale: '%c'", scale);
-            die_usage(msg);
+            usage(msg);
             break;
     }