Re-wrote the program to accept the netmask as an argument

README.md

@@ -1,6 +1,6 @@
 # subnetcalc #
 A very simple subnet calculator written in C. It's nothing fancy at all,
-just a small project of mine to get going with C.
+just a small project of mine to get me going with C.
 
 Simply build it (make), run it (./subnetcalc) and enter the subnet 
 mask in slash notation but without the slash. 
@@ -8,3 +8,11 @@ For example enter 24 for a 24-bit subnet mask and the
 program will output the total number of addresses in the range aswell
 as the total number of usable address for hosts.
 
+You can also give the netmask to the program as an argument. For example:
+
+    $> ./subnetcalc 24
+    Netmask bit: 24
+
+    256 total addresses
+    254 usable addresses for hosts
+

subnetcalc.c

@@ -2,37 +2,113 @@
  * Jack-Benny Persson (jack-benny@cyberinfo.se).
  * Released under GNU GPLv2.
 */
-
 #include <stdio.h>
 #include <math.h>
+#include <stdlib.h>
+#include <string.h>
 
-int main(void)
+int checkInput(char input[]);
+long int calcAddr(int netmask);
+long int calcHosts(int netmask);
+void printAddr(int netmask);
+void printHelp(char progname[]);
+
+int main(int argc, char *argv[])
 {
+    char input[3] = { 0 };
     int net;
-    long int hosts, addr;
-    printf("Enter netmask in slash-notation without the slash: ");
-    scanf("%2d", &net);
 
-    if (net >= 0 && net <= 32) /* Sanity check the user input */
+    if (argc == 1) /* User didn't supply an argument, go interactive */
     {
-        printf("Netmask bit: %d\n\n", net);	
-        addr = pow(2, 32-net); /* Calculate number of addresses */
-        printf("%ld total addresses\n", addr);
+        printf("Enter netmask in slash-notation without the slash: ");
+        scanf("%2c", input);
 
-        hosts = addr-2;
-        /* Check if number of usable hosts is a negative value */ 
-        if (hosts < 0)
+        if (checkInput(input))
         {
-            hosts = 0;   /* Set usable hosts to zero if it was negative */
-        }   
-        printf("%ld usable addresses for hosts\n", hosts);
-        return 0;
+            net = atoi(input);
+            printAddr(net);
+            return 0;
+        }
+        else  /* If the user entered anything else than 0-32 */
+        {
+            printHelp(argv[0]);
+            return 1;
+        }
     }
-    else  /* If the user entered anything else than 0-32 */
+    else if (argc == 2) /* User supplied exactly one argument */
     {
-        printf("Only values between 0-32 are accepted\n");
+        if (checkInput(argv[1]))
+        {
+            net = atoi(argv[1]);
+            printAddr(net);
+        }
+        else
+        {
+            printHelp(argv[0]);
+            return 1;
+        }
+    }
+    else /* User supplied to many arguments */
+    {
+        printHelp(argv[0]);
         return 1;
     }
-
-    return 1;
+    return 0;
 }
+
+int checkInput(char input[])
+{
+    int netmask;
+    /* Sanity check the user input */
+    if( strspn(input, "0123456789.-\n") == strlen(input) )
+    {
+        netmask = atoi(input);
+        if (netmask >= 0 && netmask <= 32)
+        {
+            return 1;
+        }
+        else
+        {
+            printf("Only values between 0 and 32 are accepted\n\n");
+            return 0;
+        }
+    }
+    else
+    {
+        printf("Only values between 0 and 32 are accepted\n\n");
+        return 0;
+    }
+}
+
+long int calcAddr(int netmask)
+{
+    return pow(2, 32-netmask);
+}
+
+long int calcHosts(int netmask)
+{
+    long int hosts;
+    hosts = (pow(2, 32-netmask))-2;
+    if (hosts < 0) /* If usables hosts < 0, set to 0 */
+    {
+        hosts = 0;
+    }
+    return hosts;
+}
+
+void printAddr(int netmask)
+{
+    printf("Netmask bit: %d\n\n", netmask);	
+    printf("%ld total addresses\n", calcAddr(netmask));
+    printf("%ld usable addresses for hosts\n", calcHosts(netmask));
+}
+
+void printHelp(char progname[])
+{
+    printf("Usage: %s [netmask]\n", progname);
+    printf("If the program is started without any arguments, "
+            "it's started in interactive mode.\n");
+    printf("Optionally, a netmask can be supplied as a "
+            "argument.\n");
+    printf("Examle: %s 24\n", progname);
+}