getaddrinfo() complete

.gitignore

@@ -0,0 +1 @@
+.vscode

.vscode/c_cpp_properties.json

@@ -0,0 +1,18 @@
+{
+    "configurations": [
+        {
+            "name": "Win32",
+            "includePath": [
+                "${default}"
+            ],
+            "defines": [
+                "_DEBUG",
+                "UNICODE",
+                "_UNICODE"
+            ],
+            "windowsSdkVersion": "10.0.22621.0",
+            "intelliSenseMode": "windows-msvc-x64"
+        }
+    ],
+    "version": 4
+}

System_Calls/readme.md

@@ -1,9 +1,13 @@
-# System Calls :
+# System Calls
 
 Below are the system calls that can be used to access network functionality.
 
 * **getaddrinfo()** :
   
+  * Purpose :
+
+    It helps set up the structs, specifically **struct addrinfo**. It does all the dirty work like DNS and service name lookup and fill up the structs with all the relevant information based on the parameters passed.
+  
   * Function prototye
 
     ```
@@ -16,3 +20,62 @@ Below are the system calls that can be used to access network functionality.
     1. node :
 
         Type : `const char *`
+
+        Value : Domain name or IP in form of string.
+
+        Purpose : To help find the host computer IP address for connection.
+
+    2. service :
+
+        Type : `const char *`
+
+        Value : Service/Protocol or Port in form of string.
+
+        Purpose : To help specify the port number in the host computer.
+
+    3. hints :
+
+        Type : `const struct addrinfo *`
+
+        Value : Pointer to *struct addrinfo*.
+
+        Purpose : To guide how it should fetch relevent data and store it to `struct addrinfo **res`.
+
+    4. res :
+
+        Type : `struct addrinfo **`
+
+        Value : Pointer to linked list of *struct addrinfo \**
+
+  * Return Type :
+
+      Non-zero value in case of error or zero, otherwise.
+
+    **Code Snippet**
+
+    ```
+    int status;
+    struct addrinfo hints;
+    struct addrinfo *servinfo; // will point to the results
+
+    memset(&hints, 0, sizeof hints); // make sure the struct is 
+                                     // garbage free
+    
+    
+    hints.ai_family = AF_UNSPEC;     // don't care IPv4 or IPv6
+    hints.ai_socktype = SOCK_STREAM; // TCP stream sockets
+    hints.ai_flags = AI_PASSIVE;     // fill in my IP(my host) for me
+
+    if ((status = getaddrinfo(NULL, "3490", &hints, &servinfo)) != 0){
+        fprintf(stderr, "getaddrinfo erro: %s\n", gai_strerror(status));
+        exit(1);
+    }
+
+    // servinfo now  points to a linked list of 1 or more struct 
+    // addrinfos
+
+
+    // ... do everything until you don't need servinfo anymore ...
+
+    freeaddrinfo(servinfo); // free the linked list
+    ```

System_Calls/showip.c

@@ -0,0 +1,62 @@
+/*
+*   Printing the linked list of struct addrinfo
+*/
+
+#include <stdio.h>
+#include <string.h>
+#include <sys/types.h>
+#include <sys/socket.h>
+#include <netdb.h>
+#include <arpa/inet.h>
+#include <netinet/in.h>
+
+int main(int argc, char* argv[] ){
+
+    struct addrinfo hints, *res, *p;
+    int status;
+    char ipstr[INET6_ADDRSTRLEN];
+
+    if( argc != 2 ){
+        fprintf(stderr, "usage: showip hostname\n");
+        return 1;
+    }
+
+    memset(&hints, 0, sizeof hints);
+    hints.ai_family = AF_UNSPEC; // IP version agnostic
+    hints.ai_socktype = SOCK_STREAM;
+
+    if((status = getaddrinfo(argv[1], NULL, &hints, &res)) != 0){
+        fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(status));
+        return 2;    
+    }
+
+    printf("IP address for %s:\n\n", argv[1]);
+
+    for(p = res ; p != NULL ; p = p->ai_next){
+        void *addr;
+        char *ipver;
+
+        // get the pointer to the address itself,
+        // different fields in IPv4 and IPv6:
+        if( p->ai_family == AF_INET ){
+            // IPv4
+            struct sockaddr_in *ipv4 = (struct sockaddr_in *)p->ai_addr;
+            addr = &(ipv4->sin_addr);
+            ipver = "IPv4";
+        }
+        else{
+            // IPv6
+            struct sockaddr_in6 *ipv6 = (struct sockaddr_in6 *)p->ai_addr;
+            addr = &(ipv6->sin6_addr);
+            ipver = "IPv6";
+        }
+
+       //convert the IP to a string and print it:
+       inet_ntop(p->ai_family, addr, ipstr, sizeof ipstr);
+       printf("   %s: %s\n", ipver, ipstr); 
+    }
+
+    freeaddrinfo(res); // free the linked list
+    
+    return 0;
+}