libc: Implement atoi(), atol() and atoll()

libs/libc/include/stdlib.h

@@ -21,8 +21,14 @@ extern "C"
     /* Registers a handler function to be run at normal program termination. */
     int atexit(void (*handler)(void));
 
-    /* Not implemented.*/
-    int atoi(const char*);
+    /* Returns an integer (of type int) parsed from the string str. */
+    int atoi(const char* str);
+
+    /* Returns an integer (of type long) parsed from the string str. */
+    long atol(const char* str);
+
+    /* Returns an integer (of type long long) parsed from the string str. */
+    long long atoll(const char* str);
 
     /* Not implemented. */
     char* getenv(const char*);

libs/libc/src/stdlib.cpp

@@ -1,8 +1,29 @@
+#include <ctype.h>
 #include <luna.h>
 #include <stdlib.h>
 #include <sys/syscall.h>
 #include <unistd.h>
 
+template <typename T> T string_to_integer_type(const char* str)
+{
+    bool neg = false;
+    T val = 0;
+
+    switch (*str)
+    {
+    case '-':
+        neg = true;
+        str++;
+        break;
+    case '+': str++; break;
+    default: break;
+    }
+
+    while (isdigit(*str)) { val = (10 * val) + (*str++ - '0'); }
+
+    return (neg ? -val : val);
+}
+
 extern "C"
 {
     __lc_noreturn void abort()
@@ -10,9 +31,19 @@ extern "C"
         _Exit(-1);
     }
 
-    int atoi(const char*)
+    int atoi(const char* str)
     {
-        NOT_IMPLEMENTED("atoi");
+        return string_to_integer_type<int>(str);
+    }
+
+    long atol(const char* str)
+    {
+        return string_to_integer_type<long>(str);
+    }
+
+    long long atoll(const char* str)
+    {
+        return string_to_integer_type<long long>(str);
     }
 
     char* getenv(const char*)