diff --git a/stridx.c b/stridx.c
new file mode 100644
index 0000000..1ff38c8
--- /dev/null
+++ b/stridx.c
@@ -0,0 +1,54 @@
+#include <stdio.h>
+
+int mystr_idx(char *str, char *substr) {
+    // Если подстрока пустая, возвращаем 0
+    if (substr == NULL || *substr == '\0') {
+        return 0;
+    }
+    
+    // Если строка пустая, возвращаем -1
+    if (str == NULL || *str == '\0') {
+        return -1;
+    }
+    
+    int i = 0;
+    while (str[i] != '\0') {
+        int j = 0;
+        // Проверяем, совпадает ли подстрока начиная с позиции i
+        while (substr[j] != '\0' && str[i + j] != '\0' && str[i + j] == substr[j]) {
+            j++;
+        }
+        
+        // Если дошли до конца подстроки, значит нашли полное совпадение
+        if (substr[j] == '\0') {
+            return i;
+        }
+        
+        i++;
+    }
+    
+    return -1;
+}
+
+// Функция для тестирования
+void test_mystr_idx() {
+    printf("mystr_idx(\"helloworld\", \"world\") = %d (expected: 5)\n", 
+           mystr_idx("helloworld", "world"));
+    printf("mystr_idx(\"helloworld\", \"helloworld\") = %d (expected: 0)\n", 
+           mystr_idx("helloworld", "helloworld"));
+    printf("mystr_idx(\"helloworld\", \"foo\") = %d (expected: -1)\n", 
+           mystr_idx("helloworld", "foo"));
+    printf("mystr_idx(\"helloworld\", \"hello\") = %d (expected: 0)\n", 
+           mystr_idx("helloworld", "hello"));
+    printf("mystr_idx(\"helloworld\", \"low\") = %d (expected: 3)\n", 
+           mystr_idx("helloworld", "low"));
+    printf("mystr_idx(\"\", \"test\") = %d (expected: -1)\n", 
+           mystr_idx("", "test"));
+    printf("mystr_idx(\"test\", \"\") = %d (expected: 0)\n", 
+           mystr_idx("test", ""));
+}
+
+int main() {
+    test_mystr_idx();
+    return 0;
+}