diff --git a/src/file.c b/src/file.c
index 2b871e2..37cd10e 100644
--- a/src/file.c
+++ b/src/file.c
@@ -68,7 +68,7 @@ CosmsCoreFileError cosms_core_file_open(struct cosms_core_file *file, const char
     }
     #elif defined(_WIN32)
     DWORD access_flags = 0;
-    DWORD create_flag = 0;
+    DWORD create_flag = OPEN_EXISTING;
 
     if (mode & COSMS_CORE_FILE_MODE_READ) {
         access_flags |= GENERIC_READ;
@@ -122,7 +122,7 @@ CosmsCoreFileError cosms_core_file_close(struct cosms_core_file *file) {
         return COSMS_CORE_FILE_UNKOWN_ERROR;
     }
     #elif defined(_WIN32)
-    BOOL error = CloseHandle(file);
+    BOOL error = CloseHandle(file->native_file);
     if (error == 0) {
         if (GetLastError() == ERROR_INVALID_HANDLE) {
             return COSMS_CORE_FILE_INVALID_FILE;
@@ -135,25 +135,32 @@ CosmsCoreFileError cosms_core_file_close(struct cosms_core_file *file) {
     return COSMS_CORE_FILE_OK;
 }
 
-CosmsCoreFileError cosms_core_file_get_size(unsigned long long *size) {
+CosmsCoreFileError cosms_core_file_get_size(struct cosms_core_file *file, unsigned long long *size) {
     #if defined(__GNUC__)
     struct stat file_stat;
-    if (stat(path, &file_stat) != 0) {
+    if (fstat(file->native_file, &file_stat) != 0) {
+        if (errno == EBADF) {
+            return COSMS_CORE_FILE_INVALID_FILE;
+        }
+
         return COSMS_CORE_FILE_COULD_NOT_READ_SIZE;
     }
 
     (*size) = file_stat.st_size;
     #elif defined(_WIN32)
     DWORD high_size;
-    DWORD low_size = GetFileSize(file, &high_size);
-    if (low_size == INVALID_FILE_SIZE && GetLastError() != NO_ERROR) {
-        CloseHandle(file);
+    DWORD low_size = GetFileSize(file->native_file, &high_size);
+    if (low_size == INVALID_FILE_SIZE) {
+        if (GetLastError() == ERROR_INVALID_HANDLE) {
+            return COSMS_CORE_FILE_INVALID_FILE;
+        }
+
         return COSMS_CORE_FILE_COULD_NOT_READ_SIZE;
     }
 
     (*size) = ((unsigned long long)high_size << 32) | low_size;
     #endif
-    
+
     return COSMS_CORE_FILE_OK;
 }
 
@@ -454,6 +461,9 @@ const char *cosms_core_file_error_string(CosmsCoreFileError error) {
         case COSMS_CORE_FILE_STILL_OPEN:
             return "cosms-file still open";
 
+        case COSMS_CORE_FILE_INVALID_FILE:
+            return "cosms-file invalid native file given";
+
         default:
             return NULL;
     }
diff --git a/tests/data/large-write-file.txt b/tests/data/large-write-file.txt
new file mode 100644
index 0000000..d4c5011
Binary files /dev/null and b/tests/data/large-write-file.txt differ
diff --git a/tests/unit/file.c b/tests/unit/file.c
index c6e4a42..a99c044 100644
--- a/tests/unit/file.c
+++ b/tests/unit/file.c
@@ -5,6 +5,186 @@
 #include <string.h>
 #include <stdlib.h>
 
+COSMS_CORE_TEST_TEST(file_open,
+    COSMS_CORE_TEST_SUB_TEST(opening small file for reading,
+        struct cosms_core_file file;
+        CosmsCoreFileError error = cosms_core_file_open(&file, "tests/data/small-file.txt", COSMS_CORE_FILE_MODE_READ);
+        if (error != COSMS_CORE_FILE_OK) {
+            cosms_core_test_sub_test_error = cosms_core_file_error_string(error);
+        } else {
+            error = cosms_core_file_close(&file);
+            if (error != COSMS_CORE_FILE_OK) {
+                cosms_core_test_sub_test_error = cosms_core_file_error_string(error);
+            }
+        }
+    )
+
+    COSMS_CORE_TEST_SUB_TEST(opening large file for reading,
+        struct cosms_core_file file;
+        CosmsCoreFileError error = cosms_core_file_open(&file, "tests/data/large-file.txt", COSMS_CORE_FILE_MODE_READ);
+        if (error != COSMS_CORE_FILE_OK) {
+            cosms_core_test_sub_test_error = cosms_core_file_error_string(error);
+        } else {
+            error = cosms_core_file_close(&file);
+            if (error != COSMS_CORE_FILE_OK) {
+                cosms_core_test_sub_test_error = cosms_core_file_error_string(error);
+            }
+        }
+    )
+
+    COSMS_CORE_TEST_SUB_TEST(opening small file for writting,
+        struct cosms_core_file file;
+        CosmsCoreFileError error = cosms_core_file_open(&file, "tests/data/small-file.txt", COSMS_CORE_FILE_MODE_WRITE);
+        if (error != COSMS_CORE_FILE_OK) {
+            cosms_core_test_sub_test_error = cosms_core_file_error_string(error);
+        } else {
+            error = cosms_core_file_close(&file);
+            if (error != COSMS_CORE_FILE_OK) {
+                cosms_core_test_sub_test_error = cosms_core_file_error_string(error);
+            }
+        }
+    )
+
+    COSMS_CORE_TEST_SUB_TEST(opening large file for writting,
+        struct cosms_core_file file;
+        CosmsCoreFileError error = cosms_core_file_open(&file, "tests/data/large-file.txt", COSMS_CORE_FILE_MODE_WRITE);
+        if (error != COSMS_CORE_FILE_OK) {
+            cosms_core_test_sub_test_error = cosms_core_file_error_string(error);
+        } else {
+            error = cosms_core_file_close(&file);
+            if (error != COSMS_CORE_FILE_OK) {
+                cosms_core_test_sub_test_error = cosms_core_file_error_string(error);
+            }
+        }
+    )
+
+    COSMS_CORE_TEST_SUB_TEST(opening small file for appending,
+        struct cosms_core_file file;
+        CosmsCoreFileError error = cosms_core_file_open(&file, "tests/data/small-file.txt", COSMS_CORE_FILE_MODE_APPEND);
+        if (error != COSMS_CORE_FILE_OK) {
+            cosms_core_test_sub_test_error = cosms_core_file_error_string(error);
+        } else {
+            error = cosms_core_file_close(&file);
+            if (error != COSMS_CORE_FILE_OK) {
+                cosms_core_test_sub_test_error = cosms_core_file_error_string(error);
+            }
+        }
+    )
+
+    COSMS_CORE_TEST_SUB_TEST(opening large file for appending,
+        struct cosms_core_file file;
+        CosmsCoreFileError error = cosms_core_file_open(&file, "tests/data/large-file.txt", COSMS_CORE_FILE_MODE_APPEND);
+        if (error != COSMS_CORE_FILE_OK) {
+            cosms_core_test_sub_test_error = cosms_core_file_error_string(error);
+        } else {
+            error = cosms_core_file_close(&file);
+            if (error != COSMS_CORE_FILE_OK) {
+                cosms_core_test_sub_test_error = cosms_core_file_error_string(error);
+            }
+        }
+    )
+
+    COSMS_CORE_TEST_SUB_TEST(opening small file for reading and writting,
+        struct cosms_core_file file;
+        CosmsCoreFileError error = cosms_core_file_open(&file, "tests/data/small-file.txt", COSMS_CORE_FILE_MODE_READ | COSMS_CORE_FILE_MODE_WRITE);
+        if (error != COSMS_CORE_FILE_OK) {
+            cosms_core_test_sub_test_error = cosms_core_file_error_string(error);
+        } else {
+            error = cosms_core_file_close(&file);
+            if (error != COSMS_CORE_FILE_OK) {
+                cosms_core_test_sub_test_error = cosms_core_file_error_string(error);
+            }
+        }
+    )
+
+    COSMS_CORE_TEST_SUB_TEST(opening large file for reading and writting,
+        struct cosms_core_file file;
+        CosmsCoreFileError error = cosms_core_file_open(&file, "tests/data/large-file.txt", COSMS_CORE_FILE_MODE_READ | COSMS_CORE_FILE_MODE_WRITE);
+        if (error != COSMS_CORE_FILE_OK) {
+            cosms_core_test_sub_test_error = cosms_core_file_error_string(error);
+        } else {
+            error = cosms_core_file_close(&file);
+            if (error != COSMS_CORE_FILE_OK) {
+                cosms_core_test_sub_test_error = cosms_core_file_error_string(error);
+            }
+        }
+    )
+)
+
+COSMS_CORE_TEST_TEST(file_close,
+    COSMS_CORE_TEST_SUB_TEST(closing file,
+        struct cosms_core_file file;
+        CosmsCoreFileError error = cosms_core_file_open(&file, "tests/data/small-file.txt", COSMS_CORE_FILE_MODE_READ);
+        if (error != COSMS_CORE_FILE_OK) {
+            cosms_core_test_sub_test_error = cosms_core_file_error_string(error);
+        } else {
+            error = cosms_core_file_close(&file);
+            if (error != COSMS_CORE_FILE_OK) {
+                cosms_core_test_sub_test_error = cosms_core_file_error_string(error);
+            }
+        }
+    )
+
+    COSMS_CORE_TEST_SUB_TEST(closing non-existing file,
+        struct cosms_core_file file;
+        file.native_file = 0;
+
+        CosmsCoreFileError error = cosms_core_file_close(&file);
+        if (error != COSMS_CORE_FILE_INVALID_FILE) {
+            cosms_core_test_sub_test_error = cosms_core_file_error_string(error);
+        }
+    )
+)
+
+COSMS_CORE_TEST_TEST(file_size,
+    COSMS_CORE_TEST_SUB_TEST(reading small file size, 
+        struct cosms_core_file file;
+        CosmsCoreFileError error = cosms_core_file_open(&file, "tests/data/small-file.txt", COSMS_CORE_FILE_MODE_READ);
+        if (error != COSMS_CORE_FILE_OK) {
+            cosms_core_test_sub_test_error = cosms_core_file_error_string(error);
+        } else {
+            unsigned long long size = 0;
+            error = cosms_core_file_get_size(&file, &size);
+            if (error != COSMS_CORE_FILE_OK) {
+                cosms_core_test_sub_test_error = cosms_core_file_error_string(error);
+            } else {
+                if (size != 13) {
+                    cosms_core_test_sub_test_error = "result does not match expected result";
+                }
+            }
+        }
+    )
+
+    COSMS_CORE_TEST_SUB_TEST(reading large file size, 
+        struct cosms_core_file file;
+        CosmsCoreFileError error = cosms_core_file_open(&file, "tests/data/large-file.txt", COSMS_CORE_FILE_MODE_READ);
+        if (error != COSMS_CORE_FILE_OK) {
+            cosms_core_test_sub_test_error = cosms_core_file_error_string(error);
+        } else {
+            unsigned long long size = 0;
+            error = cosms_core_file_get_size(&file, &size);
+            if (error != COSMS_CORE_FILE_OK) {
+                cosms_core_test_sub_test_error = cosms_core_file_error_string(error);
+            } else {
+                if (size != 5242880000ULL) {
+                    cosms_core_test_sub_test_error = "result does not match expected result";
+                }
+            }
+        }
+    )
+
+    COSMS_CORE_TEST_SUB_TEST(reading non-existing file size, 
+        struct cosms_core_file file;
+        file.native_file = 0;
+        
+        unsigned long long size = 0;
+        CosmsCoreFileError error = cosms_core_file_get_size(&file, &size);
+        if (error != COSMS_CORE_FILE_INVALID_FILE) {
+            cosms_core_test_sub_test_error = cosms_core_file_error_string(error);
+        }
+    )
+)
+
 COSMS_CORE_TEST_TEST(file_read, 
     COSMS_CORE_TEST_SUB_TEST(reading small file,
         unsigned long long file_size = 0;
diff --git a/tests/unit/file.h b/tests/unit/file.h
index 1a8b33c..58946da 100644
--- a/tests/unit/file.h
+++ b/tests/unit/file.h
@@ -3,11 +3,17 @@
 
 #include "test.h"
 
+COSMS_CORE_TEST_DEFINE(file_open);
+COSMS_CORE_TEST_DEFINE(file_close);
+COSMS_CORE_TEST_DEFINE(file_size);
 COSMS_CORE_TEST_DEFINE(file_read);
 COSMS_CORE_TEST_DEFINE(file_write);
 COSMS_CORE_TEST_DEFINE(file_delete);
 
 COSMS_CORE_TEST_EXPORT(file, 
+    COSMS_CORE_TEST_FUNCTION_NAME(file_open),
+    COSMS_CORE_TEST_FUNCTION_NAME(file_close),
+    COSMS_CORE_TEST_FUNCTION_NAME(file_size),
     COSMS_CORE_TEST_FUNCTION_NAME(file_read),
     COSMS_CORE_TEST_FUNCTION_NAME(file_write),
     COSMS_CORE_TEST_FUNCTION_NAME(file_delete)