diff --git a/src/file.c b/src/file.c
index 4cf538f..ff5b8fc 100644
--- a/src/file.c
+++ b/src/file.c
@@ -31,13 +31,16 @@ CosmsCoreFileError cosms_core_file_open(struct cosms_core_file *file, const char
     } else if (mode & COSMS_CORE_FILE_MODE_READ) {
         flags |= O_RDONLY;
     } else if (mode & COSMS_CORE_FILE_MODE_WRITE) {
-        flags |= (O_WRONLY | O_TRUNC);
-    } else if (mode & COSMS_CORE_FILE_MODE_APPEND) {
-        flags |= (O_WRONLY | O_APPEND);
-    }
-
-    if (mode & COSMS_CORE_FILE_MODE_CREATE) {
-        flags |= O_CREAT;
+        flags |= O_WRONLY;
+    } 
+    
+    if (mode & COSMS_CORE_FILE_MODE_APPEND) {
+        flags |= O_APPEND;
+    } else {
+        flags |= O_TRUNC;
+        if (mode & COSMS_CORE_FILE_MODE_CREATE) {
+            flags |= O_CREAT;
+        }
     }
 
     struct stat file_stat;
@@ -79,11 +82,8 @@ CosmsCoreFileError cosms_core_file_open(struct cosms_core_file *file, const char
     }
 
     if (mode & COSMS_CORE_FILE_MODE_APPEND) {
-        access_flags |= GENERIC_WRITE;
         create_flag = OPEN_ALWAYS;
-    }
-
-    if (mode & COSMS_CORE_FILE_MODE_CREATE) {
+    } else if (mode & COSMS_CORE_FILE_MODE_CREATE) {
         create_flag = CREATE_ALWAYS;
     }
 
@@ -282,6 +282,10 @@ CosmsCoreFileError cosms_core_file_write(struct cosms_core_file *file, char *buf
         (*bytes_written) = written_bytes;
     }
     #elif defined(_WIN32)
+    if (file->mode & COSMS_CORE_FILE_MODE_APPEND) {
+        SetFilePointer(file->native_file, 0, NULL, FILE_END);
+    }
+
     if (WriteFile(file->native_file, buffer, bytes_to_write, (LPDWORD)bytes_written, NULL) == 0) {
         return COSMS_CORE_FILE_FAILED_TO_WRITE;
     }
@@ -305,7 +309,7 @@ CosmsCoreFileError cosms_core_file_write_all(struct cosms_core_file *file, char
             current_bytes_to_write = (unsigned int)remaining_bytes;
         }
 
-        int written_bytes = write(file, content + (bytes_to_write - remaining_bytes), current_bytes_to_write);
+        int written_bytes = write(file->native_file, content + (bytes_to_write - remaining_bytes), current_bytes_to_write);
         if (written_bytes == -1) {
             return COSMS_CORE_FILE_FAILED_TO_WRITE;
         }
@@ -313,6 +317,13 @@ CosmsCoreFileError cosms_core_file_write_all(struct cosms_core_file *file, char
         remaining_bytes -= written_bytes;
     }
     #elif defined(_WIN32)
+    if (file->mode & COSMS_CORE_FILE_MODE_APPEND) {
+        LARGE_INTEGER zero;
+        zero.QuadPart = 0;
+
+        SetFilePointerEx(file->native_file, zero, NULL, FILE_END);
+    }
+
     unsigned long long remaining_bytes = bytes_to_write;
     while (remaining_bytes != 0) {
         DWORD current_bytes_to_write, written_bytes;
@@ -322,7 +333,7 @@ CosmsCoreFileError cosms_core_file_write_all(struct cosms_core_file *file, char
             current_bytes_to_write = (DWORD)remaining_bytes;
         }
 
-        if (WriteFile(file, buffer + (bytes_to_write - remaining_bytes), current_bytes_to_write, &written_bytes, NULL) == 0) {
+        if (WriteFile(file->native_file, buffer + (bytes_to_write - remaining_bytes), current_bytes_to_write, &written_bytes, NULL) == 0) {
             return COSMS_CORE_FILE_FAILED_TO_WRITE;
         }
 
diff --git a/tests/data/large-write-file.txt b/tests/data/large-write-file.txt
new file mode 100644
index 0000000..888ba35
Binary files /dev/null and b/tests/data/large-write-file.txt differ
diff --git a/tests/data/small-write-file.txt b/tests/data/small-write-file.txt
new file mode 100644
index 0000000..2dd9088
--- /dev/null
+++ b/tests/data/small-write-file.txt
@@ -0,0 +1 @@
+Hello, World!Hello, World!
\ No newline at end of file
diff --git a/tests/test.h b/tests/test.h
index d740e10..095cfdd 100644
--- a/tests/test.h
+++ b/tests/test.h
@@ -29,7 +29,7 @@
             if (result == NULL) {                                                                                               \
                 printf("\r[PASS] test %s completed successfully!\n", current_test.name);                                        \
             } else {                                                                                                            \
-                printf("\r[FAIL] test %s failed with error: %s\n", current_test.name, result);                                  \
+                printf("\r[FAIL] test %s failed with error:\n\t%s\n", current_test.name, result);                               \
             }                                                                                                                   \
                                                                                                                                 \
             test_index += 1;                                                                                                    \
diff --git a/tests/unit/file.c b/tests/unit/file.c
index 9a1b1b1..b2bd68d 100644
--- a/tests/unit/file.c
+++ b/tests/unit/file.c
@@ -10,6 +10,7 @@
 
 #include <cosms-core/file.h>
 
+#include <stdlib.h>
 #include <string.h>
 
 COSMS_CORE_TEST_TEST(file_open_small_file_read,
@@ -189,10 +190,12 @@ COSMS_CORE_TEST_TEST(file_size_small_file,
     unsigned long long size = 0;
     error = cosms_core_file_get_size(&file, &size);
     if (error != COSMS_CORE_FILE_OK) {
+        cosms_core_file_close(&file);
         return cosms_core_file_error_string(error);
     }
     
     if (size != 13) {
+        cosms_core_file_close(&file);
         return "result does not match expected result";
     }
 
@@ -214,10 +217,12 @@ COSMS_CORE_TEST_TEST(file_size_large,
     unsigned long long size = 0;
     error = cosms_core_file_get_size(&file, &size);
     if (error != COSMS_CORE_FILE_OK) {
+        cosms_core_file_close(&file);
         return cosms_core_file_error_string(error);
     }
     
     if (size != 5242880000ULL) {
+        cosms_core_file_close(&file);
         return "result does not match expected result";
     }
 
@@ -255,10 +260,12 @@ COSMS_CORE_TEST_TEST(file_read_small_file,
     char buffer[14];
     error = cosms_core_file_read(&file, buffer, bytes_to_read, &bytes_read);
     if (error != COSMS_CORE_FILE_OK) {
+        cosms_core_file_close(&file);
         return cosms_core_file_error_string(error);
     }
 
     if (bytes_to_read != bytes_read || strcmp(buffer, "Hello, World!") != 0) {
+        cosms_core_file_close(&file);
         return "result does not match expected result";
     }
 
@@ -283,17 +290,15 @@ COSMS_CORE_TEST_TEST(file_read_large_file,
     char *buffer = NULL;
     error = cosms_core_file_read_all(&file, &buffer, &bytes_read);
     if (error != COSMS_CORE_FILE_OK) {
+        cosms_core_file_close(&file);
         return cosms_core_file_error_string(error);
     }
 
     char *expected_result = (char*)malloc((bytes_to_read + 1) * sizeof(char));
     if (expected_result == NULL) {
+        cosms_core_file_close(&file);
         free(buffer);
 
-        error = cosms_core_file_close(&file);
-        if (error != COSMS_CORE_FILE_OK) {
-            return cosms_core_file_error_string(error);
-        }
         return "Failed to allocate memory";
     }
 
@@ -303,6 +308,8 @@ COSMS_CORE_TEST_TEST(file_read_large_file,
 
     expected_result[bytes_to_read] = '\0';
     if (bytes_to_read != bytes_read || strcmp(buffer, expected_result) != 0) {
+        cosms_core_file_close(&file);
+
         free(expected_result);
         free(buffer);
 
@@ -369,6 +376,7 @@ COSMS_CORE_TEST_TEST(file_read_file_using_wrong_mode,
     char buffer[14];
     error = cosms_core_file_read(&file, buffer, bytes_to_read, &bytes_read);
     if (error != COSMS_CORE_FILE_INVALID_OPERATION) {
+        cosms_core_file_close(&file);
         return cosms_core_file_error_string(error);
     }
 
@@ -391,6 +399,7 @@ COSMS_CORE_TEST_TEST(file_read_full_file_using_wrong_mode,
     char *buffer = NULL;
     error = cosms_core_file_read_all(&file, &buffer, &bytes_read);
     if (error != COSMS_CORE_FILE_INVALID_OPERATION) {
+        cosms_core_file_close(&file);
         if (error == COSMS_CORE_FILE_OK) {
             free(buffer);
         }
@@ -406,7 +415,291 @@ COSMS_CORE_TEST_TEST(file_read_full_file_using_wrong_mode,
     return NULL;
 )
 
-COSMS_CORE_TEST_TEST(file_write,
+COSMS_CORE_TEST_TEST(file_write_small_file,
+    struct cosms_core_file file;
+    CosmsCoreFileError error = cosms_core_file_open(&file, "tests/data/small-write-file.txt", COSMS_CORE_FILE_MODE_WRITE | COSMS_CORE_FILE_MODE_CREATE);
+    if (error != COSMS_CORE_FILE_OK) {
+        return cosms_core_file_error_string(error);
+    }
+
+    unsigned int bytes_to_write = 13;
+    unsigned int bytes_written = 0;
+
+    char *write_buffer = "Hello, World!";
+    error = cosms_core_file_write(&file, write_buffer, bytes_to_write, &bytes_written);
+    if (error != COSMS_CORE_FILE_OK) {
+        cosms_core_file_close(&file);
+        return cosms_core_file_error_string(error);
+    }
+
+    if (bytes_to_write != bytes_written) {
+        cosms_core_file_close(&file);
+        return "did not write all bytes to file";
+    }
+
+    error = cosms_core_file_close(&file);
+    if (error != COSMS_CORE_FILE_OK) {
+        return cosms_core_file_error_string(error);
+    }
+
+    error = cosms_core_file_open(&file, "tests/data/small-write-file.txt", COSMS_CORE_FILE_MODE_READ);
+    if (error != COSMS_CORE_FILE_OK) {
+        return cosms_core_file_error_string(error);
+    }
+
+    char read_buffer[14] = { 0 };
+    error = cosms_core_file_read(&file, read_buffer, bytes_to_write, NULL);
+    if (error != COSMS_CORE_FILE_OK) {
+        cosms_core_file_close(&file);
+        return cosms_core_file_error_string(error);
+    }
+
+    if (strcmp(read_buffer, write_buffer) != 0) {
+        cosms_core_file_close(&file);
+        return "result does not match expected result";
+    }
+
+    error = cosms_core_file_close(&file);
+    if (error != COSMS_CORE_FILE_OK) {
+        return cosms_core_file_error_string(error);
+    }
+
+    return NULL;
+)
+
+COSMS_CORE_TEST_TEST(file_write_large_file,
+    struct cosms_core_file file;
+    CosmsCoreFileError error = cosms_core_file_open(&file, "tests/data/large-write-file.txt", COSMS_CORE_FILE_MODE_WRITE | COSMS_CORE_FILE_MODE_CREATE);
+    if (error != COSMS_CORE_FILE_OK) {
+        return cosms_core_file_error_string(error);
+    }
+
+    unsigned long long bytes_to_write = 5242880000ULL;
+    char *write_buffer = (char*)malloc((bytes_to_write + 1) * sizeof(char));
+    for (unsigned long long index = 0; index < bytes_to_write; index += 1) {
+        write_buffer[index] = 'a';
+    }
+
+    write_buffer[bytes_to_write] = '\0';
+    error = cosms_core_file_write_all(&file, write_buffer, bytes_to_write);
+    if (error != COSMS_CORE_FILE_OK) {
+        cosms_core_file_close(&file);
+        free(write_buffer);
+
+        return cosms_core_file_error_string(error);
+    }
+
+    error = cosms_core_file_close(&file);
+    if (error != COSMS_CORE_FILE_OK) {
+        free(write_buffer);
+        return cosms_core_file_error_string(error);
+    }
+
+    error = cosms_core_file_open(&file, "tests/data/large-write-file.txt", COSMS_CORE_FILE_MODE_READ);
+    if (error != COSMS_CORE_FILE_OK) {
+        free(write_buffer);
+        return cosms_core_file_error_string(error);
+    }
+
+    char *read_buffer = NULL;
+    error = cosms_core_file_read_all(&file, &read_buffer, NULL);
+    if (error != COSMS_CORE_FILE_OK) {
+        cosms_core_file_close(&file);
+        free(write_buffer);
+
+        return cosms_core_file_error_string(error);
+    }
+
+    if (strcmp(read_buffer, write_buffer) != 0) {
+        cosms_core_file_close(&file);
+
+        free(read_buffer);
+        free(write_buffer);
+
+        return "result does not match expected result";
+    }
+
+    free(read_buffer);
+    free(write_buffer);
+
+    error = cosms_core_file_close(&file);
+    if (error != COSMS_CORE_FILE_OK) {
+        return cosms_core_file_error_string(error);
+    }
+
+    return NULL;
+)
+
+COSMS_CORE_TEST_TEST(file_write_append_small_file,
+    struct cosms_core_file file;
+    CosmsCoreFileError error = cosms_core_file_open(&file, "tests/data/small-write-file.txt", COSMS_CORE_FILE_MODE_WRITE | COSMS_CORE_FILE_MODE_APPEND);
+    if (error != COSMS_CORE_FILE_OK) {
+        return cosms_core_file_error_string(error);
+    }
+
+    unsigned int bytes_to_write = 13;
+    unsigned int bytes_written = 0;
+
+    char *write_buffer = "Hello, World!";
+    error = cosms_core_file_write(&file, write_buffer, bytes_to_write, &bytes_written);
+    if (error != COSMS_CORE_FILE_OK) {
+        cosms_core_file_close(&file);
+        return cosms_core_file_error_string(error);
+    }
+
+    if (bytes_to_write != bytes_written) {
+        cosms_core_file_close(&file);
+        return "did not write all bytes to file";
+    }
+
+    error = cosms_core_file_close(&file);
+    if (error != COSMS_CORE_FILE_OK) {
+        return cosms_core_file_error_string(error);
+    }
+
+    error = cosms_core_file_open(&file, "tests/data/small-write-file.txt", COSMS_CORE_FILE_MODE_READ);
+    if (error != COSMS_CORE_FILE_OK) {
+        return cosms_core_file_error_string(error);
+    }
+
+    unsigned int bytes_to_read = 26;
+    char read_buffer[27] = { 0 };
+    error = cosms_core_file_read(&file, read_buffer, bytes_to_read, NULL);
+    if (error != COSMS_CORE_FILE_OK) {
+        cosms_core_file_close(&file);
+        return cosms_core_file_error_string(error);
+    }
+
+    if (strcmp(read_buffer, "Hello, World!Hello, World!") != 0) {
+        cosms_core_file_close(&file);
+        return "result does not match expected result";
+    }
+
+    error = cosms_core_file_close(&file);
+    if (error != COSMS_CORE_FILE_OK) {
+        return cosms_core_file_error_string(error);
+    }
+
+    return NULL;
+)
+
+COSMS_CORE_TEST_TEST(file_write_append_large_file,
+    struct cosms_core_file file;
+    CosmsCoreFileError error = cosms_core_file_open(&file, "tests/data/large-write-file.txt", COSMS_CORE_FILE_MODE_WRITE | COSMS_CORE_FILE_MODE_APPEND);
+    if (error != COSMS_CORE_FILE_OK) {
+        return cosms_core_file_error_string(error);
+    }
+
+    unsigned long long bytes_to_write = 2;
+    char *write_buffer = "bb";
+    error = cosms_core_file_write_all(&file, write_buffer, bytes_to_write);
+    if (error != COSMS_CORE_FILE_OK) {
+        cosms_core_file_close(&file);
+        return cosms_core_file_error_string(error);
+    }
+
+    error = cosms_core_file_close(&file);
+    if (error != COSMS_CORE_FILE_OK) {
+        return cosms_core_file_error_string(error);
+    }
+
+    error = cosms_core_file_open(&file, "tests/data/large-write-file.txt", COSMS_CORE_FILE_MODE_READ);
+    if (error != COSMS_CORE_FILE_OK) {
+        return cosms_core_file_error_string(error);
+    }
+
+    unsigned long long bytes_to_read = 5242880002ULL;
+    unsigned long long bytes_read = 0;
+    char *read_buffer = NULL;
+    error = cosms_core_file_read_all(&file, &read_buffer, &bytes_read);
+    if (error != COSMS_CORE_FILE_OK) {
+        cosms_core_file_close(&file);
+        return cosms_core_file_error_string(error);
+    }
+
+    char *expected_result = (char*)malloc((bytes_to_read + 1) * sizeof(char));
+    for (unsigned long long index = 0; index < bytes_to_read; index += 1) { 
+        expected_result[index] = 'a';
+    }
+
+    expected_result[bytes_to_read - 2] = 'b';
+    expected_result[bytes_to_read - 1] = 'b';
+    expected_result[bytes_to_read] = '\0';
+
+    if (strcmp(read_buffer, expected_result) != 0) {
+        cosms_core_file_close(&file);
+
+        free(expected_result);
+        free(read_buffer);
+        
+        return "result does not match expected result";
+    }
+
+    free(expected_result);
+    free(read_buffer);
+
+    error = cosms_core_file_close(&file);
+    if (error != COSMS_CORE_FILE_OK) {
+        return cosms_core_file_error_string(error);
+    }
+
+    return NULL;
+)
+
+COSMS_CORE_TEST_TEST(file_write_using_wrong_mode,
+    struct cosms_core_file file;
+    CosmsCoreFileError error = cosms_core_file_open(&file, "tests/data/small-write-file.txt", COSMS_CORE_FILE_MODE_READ);
+    if (error != COSMS_CORE_FILE_OK) {
+        return cosms_core_file_error_string(error);
+    }
+
+    unsigned int bytes_to_write = 13;
+    unsigned int bytes_written = 0;
+
+    char *write_buffer = "Hello, World!";
+    error = cosms_core_file_write(&file, write_buffer, bytes_to_write, &bytes_written);
+    if (error != COSMS_CORE_FILE_INVALID_OPERATION) {
+        cosms_core_file_close(&file);
+        return cosms_core_file_error_string(error);
+    }
+
+    error = cosms_core_file_close(&file);
+    if (error != COSMS_CORE_FILE_OK) {
+        free(write_buffer);
+        return cosms_core_file_error_string(error);
+    }
+
+    return NULL;
+)
+
+COSMS_CORE_TEST_TEST(file_write_full_file_using_wrong_mode,
+    struct cosms_core_file file;
+    CosmsCoreFileError error = cosms_core_file_open(&file, "tests/data/large-write-file.txt", COSMS_CORE_FILE_MODE_READ);
+    if (error != COSMS_CORE_FILE_OK) {
+        return cosms_core_file_error_string(error);
+    }
+
+    unsigned long long bytes_to_write = 5242880000ULL;
+    char *write_buffer = (char*)malloc((bytes_to_write + 1) * sizeof(char));
+    for (unsigned long long index = 0; index < bytes_to_write; index += 1) {
+        write_buffer[index] = 'a';
+    }
+
+    write_buffer[bytes_to_write] = '\0';
+    error = cosms_core_file_write_all(&file, write_buffer, bytes_to_write);
+    if (error != COSMS_CORE_FILE_INVALID_OPERATION) {
+        cosms_core_file_close(&file);
+        free(write_buffer);
+
+        return cosms_core_file_error_string(error);
+    }
+
+    error = cosms_core_file_close(&file);
+    if (error != COSMS_CORE_FILE_OK) {
+        free(write_buffer);
+        return cosms_core_file_error_string(error);
+    }
+
     return NULL;
 )
 
diff --git a/tests/unit/file.h b/tests/unit/file.h
index 9c662a7..adbe567 100644
--- a/tests/unit/file.h
+++ b/tests/unit/file.h
@@ -35,7 +35,13 @@ COSMS_CORE_TEST_DEFINE(file_read_full_non_existing_file);
 COSMS_CORE_TEST_DEFINE(file_read_file_using_wrong_mode);
 COSMS_CORE_TEST_DEFINE(file_read_full_file_using_wrong_mode);
 
-COSMS_CORE_TEST_DEFINE(file_write);
+COSMS_CORE_TEST_DEFINE(file_write_small_file);
+COSMS_CORE_TEST_DEFINE(file_write_large_file);
+COSMS_CORE_TEST_DEFINE(file_write_append_small_file);
+COSMS_CORE_TEST_DEFINE(file_write_append_large_file);
+COSMS_CORE_TEST_DEFINE(file_write_using_wrong_mode);
+COSMS_CORE_TEST_DEFINE(file_write_full_file_using_wrong_mode);
+
 COSMS_CORE_TEST_DEFINE(file_delete);
 
 COSMS_CORE_TEST_EXPORT(file, 
@@ -63,7 +69,13 @@ COSMS_CORE_TEST_EXPORT(file,
     COSMS_CORE_TEST_EXPORT_TEST(file_read_file_using_wrong_mode),
     COSMS_CORE_TEST_EXPORT_TEST(file_read_full_file_using_wrong_mode),
 
-    COSMS_CORE_TEST_EXPORT_TEST(file_write),
+    COSMS_CORE_TEST_EXPORT_TEST(file_write_small_file),
+    COSMS_CORE_TEST_EXPORT_TEST(file_write_large_file),
+    COSMS_CORE_TEST_EXPORT_TEST(file_write_append_small_file),
+    COSMS_CORE_TEST_EXPORT_TEST(file_write_append_large_file),
+    COSMS_CORE_TEST_EXPORT_TEST(file_write_using_wrong_mode),
+    COSMS_CORE_TEST_EXPORT_TEST(file_write_full_file_using_wrong_mode),
+
     COSMS_CORE_TEST_EXPORT_TEST(file_delete)
 );