[SOLVED] WriteFile fails to append but always overwrites

WriteFile fails to append but always overwrites

HANDLE hFile = CreateFile("test.txt", FILE_GENERIC_WRITE, FILE_SHARE_READ, NULL, OPEN_ALWAYS, FILE_ATTRIBUTE_NORMAL, NULL);

ULONG nWritten = 0;
WriteFile(hFile, "line-to-be-appened\r\n", strlen("line-to-be-appened\r\n"), &nWritten, NULL);

CloseFile(hFile);

When the above code snippet opens an existing file test.txt and is expected to append "line-to-be-appened\r\n" to the end of the file. However, it just overwrites the file.

How can I make it append at the end?

Solution

When you are opening the file, its read/write pointer is at the beginning of the file by default. Thus, you are writing at the front of the file, overwriting whatever is already there.

To append at the end of the file, you need to seek to the end of the file before writing, either by:

seeking manually via SetFilePointer/Ex(), eg:

HANDLE hFile = CreateFile("test.txt", FILE_GENERIC_WRITE, FILE_SHARE_READ, NULL, OPEN_ALWAYS, FILE_ATTRIBUTE_NORMAL, NULL);

SetFilePointer(hFile, 0, NULL, FILE_END); // <-- add this

ULONG nWritten = 0;
WriteFile(hFile, "line-to-be-appened\r\n", strlen("line-to-be-appened\r\n"), &nWritten, NULL);

CloseFile(hFile);

open the file with just the FILE_APPEND_DATA flag by itself, which will then handle the seeking for you, eg:

HANDLE hFile = CreateFile("test.txt", FILE_APPEND_DATA, FILE_SHARE_READ, NULL, OPEN_ALWAYS, FILE_ATTRIBUTE_NORMAL, NULL);

ULONG nWritten = 0;
WriteFile(hFile, "line-to-be-appened\r\n", strlen("line-to-be-appened\r\n"), &nWritten, NULL);

CloseFile(hFile);

FILE_GENERIC_WRITE includes both FILE_WRITE_DATA and FILE_APPEND_DATA, but FILE_WRITE_DATA prevents FILE_APPEND_DATA from seeking automatically:

For local files, write operations will not overwrite existing data if [FILE_APPEND_DATA] is specified without FILE_WRITE_DATA.