Check the file-size without opening file in C++?

Question

I'm trying to get the filesize of a large file (12gb+) and I don't want to open the file to do so as I assume this would eat a lot of resources. Is there any good API to do so with? I'm in a Windows environment.

Answer 1

You should call GetFileSizeEx which is easier to use than the older GetFileSize. You will need to open the file by calling CreateFile but that's a cheap operation. Your assumption that opening a file is expensive, even a 12GB file, is false.

You could use the following function to get the job done:

__int64 FileSize(const wchar_t* name) {     HANDLE hFile = CreateFile(name, GENERIC_READ,          FILE_SHARE_READ | FILE_SHARE_WRITE, NULL, OPEN_EXISTING,          FILE_ATTRIBUTE_NORMAL, NULL);     if (hFile==INVALID_HANDLE_VALUE)         return -1; // error condition, could call GetLastError to find out more      LARGE_INTEGER size;     if (!GetFileSizeEx(hFile, &size))     {         CloseHandle(hFile);         return -1; // error condition, could call GetLastError to find out more     }      CloseHandle(hFile);     return size.QuadPart; } 

There are other API calls that will return you the file size without forcing you to create a file handle, notably GetFileAttributesEx. However, it's perfectly plausible that this function will just open the file behind the scenes.

__int64 FileSize(const wchar_t* name) {     WIN32_FILE_ATTRIBUTE_DATA fad;     if (!GetFileAttributesEx(name, GetFileExInfoStandard, &fad))         return -1; // error condition, could call GetLastError to find out more     LARGE_INTEGER size;     size.HighPart = fad.nFileSizeHigh;     size.LowPart = fad.nFileSizeLow;     return size.QuadPart; } 

If you are compiling with Visual Studio and want to avoid calling Win32 APIs then you can use _wstat64.

Here is a _wstat64 based version of the function:

__int64 FileSize(const wchar_t* name) {     __stat64 buf;     if (_wstat64(name, &buf) != 0)         return -1; // error, could use errno to find out more      return buf.st_size; }  

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If performance ever became an issue for you then you should time the various options on all the platforms that you target in order to reach a decision. Don't assume that the APIs that don't require you to call CreateFile will be faster. They might be but you won't know until you have timed it.