Clearing Contents of a File in C++ knowing only the FILE *

Question

Is it possible to clear the contents (ie. set EOF to the beginning/reset the file) in C++ knowing just the FILE*? I'm writing to a temp file with wb+ access and wish to sometimes clear it and truncate it without adding the calls to fclose and fopen. I dont think it's possible... but if not, why not?

Thanks in advance!

Answer 1

I don't believe this can be done using just the FILE*. You can always write null data through the end of the file but that won't truncate it.

Alternately if you have access to the filename (I can't tell from the question) you could use freopen which hides the close/open/truncate into a single function call.

Answer 2

It will depend on your platform. The POSIX standard provides ftruncate(), which requires a file descriptor, not a FILE pointer, but it also provides fileno() to get the file descriptor from the FILE pointer.

The analogous facilities will be available in Windows environments - but under different names.

Answer 3

#include <cstdio>

freopen(null, "w", filePtr);

see http://www.cplusplus.com/reference/clibrary/cstdio/freopen/ for more. espacialy the description for the parameter filename.