#Can you safely delete zero byte files portable#

You might consider putting the call to fileno() right in the parameter list for the ftruncate() call so that you don't keep the file descriptor around and accidentally use it yet other ways which might further confuse the internal state of STDIO. You can then use fileno() on the STDIO stream's FILE handle to find the underlying file descriptor for the open STDIO stream, and you would then use that file descriptor with ftruncate(). Linc no longer participates in the forums. Can you offer a suggestion Thanks in advance. When I try to just delete it, I get 'you don't have permission, but the file itself in Get Info says 'You have custom access' and there is no lock icon to unlock the file permissions and adjust them.

This will avoid STDIO trying to flush the otherwise unwritten buffer to the file after the truncation has been done. It's a zero byte file, but it's not greyed out. So, to use ftruncate() safely with STDIO it may be necessary to first flush any STDIO buffers (with fflush()) if your program may have already written to the stream in question. Note also that mixing operations on the STDIO stream and on the underlying OS calls which operate on the file descriptor for the open stream can confuse the internal runtime state of the STDIO library. Note that ftruncate() operates on a POSIX file descriptor (despite its potentially misleading name), not a STDIO stream FILE handle. I conclude that current possible best practice is: Set the stream to unbuffered mode by calling setbuf(stream, NULL), then truncate() or _chsize_s() can work peacefully with the stream.Īnyhow, no official document seems to explicitly confirm this behavior, whether Microsoft CRT or GNU glibc.įtruncate() is what you're looking for, and it's been in POSIX base specifications since 2001, so it should be in every modern POSIX-compatible system by now. Similar question: When querying file size from a file-handle returned by fileno, is it the accurate size when I later call fclose() - without further fwrite()?Īccording to Joshua's suggestion.

#Can you safely delete zero byte files portable#

If no guarantee, what is the best practice of using file I/O functions accompanied with truncate operation when write a Windows-Linux portable program? In case the buffered bytes is beyond the truncated point, will the buffered content be flushed to the file when doing fclose() ? My question is: Is it reliable to continue using FILE* after truncating the file? I mean, ANSI C FILE layer has its own buffer and does not know the file is truncated from beneath. But before we can call those OS specific functions, we have to obtain the file-handle from FILE pointer, by calling fileno, also an non-ANSI-C one. However, when it comes to truncating a file, we have to turn to OS specific function, e.g, truncate() on Linux, _chsize_s_() on Windows.

I know ANSI C defines fopen, fwrite, fread, fclose to modify a file's content.