How to truncate a file in C?

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If you want to preserve the previous contents of the file up to some length (a length bigger than zero, which other answers provide), then POSIX provides the truncate() and ftruncate() functions for the job.

#include <unistd.h>
int ftruncate(int fildes, off_t length);
int truncate(const char *path, off_t length);

The name indicates the primary purpose – shortening a file. But if the specified length is longer than the previous length, the file grows (zero padding) to the new size. Note that ftruncate() works on a file descriptor, not a FILE *; you could use:

if (ftruncate(fileno(fp), new_length) != 0) ...error handling...

However, you should be aware that mixing file stream (FILE *) and file descriptor (int) access to a single file is apt to lead to confusion — see the comments for some of the issues. This should be a last resort.

It is likely, though, that for your purposes, truncate on open is all you need, and for that, the options given by others will be sufficient.

For Windows, there is a function SetEndOfFile() and a related function SetFileValidData() function that can do a similar job, but using a different interface. Basically, you seek to where you want to set the end of file and then call the function.

There’s also a function _chsize() as documented in the answer by sofr.

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