4.13. File Truncation

There are times when we would like to truncate a file by chopping off data at the end of the file. Emptying a file, which we can do with the O_TRUNC flag to open, is a special case of truncation.

#include <unistd.h> int truncate(const char *pathname, off_t length); int ftruncate(int filedes, off_t length);
Both return: 0 if OK, 1 on error

These two functions truncate an existing file to length bytes. If the previous size of the file was greater than length, the data beyond length is no longer accessible. If the previous size was less than length, the effect is system dependent, but XSI-conforming systems will increase the file size. If the implementation does extend a file, data between the old end of file and the new end of file will read as 0 (i.e., a hole is probably created in the file).

The ftruncate function is part of POSIX.1. The truncate function is an XSI extension to the POSIX.1 functionality defined in the Single UNIX Specification.

BSD releases prior to 4.4BSD could only make a file smaller with TRuncate.

Solaris also includes an extension to fcntl (F_FREESP) that allows us to free any part of a file, not just a chunk at the end of the file.

We use ftruncate in the program shown in Figure 13.6 when we need to empty a file after obtaining a lock on the file.