Truncating a File: truncate() and ftruncate()

The truncate() and ftruncate() system calls set the size of a file to the value specified by length.

#include <unistd.h>

int truncate(const char *pathname, off_t length);
int ftruncate(int fd, off_t length);

Note

Both return 0 on success, or -1 on error

If the file is longer than length, the excess data is lost. If the file is currently shorter than length, it is extended by padding with a sequence of null bytes or a hole.

The difference between the two system calls lies in how the file is specified. With truncate(), the file, which must be accessible and writable, is specified as a pathname string. If pathname is a symbolic link, it is dereferenced. The ftruncate() system call takes a descriptor for a file that has been opened for writing. It doesn’t change the file offset for the file.

If the length argument to ftruncate() exceeds the current file size, SUSv3 allows two possible behaviors: either the file is extended (as on Linux) or the system call returns an error. XSI-conformant systems must adopt the former behavior. SUSv3 requires that truncate() always extend the file if length is greater than the current file size.

Note

The truncate() system call is unique in being the only system call that can change the contents of a file without first obtaining a descriptor for the file via open() (or by some other means).