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TRUNCATE(2) System Calls Manual TRUNCATE(2)

truncate, ftruncate — truncate or extend a file to a specified length

#include <unistd.h>

int
truncate(const char *path, off_t length);

int
ftruncate(int fd, off_t length);

() causes the file named by path or referenced by fd to be truncated or extended to length bytes in size. If the file was larger than this size, the extra data is lost. If the file was smaller than this size, it will be extended as if by writing bytes with the value zero. With (), the file must be open for writing.

A value of 0 is returned if the call succeeds. If the call fails a -1 is returned, and the global variable errno specifies the error.

truncate() and ftruncate() will succeed unless:

[]
The length is a negative value.
[]
An I/O error occurred updating the inode.

In addition, truncate() may return the following errors:

[]
A component of the path prefix is not a directory.
[]
A component of a pathname exceeded {NAME_MAX} characters, or an entire path name exceeded {PATH_MAX} characters.
[]
The named file does not exist.
[]
Search permission is denied for a component of the path prefix.
[]
The named file is not writable by the user.
[]
Too many symbolic links were encountered in translating the pathname.
[]
The named file is a directory.
[]
The named file resides on a read-only file system.
[]
The file is a pure procedure (shared text) file that is being executed.
[]
path points outside the process's allocated address space.

ftruncate() may return the following errors:

[]
The fd is not a valid descriptor.
[]
The fd references a socket, not a file.
[]
The fd is not open for writing.

open(2)

The truncate() and ftruncate() functions conform to IEEE Std 1003.1-2008 (“POSIX.1”).

The truncate() and ftruncate() system calls first appeared in 4.1cBSD.

These calls should be generalized to allow ranges of bytes in a file to be discarded.

Use of truncate() to extend a file is not portable.

July 17, 2013 OpenBSD-5.6