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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() succeeds unless:

[]
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.
[]
An I/O error occurred updating the inode.
[]
path points outside the process's allocated address space.
[]
The effective user ID does not match the owner of the file and the effective user ID is not the superuser.

ftruncate() succeeds unless:

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

open(2)

The truncate() and ftruncate() function calls appeared in 4.2BSD.

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.

May 31, 2007 OpenBSD-5.1