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TRUNCATE(2) System Calls Manual TRUNCATE(2)

truncate, ftruncate — truncate a file to a specified length

library “libc”

#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 have a size of length bytes. If the file previously was larger than this size, the extra data is discarded. If it was previously shorter than length, its size is increased to the specified value and the extended area appears as if it were zero-filled.

With (), the file must be open for writing; for truncate(), the process must have write permissions for the file.

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.

Error return codes common to truncate() and ftruncate() are:

[]
The length argument was less than 0.
[]
An I/O error occurred updating the inode.
[]
The named file is a directory.
[]
There was no space in the file system to complete the operation.
[]
The named file resides on a read-only file system.
[]
The file is a pure procedure (shared text) file that is being executed.

Error codes specific to truncate() are:

[]
Search permission is denied for a component of the path prefix, or the named file is not writable by the user.
[]
path points outside the process's allocated address space.
[]
Too many symbolic links were encountered in translating the pathname.
[]
A component of a pathname exceeded {NAME_MAX} characters, or an entire path name exceeded {PATH_MAX} characters.
[]
The named file does not exist.
[]
A component of the path prefix is not a directory.

Error codes specific to ftruncate() are:

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

fdiscard(2), open(2)

Use of truncate() to extend a file is an IEEE Std 1003.1-2004 (“POSIX.1”) extension, and is thus not portable. Files can be extended in a portable way seeking (using lseek(2)) to the required size and writing a single character with write(2).

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

July 23, 2014 NetBSD-9.2