TRUNCATE(2) System Calls Manual TRUNCATE(2)

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

#include <unistd.h>

int
ftruncate(int fildes, off_t length);

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

() and () cause the file named by path, or referenced by fildes, to be truncated (or extended) to length bytes in size. If the file size exceeds length, any extra data is discarded. If the file size is smaller than length, the file is extended and filled with zeros to the indicated length. The ftruncate() form requires the file to be open for writing.

Note: () and () do not modify the current file offset for any open file descriptions associated with 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.

The ftruncate() system call will fail if:

[]
fildes is not a valid descriptor open for writing.
[]
The file is a regular file and length is greater than the offset maximum established in the open file description associated with fildes.
[]
fildes references a socket, not a file.
[]
fildes references a symbolic link.
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fildes is not open for writing.
[]
The named file has its append-only flag set, see the chflags(2) manual page for more information.
[]
The named file resides on a read-only file system.

The truncate() system call will fail if:

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Search permission is denied for a component of the path prefix.
[]
The named file is not writable by the user.
[]
Path points outside the process's allocated address space.
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The named file is a directory.
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Too many symbolic links are encountered in translating the pathname. This is taken to be indicative of a looping symbolic link.
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A component of a pathname exceeds {NAME_MAX} characters, or an entire path name exceeds {PATH_MAX} characters.
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The named file does not exist.
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A component of the path prefix of path names an existing file that is neither a directory nor a symbolic link to a directory, or path contains at least one non-<slash> character and ends with one or more trailing <slash> characters and the last pathname component names an existing file that is neither a directory nor a symbolic link to a directory.
[]
A component of the pathname refers to a “dataless” directory that requires materialization and the I/O policy of the current thread or process disallows dataless directory materialization (see getiopolicy_np(3)).
[]
The named file resides on a read-only file system.
[]
The file is a pure procedure (shared text) file that is being executed.

The ftruncate() and truncate() system calls will fail if:

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The length argument was greater than the maximum file size.
[]
A signal is caught during execution.
[]
The length argument is less than 0.
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An I/O error occurred while reading from or writing to a file system.
[]
The file is a “dataless” file that requires materialization and the I/O policy of the current thread or process disallows dataless file materialization (see getiopolicy_np(3)).

open(2)

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.

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

June 3, 2021 BSD 4.2