The Open Group Base Specifications Issue 7, 2018 edition
IEEE Std 1003.1-2017 (Revision of IEEE Std 1003.1-2008)
Copyright © 2001-2018 IEEE and The Open Group


truncate - truncate a file to a specified length


#include <unistd.h>

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


The truncate() function shall cause the regular file named by path to have a size which shall be equal to length bytes.

If the file previously was larger than length, the extra data is discarded. If the file was previously shorter than length, its size is increased, and the extended area appears as if it were zero-filled.

The application shall ensure that the process has write permission for the file.

[XSI] [Option Start] If the request would cause the file size to exceed the soft file size limit for the process, the request shall fail and the implementation shall generate the SIGXFSZ signal for the process. [Option End]

The truncate() function shall not modify the file offset for any open file descriptions associated with the file. Upon successful completion, truncate() shall mark for update the last data modification and last file status change timestamps of the file, and the S_ISUID and S_ISGID bits of the file mode may be cleared.


Upon successful completion, truncate() shall return 0. Otherwise, -1 shall be returned, and errno set to indicate the error.


The truncate() function shall fail if:

A signal was caught during execution.
The length argument was less than 0.
The length argument was greater than the maximum file size.
An I/O error occurred while reading from or writing to a file system.
A component of the path prefix denies search permission, or write permission is denied on the file.
The named file is a directory.
A loop exists in symbolic links encountered during resolution of the path argument.
The length of a component of a pathname is longer than {NAME_MAX}.
A component of path does not name an existing file or path is an empty string.
A component of the path prefix names an existing file that is neither a directory nor a symbolic link to a directory, or the path argument 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.
The named file resides on a read-only file system.

The truncate() function may fail if:

More than {SYMLOOP_MAX} symbolic links were encountered during resolution of the path argument.
The length of a pathname exceeds {PATH_MAX}, or pathname resolution of a symbolic link produced an intermediate result with a length that exceeds {PATH_MAX}.

The following sections are informative.











XBD <unistd.h>


First released in Issue 4, Version 2.

Issue 5

Moved from X/OPEN UNIX extension to BASE.

Large File Summit extensions are added.

Issue 6

This reference page is split out from the ftruncate() reference page.

The normative text is updated to avoid use of the term "must" for application requirements.

The wording of the mandatory [ELOOP] error condition is updated, and a second optional [ELOOP] error condition is added.

Issue 7

Austin Group Interpretation 1003.1-2001 #143 is applied.

The truncate() function is moved from the XSI option to the Base.

Changes are made related to support for finegrained timestamps.

The [ENOTDIR] error condition is clarified to cover the condition where the last component of a pathname exists but is not a directory or a symbolic link to a directory.

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0685 [324] is applied.

POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0375 [489] is applied.

End of informative text.


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