The Open Group Base Specifications Issue 6
IEEE Std 1003.1, 2004 Edition
Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.
A newer edition of this document exists here


ftruncate - truncate a file to a specified length


#include <unistd.h>

int ftruncate(int
fildes, off_t length);


If fildes is not a valid file descriptor open for writing, the ftruncate() function shall fail.

If fildes refers to a regular file, the ftruncate() function shall cause the size of the file to be truncated to length. If the size of the file previously exceeded length, the extra data shall no longer be available to reads on the file. If the file previously was smaller than this size, ftruncate() shall either increase the size of the file or fail. [XSI] [Option Start]  XSI-conformant systems shall increase the size of the file. [Option End] If the file size is increased, the extended area shall appear as if it were zero-filled. The value of the seek pointer shall not be modified by a call to ftruncate().

Upon successful completion, if fildes refers to a regular file, the ftruncate() function shall mark for update the st_ctime and st_mtime fields of the file and the S_ISUID and S_ISGID bits of the file mode may be cleared. If the ftruncate() function is unsuccessful, the file is unaffected.

[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 thread. [Option End]

If fildes refers to a directory, ftruncate() shall fail.

If fildes refers to any other file type, except a shared memory object, the result is unspecified.

[SHM] [Option Start] If fildes refers to a shared memory object, ftruncate() shall set the size of the shared memory object to length. [Option End]

[MF|SHM] [Option Start] If the effect of ftruncate() is to decrease the size of a shared memory object or memory mapped file and whole pages beyond the new end were previously mapped, then the whole pages beyond the new end shall be discarded. [Option End]

[MPR] [Option Start] If the Memory Protection option is supported, references to discarded pages shall result in the generation of a SIGBUS signal; otherwise, the result of such references is undefined. [Option End]

[MF|SHM] [Option Start] If the effect of ftruncate() is to increase the size of a shared memory object, it is unspecified whether the contents of any mapped pages between the old end-of-file and the new are flushed to the underlying object. [Option End]


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


The ftruncate() 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.
[XSI] [Option Start] The file is a regular file and length is greater than the offset maximum established in the open file description associated with fildes. [Option End]
An I/O error occurred while reading from or writing to a file system.
The fildes argument is not a file descriptor open for writing.
The fildes argument references a file that was opened without write permission.
The named file resides on a read-only file system.

The following sections are informative.






The ftruncate() function is part of IEEE Std 1003.1-2001 as it was deemed to be more useful than truncate(). The truncate() function is provided as an XSI extension.




open(), truncate(), the Base Definitions volume of IEEE Std 1003.1-2001, <unistd.h>


First released in Issue 4, Version 2.

Issue 5

Moved from X/OPEN UNIX extension to BASE and aligned with ftruncate() in the POSIX Realtime Extension. Specifically, the DESCRIPTION is extensively reworded and [EROFS] is added to the list of mandatory errors that can be returned by ftruncate().

Large File Summit extensions are added.

Issue 6

The truncate() function is split out into a separate reference page.

The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:

The following changes were made to align with the IEEE P1003.1a draft standard:

XSI-conformant systems are required to increase the size of the file if the file was previously smaller than the size requested.

End of informative text.

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