The Open Group Base Specifications Issue 7
IEEE Std 1003.1-2008, 2016 Edition
Copyright © 2001-2016 The IEEE and The Open Group


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 increase the size of the file. 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, ftruncate() 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. 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]

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

References to discarded pages shall result in the generation of a SIGBUS signal.

If the effect of ftruncate() is to increase the size of a 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.


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.
The file is a regular file and length is greater than the offset maximum established in the open file description associated with fildes.
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 following sections are informative.










open, truncate

XBD <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.

Issue 7

Austin Group Interpretation 1003.1-2001 #056 is applied, revising the ERRORS section (although the [EINVAL] ``may fail'' error was subsequently removed during review of the XSI option).

Functionality relating to the Memory Protection and Memory Mapped Files options is moved to the Base.

The DESCRIPTION is updated so that a call to ftruncate() when the file is smaller than the size requested will increase the size of the file. Previously, non-XSI-conforming implementations were allowed to increase the size of the file or fail.

Changes are made related to support for finegrained timestamps.

End of informative text.


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