ftruncate (original) (raw)

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] XSI-conformant systems 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, 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] 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.

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] If fildes refers to a shared memory object, ftruncate() shall set the size of the shared memory object to_length_.

[MF|SHM] 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.

[MPR] 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.

[MF|SHM] 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.

The following sections are informative.

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 DESCRIPTION is changed to indicate that if the file size is changed, and if the file is a regular file, the S_ISUID and S_ISGID bits in the file mode may be cleared.

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

• The DESCRIPTION text is updated.

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.