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-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
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.
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.
If fildes refers to a shared memory object, ftruncate() shall set
the size of the shared memory object to length.
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.
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.
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.
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard, the original
IEEE and The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.opengroup.org/unix/online.html