NAME

       ftruncate - truncate a file to a specified length

SYNOPSIS

       #include <unistd.h>

       int ftruncate(int fildes, off_t length);

DESCRIPTION

       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 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.

       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.

RETURN VALUE

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

ERRORS

       The ftruncate() function shall fail if:

       EINTR  A signal was caught during execution.

       EINVAL The length argument was less than 0.

       EFBIG or EINVAL

              The length argument was greater than the maximum file size.

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

       EIO    An I/O error occurred while reading from or writing to a file system.

       EBADF or EINVAL

              The fildes argument is not a file descriptor open for writing.

       EINVAL The fildes argument references a file that was opened without write permission.

       EROFS  The named file resides on a read-only file system.

       The following sections are informative.

EXAMPLES

       None.

APPLICATION USAGE

       None.

RATIONALE

       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.

FUTURE DIRECTIONS

       None.

SEE ALSO

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

COPYRIGHT

       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 .