PROLOG

       This  manual  page  is part of the POSIX Programmer's Manual.  The Linux implementation of
       this interface may differ (consult the corresponding Linux  manual  page  for  details  of
       Linux behavior), or the interface may not be implemented on Linux.

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 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.

       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 memory mapped file or  a  shared
       memory  object  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.

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.

       The following sections are informative.

EXAMPLES

       None.

APPLICATION USAGE

       None.

RATIONALE

       None.

FUTURE DIRECTIONS

       None.

SEE ALSO

       open(), truncate()

       The Base Definitions volume of POSIX.1‐2017, <unistd.h>

COPYRIGHT

       Portions of this text are reprinted and  reproduced  in  electronic  form  from  IEEE  Std
       1003.1-2017,  Standard  for  Information Technology -- Portable Operating System Interface
       (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C)  2018  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 .

       Any typographical or formatting errors that appear in this page are most  likely  to  have
       been  introduced  during  the conversion of the source files to man page format. To report
       such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .