1FTRUNCATE(3P)              POSIX Programmer's Manual             FTRUNCATE(3P)
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PROLOG

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

12       ftruncate - truncate a file to a specified length
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SYNOPSIS

15       #include <unistd.h>
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17       int ftruncate(int fildes, off_t length);
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DESCRIPTION

21       If fildes is not a valid file descriptor open for writing,  the  ftrun‐
22       cate() function shall fail.
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24       If  fildes  refers  to  a  regular file, the ftruncate() function shall
25       cause the size of the file to be truncated to length. If  the  size  of
26       the  file previously exceeded length, the extra data shall no longer be
27       available to reads on the file. If the file previously was smaller than
28       this  size,  ftruncate()  shall either increase the size of the file or
29       fail.   XSI-conformant systems shall increase the size of the file.  If
30       the  file  size  is  increased, the extended area shall appear as if it
31       were zero-filled. The value of the seek pointer shall not  be  modified
32       by a call to ftruncate().
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34       Upon  successful  completion,  if  fildes refers to a regular file, the
35       ftruncate() function shall mark for update the  st_ctime  and  st_mtime
36       fields  of  the  file and the S_ISUID and S_ISGID bits of the file mode
37       may be cleared. If the ftruncate() function is unsuccessful,  the  file
38       is unaffected.
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40       If  the  request would cause the file size to exceed the soft file size
41       limit for the process, the request shall fail  and  the  implementation
42       shall generate the SIGXFSZ signal for the thread.
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44       If fildes refers to a directory, ftruncate() shall fail.
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46       If fildes refers to any other file type, except a shared memory object,
47       the result is unspecified.
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49       If fildes refers to a shared memory object, ftruncate() shall  set  the
50       size of the shared memory object to length.
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52       If the effect of ftruncate() is to decrease the size of a shared memory
53       object or memory mapped file and whole pages beyond the  new  end  were
54       previously  mapped,  then  the  whole pages beyond the new end shall be
55       discarded.
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57       If the Memory Protection option is supported, references  to  discarded
58       pages shall result in the generation of a SIGBUS signal; otherwise, the
59       result of such references is undefined.
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61       If the effect of ftruncate() is to increase the size of a shared memory
62       object,  it  is  unspecified  whether  the contents of any mapped pages
63       between the old end-of-file and the new are flushed to  the  underlying
64       object.
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RETURN VALUE

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

71       The ftruncate() function shall fail if:
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73       EINTR  A signal was caught during execution.
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75       EINVAL The length argument was less than 0.
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77       EFBIG or EINVAL
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79              The length argument was greater than the maximum file size.
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81       EFBIG  The file is a regular file and length is greater than the offset
82              maximum established in the open file description associated with
83              fildes.
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85       EIO    An I/O error occurred while reading from or writing  to  a  file
86              system.
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88       EBADF or EINVAL
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90              The fildes argument is not a file descriptor open for writing.
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92       EINVAL The  fildes  argument  references a file that was opened without
93              write permission.
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95       EROFS  The named file resides on a read-only file system.
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98       The following sections are informative.
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EXAMPLES

101       None.
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APPLICATION USAGE

104       None.
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RATIONALE

107       The ftruncate() function is part  of  IEEE Std 1003.1-2001  as  it  was
108       deemed  to  be  more useful than truncate(). The truncate() function is
109       provided as an XSI extension.
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FUTURE DIRECTIONS

112       None.
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SEE ALSO

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

119       Portions  of  this text are reprinted and reproduced in electronic form
120       from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
121       --  Portable  Operating  System  Interface (POSIX), The Open Group Base
122       Specifications Issue 6, Copyright (C) 2001-2003  by  the  Institute  of
123       Electrical  and  Electronics  Engineers, Inc and The Open Group. In the
124       event of any discrepancy between this version and the original IEEE and
125       The  Open Group Standard, the original IEEE and The Open Group Standard
126       is the referee document. The original Standard can be  obtained  online
127       at http://www.opengroup.org/unix/online.html .
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131IEEE/The Open Group                  2003                        FTRUNCATE(3P)