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

13       ftruncate — truncate a file to a specified length
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SYNOPSIS

16       #include <unistd.h>
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18       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 increase the size of the file. If the file
29       size is increased, the extended area shall appear as if it  were  zero-
30       filled.  The  value of the seek pointer shall not be modified by a call
31       to ftruncate().
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33       Upon successful completion, if fildes refers to a regular file,  ftrun‐
34       cate()  shall  mark for update the last data modification and last file
35       status change timestamps of the file and the S_ISUID and  S_ISGID  bits
36       of  the file mode may be cleared. If the ftruncate() function is unsuc‐
37       cessful, the file is unaffected.
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39       If the request would cause the file size to exceed the soft  file  size
40       limit  for  the  process, the request shall fail and the implementation
41       shall generate the SIGXFSZ signal for the thread.
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43       If fildes refers to a directory, ftruncate() shall fail.
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45       If fildes refers to any other file type, except a shared memory object,
46       the result is unspecified.
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48       If  fildes  refers to a shared memory object, ftruncate() shall set the
49       size of the shared memory object to length.
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51       If the effect of ftruncate() is to decrease the size of a memory mapped
52       file  or a shared memory object and whole pages beyond the new end were
53       previously mapped, then the whole pages beyond the  new  end  shall  be
54       discarded.
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56       References  to discarded pages shall result in the generation of a SIG‐
57       BUS signal.
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59       If the effect of ftruncate() is  to  increase  the  size  of  a  memory
60       object,  it  is  unspecified  whether  the contents of any mapped pages
61       between the old end-of-file and the new are flushed to  the  underlying
62       object.
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RETURN VALUE

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

69       The ftruncate() function shall fail if:
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71       EINTR  A signal was caught during execution.
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73       EINVAL The length argument was less than 0.
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75       EFBIG or EINVAL
76              The length argument was greater than the maximum file size.
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78       EFBIG  The file is a regular file and length is greater than the offset
79              maximum established in the open file description associated with
80              fildes.
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82       EIO    An I/O error occurred while reading from or writing  to  a  file
83              system.
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85       EBADF or EINVAL
86              The fildes argument is not a file descriptor open for writing.
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88       The following sections are informative.
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EXAMPLES

91       None.
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APPLICATION USAGE

94       None.
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RATIONALE

97       None.
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FUTURE DIRECTIONS

100       None.
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SEE ALSO

103       open(), truncate()
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105       The Base Definitions volume of POSIX.1‐2008, <unistd.h>
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108       Portions  of  this text are reprinted and reproduced in electronic form
109       from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
110       --  Portable  Operating  System  Interface (POSIX), The Open Group Base
111       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
112       cal  and  Electronics  Engineers,  Inc  and  The  Open Group.  (This is
113       POSIX.1-2008 with the 2013 Technical Corrigendum  1  applied.)  In  the
114       event of any discrepancy between this version and the original IEEE and
115       The Open Group Standard, the original IEEE and The Open Group  Standard
116       is  the  referee document. The original Standard can be obtained online
117       at http://www.unix.org/online.html .
118
119       Any typographical or formatting errors that appear  in  this  page  are
120       most likely to have been introduced during the conversion of the source
121       files to man page format. To report such errors,  see  https://www.ker‐
122       nel.org/doc/man-pages/reporting_bugs.html .
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126IEEE/The Open Group                  2013                        FTRUNCATE(3P)
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