1LSEEK(3P)                  POSIX Programmer's Manual                 LSEEK(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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11

NAME

13       lseek — move the read/write file offset
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SYNOPSIS

16       #include <unistd.h>
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18       off_t lseek(int fildes, off_t offset, int whence);
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DESCRIPTION

21       The lseek() function shall set  the  file  offset  for  the  open  file
22       description associated with the file descriptor fildes, as follows:
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24        *  If  whence  is  SEEK_SET,  the  file  offset shall be set to offset
25           bytes.
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27        *  If whence is SEEK_CUR, the file offset shall be set to its  current
28           location plus offset.
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30        *  If  whence is SEEK_END, the file offset shall be set to the size of
31           the file plus offset.
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33       The symbolic constants SEEK_SET, SEEK_CUR, and SEEK_END are defined  in
34       <unistd.h>.
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36       The  behavior  of  lseek() on devices which are incapable of seeking is
37       implementation-defined.  The value of the file offset  associated  with
38       such a device is undefined.
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40       The  lseek()  function shall allow the file offset to be set beyond the
41       end of the existing data in the file. If data is later written at  this
42       point,  subsequent reads of data in the gap shall return bytes with the
43       value 0 until data is actually written into the gap.
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45       The lseek() function shall not, by itself, extend the size of a file.
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47       If fildes refers to a shared memory object, the result of  the  lseek()
48       function is unspecified.
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50       If  fildes  refers  to a typed memory object, the result of the lseek()
51       function is unspecified.
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RETURN VALUE

54       Upon successful completion, the resulting offset, as measured in  bytes
55       from  the beginning of the file, shall be returned. Otherwise, −1 shall
56       be returned, errno shall be set to indicate the  error,  and  the  file
57       offset shall remain unchanged.
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ERRORS

60       The lseek() function shall fail if:
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62       EBADF  The fildes argument is not an open file descriptor.
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64       EINVAL The whence argument is not a proper value, or the resulting file
65              offset would be negative for a regular file, block special file,
66              or directory.
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68       EOVERFLOW
69              The  resulting file offset would be a value which cannot be rep‐
70              resented correctly in an object of type off_t.
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72       ESPIPE The fildes argument is associated with a pipe, FIFO, or socket.
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74       The following sections are informative.
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EXAMPLES

77       None.
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APPLICATION USAGE

80       None.
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RATIONALE

83       The ISO C standard includes  the  functions  fgetpos()  and  fsetpos(),
84       which work on very large files by use of a special positioning type.
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86       Although  lseek()  may  position  the file offset beyond the end of the
87       file, this function does not itself extend the size of the file.  While
88       the  only function in POSIX.1‐2008 that may directly extend the size of
89       the file is write(), truncate(),  and  ftruncate(),  several  functions
90       originally   derived   from  the  ISO C  standard,  such  as  fwrite(),
91       fprintf(), and so on, may do so (by causing calls on write()).
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93       An invalid file offset that would cause [EINVAL] to be returned may  be
94       both  implementation-defined  and device-dependent (for example, memory
95       may have few invalid values). A negative file offset may be  valid  for
96       some devices in some implementations.
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98       The  POSIX.1‐1990  standard  did not specifically prohibit lseek() from
99       returning a negative offset. Therefore, an application was required  to
100       clear  errno prior to the call and check errno upon return to determine
101       whether a return value of (off_t)−1 is a negative offset or an  indica‐
102       tion  of  an  error  condition. The standard developers did not wish to
103       require this action on the part of a conforming application, and  chose
104       to require that errno be set to [EINVAL] when the resulting file offset
105       would be negative for a regular file, block special file, or directory.
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FUTURE DIRECTIONS

108       None.
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SEE ALSO

111       open()
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113       The Base Definitions volume of POSIX.1‐2008, <sys_types.h>, <unistd.h>
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116       Portions of this text are reprinted and reproduced in  electronic  form
117       from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
118       -- Portable Operating System Interface (POSIX),  The  Open  Group  Base
119       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
120       cal and Electronics Engineers,  Inc  and  The  Open  Group.   (This  is
121       POSIX.1-2008  with  the  2013  Technical Corrigendum 1 applied.) In the
122       event of any discrepancy between this version and the original IEEE and
123       The  Open Group Standard, the original IEEE and The Open Group Standard
124       is the referee document. The original Standard can be  obtained  online
125       at http://www.unix.org/online.html .
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127       Any  typographical  or  formatting  errors that appear in this page are
128       most likely to have been introduced during the conversion of the source
129       files  to  man page format. To report such errors, see https://www.ker
130       nel.org/doc/man-pages/reporting_bugs.html .
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134IEEE/The Open Group                  2013                            LSEEK(3P)
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