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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NAME

12       lseek - move the read/write file offset
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SYNOPSIS

15       #include <unistd.h>
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17       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 bytes.
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26        * If  whence  is SEEK_CUR, the file offset shall be set to its current
27          location plus offset.
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29        * If whence is SEEK_END, the file offset shall be set to the  size  of
30          the file plus offset.
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32       The  symbolic constants SEEK_SET, SEEK_CUR, and SEEK_END are defined in
33       <unistd.h>.
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35       The behavior of lseek() on devices which are incapable  of  seeking  is
36       implementation-defined.  The  value  of the file offset associated with
37       such a device is undefined.
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39       The lseek() function shall allow the file offset to be set  beyond  the
40       end  of the existing data in the file. If data is later written at this
41       point, subsequent reads of data in the gap shall return bytes with  the
42       value 0 until data is actually written into the gap.
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44       The lseek() function shall not, by itself, extend the size of a file.
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46       If  fildes  refers to a shared memory object, the result of the lseek()
47       function is unspecified.
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49       If fildes refers to a typed memory object, the result  of  the  lseek()
50       function is unspecified.
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RETURN VALUE

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

59       The lseek() function shall fail if:
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61       EBADF  The fildes argument is not an open file descriptor.
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63       EINVAL The whence argument is not a proper value, or the resulting file
64              offset would be negative for a regular file, block special file,
65              or directory.
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67       EOVERFLOW
68              The resulting file offset would be a value which cannot be  rep‐
69              resented correctly in an object of type off_t.
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71       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 IEEE Std 1003.1-2001 that may directly extend  the
89       size of the file is write(), truncate(), and ftruncate(), several func‐
90       tions 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(),   the   Base   Definitions   volume   of  IEEE Std 1003.1-2001,
112       <sys/types.h>, <unistd.h>
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115       Portions of this text are reprinted and reproduced in  electronic  form
116       from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
117       -- Portable Operating System Interface (POSIX),  The  Open  Group  Base
118       Specifications  Issue  6,  Copyright  (C) 2001-2003 by the Institute of
119       Electrical and Electronics Engineers, Inc and The Open  Group.  In  the
120       event of any discrepancy between this version and the original IEEE and
121       The Open Group Standard, the original IEEE and The Open Group  Standard
122       is  the  referee document. The original Standard can be obtained online
123       at http://www.opengroup.org/unix/online.html .
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127IEEE/The Open Group                  2003                            LSEEK(3P)
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