lseek(2)

1LSEEK(2)                   Linux Programmer's Manual                  LSEEK(2)
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NAME

6       lseek - reposition read/write file offset
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SYNOPSIS

9       #include <sys/types.h>
10       #include <unistd.h>
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12       off_t lseek(int fd, off_t offset, int whence);
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DESCRIPTION

15       The lseek() function repositions the offset of the open file associated
16       with the file descriptor fd to the argument  offset  according  to  the
17       directive whence as follows:
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19       SEEK_SET
20              The offset is set to offset bytes.
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22       SEEK_CUR
23              The offset is set to its current location plus offset bytes.
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25       SEEK_END
26              The offset is set to the size of the file plus offset bytes.
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28       The lseek() function allows the file offset to be set beyond the end of
29       the file (but this does not change the size of the file).  If  data  is
30       later written at this point, subsequent reads of the data in the gap (a
31       "hole") return null bytes ('\0') until data is  actually  written  into
32       the gap.
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34   Seeking file data and holes
35       Since  version  3.1, Linux supports the following additional values for
36       whence:
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38       SEEK_DATA
39              Adjust the file offset to the next location in the file  greater
40              than  or  equal  to offset containing data.  If offset points to
41              data, then the file offset is set to offset.
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43       SEEK_HOLE
44              Adjust the file offset to the next hole in the file greater than
45              or equal to offset.  If offset points into the middle of a hole,
46              then the file offset is set to offset.  If there is no hole past
47              offset,  then the file offset is adjusted to the end of the file
48              (i.e., there is an implicit hole at the end of any file).
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50       In both of the above cases, lseek() fails if offset points past the end
51       of the file.
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53       These  operations  allow  applications to map holes in a sparsely allo‐
54       cated file.  This can be useful for applications such  as  file  backup
55       tools,  which  can save space when creating backups and preserve holes,
56       if they have a mechanism for discovering holes.
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58       For the purposes of these operations, a hole is  a  sequence  of  zeros
59       that  (normally) has not been allocated in the underlying file storage.
60       However, a file system is not obliged to report holes, so these  opera‐
61       tions  are  not  a  guaranteed  mechanism for mapping the storage space
62       actually allocated to a file.  (Furthermore, a sequence of  zeros  that
63       actually has been written to the underlying storage may not be reported
64       as a hole.)  In the simplest implementation, a file system can  support
65       the  operations by making SEEK_HOLE always return the offset of the end
66       of the file, and making SEEK_DATA always return offset (i.e.,  even  if
67       the  location  referred to by offset is a hole, it can be considered to
68       consist of data that is a sequence of zeros).
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70       The _GNU_SOURCE feature test macro must be defined in order  to  obtain
71       the definitions of SEEK_DATA and SEEK_HOLE from <unistd.h>.
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RETURN VALUE

74       Upon  successful completion, lseek() returns the resulting offset loca‐
75       tion as measured in bytes from the beginning of the  file.   On  error,
76       the  value  (off_t) -1  is  returned  and  errno is set to indicate the
77       error.
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ERRORS

80       EBADF  fd is not an open file descriptor.
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82       EINVAL whence is not valid.  Or: the resulting  file  offset  would  be
83              negative, or beyond the end of a seekable device.
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85       EOVERFLOW
86              The resulting file offset cannot be represented in an off_t.
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88       ESPIPE fd is associated with a pipe, socket, or FIFO.
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90       ENXIO  whence is SEEK_DATA or SEEK_HOLE, and the current file offset is
91              beyond the end of the file.
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CONFORMING TO

94       SVr4, 4.3BSD, POSIX.1-2001.
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96       SEEK_DATA and SEEK_HOLE are  nonstandard  extensions  also  present  in
97       Solaris, FreeBSD, and DragonFly BSD; they are proposed for inclusion in
98       the next POSIX revision (Issue 8).
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NOTES

101       Some devices are incapable of seeking and POSIX does not specify  which
102       devices must support lseek().
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104       On Linux, using lseek() on a terminal device returns ESPIPE.
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106       When converting old code, substitute values for whence with the follow‐
107       ing macros:
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109        old       new
110       0        SEEK_SET
111       1        SEEK_CUR
112       2        SEEK_END
113       L_SET    SEEK_SET
114       L_INCR   SEEK_CUR
115       L_XTND   SEEK_END
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117       Note that file descriptors created by dup(2) or fork(2) share the  cur‐
118       rent  file position pointer, so seeking on such files may be subject to
119       race conditions.
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COLOPHON

125       This page is part of release 3.53 of the Linux  man-pages  project.   A
126       description  of  the project, and information about reporting bugs, can
127       be found at http://www.kernel.org/doc/man-pages/.
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131Linux                             2013-03-27                          LSEEK(2)