lseek(2) - f31

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       lseek()  repositions the file offset of the open file description asso‐
16       ciated with the file descriptor fd to the argument offset according  to
17       the directive whence as follows:
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19       SEEK_SET
20              The file offset is set to offset bytes.
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22       SEEK_CUR
23              The  file  offset  is  set  to  its current location plus offset
24              bytes.
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26       SEEK_END
27              The file offset is set to the  size  of  the  file  plus  offset
28              bytes.
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30       lseek()  allows  the  file  offset to be set beyond the end of the file
31       (but this does not change the size of the  file).   If  data  is  later
32       written  at  this  point,  subsequent  reads  of the data in the gap (a
33       "hole") return null bytes ('\0') until data is  actually  written  into
34       the gap.
35
36   Seeking file data and holes
37       Since  version  3.1, Linux supports the following additional values for
38       whence:
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40       SEEK_DATA
41              Adjust the file offset to the next location in the file  greater
42              than  or  equal  to offset containing data.  If offset points to
43              data, then the file offset is set to offset.
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45       SEEK_HOLE
46              Adjust the file offset to the next hole in the file greater than
47              or equal to offset.  If offset points into the middle of a hole,
48              then the file offset is set to offset.  If there is no hole past
49              offset,  then the file offset is adjusted to the end of the file
50              (i.e., there is an implicit hole at the end of any file).
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52       In both of the above cases, lseek() fails if offset points past the end
53       of the file.
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55       These  operations  allow  applications to map holes in a sparsely allo‐
56       cated file.  This can be useful for applications such  as  file  backup
57       tools,  which  can save space when creating backups and preserve holes,
58       if they have a mechanism for discovering holes.
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60       For the purposes of these operations, a hole is  a  sequence  of  zeros
61       that  (normally) has not been allocated in the underlying file storage.
62       However, a filesystem is not obliged to report holes, so  these  opera‐
63       tions  are  not  a  guaranteed  mechanism for mapping the storage space
64       actually allocated to a file.  (Furthermore, a sequence of  zeros  that
65       actually has been written to the underlying storage may not be reported
66       as a hole.)  In the simplest implementation, a filesystem  can  support
67       the  operations by making SEEK_HOLE always return the offset of the end
68       of the file, and making SEEK_DATA always return offset (i.e.,  even  if
69       the  location  referred to by offset is a hole, it can be considered to
70       consist of data that is a sequence of zeros).
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72       The _GNU_SOURCE feature test macro must be defined in order  to  obtain
73       the definitions of SEEK_DATA and SEEK_HOLE from <unistd.h>.
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75       The  SEEK_HOLE and SEEK_DATA operations are supported for the following
76       filesystems:
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78       *  Btrfs (since Linux 3.1)
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80       *  OCFS (since Linux 3.2)
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82       *  XFS (since Linux 3.5)
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84       *  ext4 (since Linux 3.8)
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86       *  tmpfs(5) (since Linux 3.8)
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88       *  NFS (since Linux 3.18)
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90       *  FUSE (since Linux 4.5)
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RETURN VALUE

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

99       EBADF  fd is not an open file descriptor.
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101       EINVAL whence  is  not  valid.   Or: the resulting file offset would be
102              negative, or beyond the end of a seekable device.
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104       ENXIO  whence is SEEK_DATA or SEEK_HOLE, and the file offset is  beyond
105              the end of the file.
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107       EOVERFLOW
108              The resulting file offset cannot be represented in an off_t.
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110       ESPIPE fd is associated with a pipe, socket, or FIFO.
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CONFORMING TO

113       POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
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115       SEEK_DATA  and  SEEK_HOLE  are  nonstandard  extensions also present in
116       Solaris, FreeBSD, and DragonFly BSD; they are proposed for inclusion in
117       the next POSIX revision (Issue 8).
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NOTES

120       See  open(2) for a discussion of the relationship between file descrip‐
121       tors, open file descriptions, and files.
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123       If the O_APPEND file status flag is set on the open  file  description,
124       then  a  write(2)  always moves the file offset to the end of the file,
125       regardless of the use of lseek().
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127       The off_t data type is a signed integer data type specified by POSIX.1.
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129       Some devices are incapable of seeking and POSIX does not specify  which
130       devices must support lseek().
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132       On  Linux,  using  lseek()  on  a  terminal device fails with the error
133       ESPIPE.
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COLOPHON

140       This  page  is  part of release 5.02 of the Linux man-pages project.  A
141       description of the project, information about reporting bugs,  and  the
142       latest     version     of     this    page,    can    be    found    at
143       https://www.kernel.org/doc/man-pages/.
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147Linux                             2019-03-06                          LSEEK(2)