lseek(2) - f39

1lseek(2)                      System Calls Manual                     lseek(2)
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NAME

6       lseek - reposition read/write file offset
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LIBRARY

9       Standard C library (libc, -lc)
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SYNOPSIS

12       #include <unistd.h>
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14       off_t lseek(int fd, off_t offset, int whence);
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DESCRIPTION

17       lseek()  repositions the file offset of the open file description asso‐
18       ciated with the file descriptor fd to the argument offset according  to
19       the directive whence as follows:
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21       SEEK_SET
22              The file offset is set to offset bytes.
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24       SEEK_CUR
25              The  file  offset  is  set  to  its current location plus offset
26              bytes.
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28       SEEK_END
29              The file offset is set to the  size  of  the  file  plus  offset
30              bytes.
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32       lseek()  allows  the  file  offset to be set beyond the end of the file
33       (but this does not change the size of the  file).   If  data  is  later
34       written  at  this  point,  subsequent  reads  of the data in the gap (a
35       "hole") return null bytes ('\0') until data is  actually  written  into
36       the gap.
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38   Seeking file data and holes
39       Since  Linux  3.1,  Linux  supports the following additional values for
40       whence:
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42       SEEK_DATA
43              Adjust the file offset to the next location in the file  greater
44              than  or  equal  to offset containing data.  If offset points to
45              data, then the file offset is set to offset.
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47       SEEK_HOLE
48              Adjust the file offset to the next hole in the file greater than
49              or equal to offset.  If offset points into the middle of a hole,
50              then the file offset is set to offset.  If there is no hole past
51              offset,  then the file offset is adjusted to the end of the file
52              (i.e., there is an implicit hole at the end of any file).
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54       In both of the above cases, lseek() fails if offset points past the end
55       of the file.
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57       These  operations  allow  applications to map holes in a sparsely allo‐
58       cated file.  This can be useful for applications such  as  file  backup
59       tools,  which  can save space when creating backups and preserve holes,
60       if they have a mechanism for discovering holes.
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62       For the purposes of these operations, a hole is  a  sequence  of  zeros
63       that  (normally) has not been allocated in the underlying file storage.
64       However, a filesystem is not obliged to report holes, so  these  opera‐
65       tions  are not a guaranteed mechanism for mapping the storage space ac‐
66       tually allocated to a file.  (Furthermore, a sequence of zeros that ac‐
67       tually  has  been written to the underlying storage may not be reported
68       as a hole.)  In the simplest implementation, a filesystem  can  support
69       the  operations by making SEEK_HOLE always return the offset of the end
70       of the file, and making SEEK_DATA always return offset (i.e.,  even  if
71       the  location  referred to by offset is a hole, it can be considered to
72       consist of data that is a sequence of zeros).
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74       The _GNU_SOURCE feature test macro must be defined in order  to  obtain
75       the definitions of SEEK_DATA and SEEK_HOLE from <unistd.h>.
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77       The  SEEK_HOLE and SEEK_DATA operations are supported for the following
78       filesystems:
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80       •  Btrfs (since Linux 3.1)
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82       •  OCFS (since Linux 3.2)
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84       •  XFS (since Linux 3.5)
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86       •  ext4 (since Linux 3.8)
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88       •  tmpfs(5) (since Linux 3.8)
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90       •  NFS (since Linux 3.18)
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92       •  FUSE (since Linux 4.5)
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94       •  GFS2 (since Linux 4.15)
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RETURN VALUE

97       Upon successful completion, lseek() returns the resulting offset  loca‐
98       tion  as  measured  in bytes from the beginning of the file.  On error,
99       the value (off_t) -1 is returned and errno is set to indicate  the  er‐
100       ror.
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ERRORS

103       EBADF  fd is not an open file descriptor.
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105       EINVAL whence  is  not  valid.   Or: the resulting file offset would be
106              negative, or beyond the end of a seekable device.
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108       ENXIO  whence is SEEK_DATA or SEEK_HOLE, and offset is beyond  the  end
109              of  the file, or whence is SEEK_DATA and offset is within a hole
110              at the end of the file.
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112       EOVERFLOW
113              The resulting file offset cannot be represented in an off_t.
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115       ESPIPE fd is associated with a pipe, socket, or FIFO.
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VERSIONS

118       On Linux, using lseek() on a terminal device fails with the  error  ES‐
119       PIPE.
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STANDARDS

122       POSIX.1-2008.
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HISTORY

125       POSIX.1-2001, SVr4, 4.3BSD.
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127       SEEK_DATA  and SEEK_HOLE are nonstandard extensions also present in So‐
128       laris, FreeBSD, and DragonFly BSD; they are proposed for  inclusion  in
129       the next POSIX revision (Issue 8).
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NOTES

132       See  open(2) for a discussion of the relationship between file descrip‐
133       tors, open file descriptions, and files.
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135       If the O_APPEND file status flag is set on the open  file  description,
136       then  a  write(2)  always moves the file offset to the end of the file,
137       regardless of the use of lseek().
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139       Some devices are incapable of seeking and POSIX does not specify  which
140       devices must support lseek().
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