lseek(2) - osol9

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

6       lseek - move read/write file pointer
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SYNOPSIS

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

16       The  lseek()  function  sets  the file pointer associated with the open
17       file descriptor specified by fildes as follows:
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19           o      If whence is SEEK_SET, the pointer is set to offset bytes.
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21           o      If whence is SEEK_CUR, the pointer is  set  to  its  current
22                  location plus offset.
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24           o      If whence is SEEK_END, the pointer is set to the size of the
25                  file plus offset.
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27           o      If whence is SEEK_HOLE, the offset of the start of the  next
28                  hole  greater  than  or  equal  to  the  supplied  offset is
29                  returned. The definition of a hole is provided near the  end
30                  of the DESCRIPTION.
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32           o      If whence is SEEK_DATA, the file pointer is set to the start
33                  of the next non-hole file region greater than  or  equal  to
34                  the supplied offset.
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37       The  symbolic  constants  SEEK_SET,  SEEK_CUR, SEEK_END, SEEK_HOLE, and
38       SEEK_DATA are defined in the header <unistd.h>.
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41       Some devices are incapable of seeking. The value of  the  file  pointer
42       associated with such a device is undefined.
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45       The  lseek()  function  allows  the  file  pointer to be set beyond the
46       existing data in the file. If data are later  written  at  this  point,
47       subsequent  reads  in  the gap between the previous end of data and the
48       newly written data will return bytes of value 0 until data are  written
49       into the gap.
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52       If  fildes  is a remote file descriptor and offset is negative, lseek()
53       returns the file pointer  even if it is negative. The lseek()  function
54       will not, by itself, extend the size of a file.
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57       If  fildes  refers  to  a  shared  memory object, lseek() behaves as if
58       fildes referred to a regular file.
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61       A "hole" is defined as a contiguous range of bytes in a file, all  hav‐
62       ing the value of zero, but not all zeros in a file are guaranteed to be
63       represented as holes returned with SEEK_HOLE. Filesystems  are  allowed
64       to expose ranges of zeros with SEEK_HOLE, but not required to. Applica‐
65       tions can use SEEK_HOLE to optimise their behavior for ranges of zeros,
66       but  must not depend on it to find all such ranges in a file. The exis‐
67       tence of a hole at the end of every data region allows  for  easy  pro‐
68       gramming and implies that a virtual hole exists at the end of the file.
69       Applications   should   use   fpathconf(_PC_MIN_HOLE_SIZE)   or   path‐
70       conf(_PC_MIN_HOLE_SIZE)   to   determine   if   a  filesystem  supports
71       SEEK_HOLE. See fpathconf(2).
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74       For filesystems that do not supply information about  holes,  the  file
75       will be represented as one entire data region.
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RETURN VALUES

78       Upon  successful completion, the resulting offset, as measured in bytes
79       from the beginning of the file, is returned.  Otherwise,  (off_t)−1  is
80       returned,  the file offset remains unchanged, and errno is set to indi‐
81       cate the error.
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ERRORS

84       The lseek() function will fail if:
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86       EBADF        The fildes argument is not an open file descriptor.
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89       EINVAL       The  whence  argument  is  not  SEEK_SET,   SEEK_CUR,   or
90                    SEEK_END;  or  the  fildes  argument  is not a remote file
91                    descriptor and the resulting file pointer would  be  nega‐
92                    tive.
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95       ENXIO        For  SEEK_DATA,  there  are  no more data regions past the
96                    supplied offset. For SEEK_HOLE, there are  no  more  holes
97                    past the supplied offset.
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100       EOVERFLOW    The resulting file offset would be a value which cannot be
101                    represented correctly in an object of type off_t for regu‐
102                    lar files.
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105       ESPIPE       The  fildes argument is associated with a pipe, a FIFO, or
106                    a socket.
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USAGE

110       The lseek() function has a transitional interface for 64-bit file  off‐
111       sets.  See lf64(5).
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114       In  multithreaded  applications,  using  lseek()  in conjunction with a
115       read(2) or write(2) call on a file descriptor shared by more  than  one
116       thread is not an atomic operation.  To ensure atomicity, use pread() or
117       pwrite().
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ATTRIBUTES

120       See attributes(5) for descriptions of the following attributes:
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125       ┌─────────────────────────────┬─────────────────────────────┐
126       │      ATTRIBUTE TYPE         │      ATTRIBUTE VALUE        │
127       ├─────────────────────────────┼─────────────────────────────┤
128       │Interface Stability          │Standard                     │
129       ├─────────────────────────────┼─────────────────────────────┤
130       │MT-Level                     │Async-Signal-Safe            │
131       └─────────────────────────────┴─────────────────────────────┘
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