lseek

LSEEK(2)                     BSD System Calls Manual                    LSEEK(2)

NAME
     lseek — reposition read/write file offset

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <unistd.h>

     off_t
     lseek(int fildes, off_t offset, int whence);

DESCRIPTION
     The lseek() system call repositions the offset of the file descriptor
     fildes to the argument offset according to the directive whence.  The
     argument fildes must be an open file descriptor.  The lseek() system call
     repositions the file position pointer associated with the file descriptor
     fildes as follows:

           If whence is SEEK_SET, the offset is set to offset bytes.

           If whence is SEEK_CUR, the offset is set to its current location plus
           offset bytes.

           If whence is SEEK_END, the offset is set to the size of the file plus
           offset bytes.

           If whence is SEEK_HOLE, the offset is set to the start of the next
           hole greater than or equal to the supplied offset.  The definition of
           a hole is provided below.

           If whence is SEEK_DATA, the offset is set to the start of the next
           non-hole file region greater than or equal to the supplied offset.

     The lseek() system call allows the file offset to be set beyond the end of
     the existing end-of-file of the file.  If data is later written at this
     point, subsequent reads of the data in the gap return bytes of zeros (until
     data is actually written into the gap).

     Some devices are incapable of seeking.  The value of the pointer associated
     with such a device is undefined.

     A "hole" is defined as a contiguous range of bytes in a file, all having
     the value of zero, but not all zeros in a file are guaranteed to be
     represented as holes returned with SEEK_HOLE.  File systems are allowed to
     expose ranges of zeros with SEEK_HOLE, but not required to.  Applications
     can use SEEK_HOLE to optimise their behavior for ranges of zeros, but must
     not depend on it to find all such ranges in a file.  Each file is presented
     as having a zero-size virtual hole at the very end of the file.  The
     existence of a hole at the end of every data region allows for easy
     programming and also provides compatibility to the original implementation
     in Solaris.  It also causes the current file size (i.e., end-of-file
     offset) to be returned to indicate that there are no more holes past the
     supplied offset.  Applications should use fpathconf(_PC_MIN_HOLE_SIZE) or
     pathconf(_PC_MIN_HOLE_SIZE) to determine if a file system supports
     SEEK_HOLE.  See pathconf(2).

     For file systems that do not supply information about holes, the file will
     be represented as one entire data region.

RETURN VALUES
     Upon successful completion, lseek() returns the resulting offset location
     as measured in bytes from the beginning of the file.  Otherwise, a value of
     -1 is returned and errno is set to indicate the error.

ERRORS
     The lseek() system call will fail and the file position pointer will remain
     unchanged if:

     [EBADF]            The fildes argument is not an open file descriptor.

     [EINVAL]           The whence argument is not a proper value or the
                        resulting file offset would be negative for a non-
                        character special file.

     [ENXIO]            For SEEK_DATA, there are no more data regions past the
                        supplied offset.  Due to existence of the hole at the
                        end of the file, for SEEK_HOLE this error is only
                        returned when the offset already points to the end-of-
                        file position.

     [EOVERFLOW]        The resulting file offset would be a value which cannot
                        be represented correctly in an object of type off_t.

     [ESPIPE]           The fildes argument is associated with a pipe, socket,
                        or FIFO.

SEE ALSO
     dup(2), open(2), pathconf(2)

STANDARDS
     The lseek() system call is expected to conform to ISO/IEC 9945-1:1990
     (“POSIX.1”).

HISTORY
     The lseek() function appeared in Version 7 AT&T UNIX.

BUGS
     This document's use of whence is incorrect English, but is maintained for
     historical reasons.

BSD                             February 18, 2016                            BSD