DragonFly On-Line Manual Pages

LSEEK(2)                 DragonFly 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() function 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.  Lseek() 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.

     The lseek() function 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 associ-
     ated with such a device is undefined.

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
     Lseek() will fail and the file position pointer will remain unchanged if:

     [EBADF]            Fildes is not an open file descriptor.

     [ESPIPE]           Fildes is associated with a pipe, socket, or FIFO.

     [EINVAL]           Whence is not a proper value.

SEE ALSO
     dup(2), open(2)

STANDARDS
     The lseek() function call is expected to conform to ISO/IEC 9945-1:1990
     (``POSIX.1'').

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

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

DragonFly 3.5                   April 19, 1994                   DragonFly 3.5
seek(n)                      Tcl Built-In Commands                     seek(n)

______________________________________________________________________________

NAME
       seek - Change the access position for an open channel

SYNOPSIS
       seek channelId offset ?origin?
______________________________________________________________________________

DESCRIPTION
       Changes the current access position for channelId.

       ChannelId must be an identifier for an open channel such as a Tcl
       standard channel (stdin, stdout, or stderr), the return value from an
       invocation of open or socket, or the result of a channel creation
       command provided by a Tcl extension.

       The offset and origin arguments specify the position at which the next
       read or write will occur for channelId. Offset must be an integer
       (which may be negative) and origin must be one of the following:

       start     The new access position will be offset bytes from the start
                 of the underlying file or device.

       current   The new access position will be offset bytes from the current
                 access position; a negative offset moves the access position
                 backwards in the underlying file or device.

       end       The new access position will be offset bytes from the end of
                 the file or device.  A negative offset places the access
                 position before the end of file, and a positive offset places
                 the access position after the end of file.

       The origin argument defaults to start.

       The command flushes all buffered output for the channel before the
       command returns, even if the channel is in non-blocking mode.  It also
       discards any buffered and unread input.  This command returns an empty
       string.  An error occurs if this command is applied to channels whose
       underlying file or device does not support seeking.

       Note that offset values are byte offsets, not character offsets.  Both
       seek and tell operate in terms of bytes, not characters, unlike read.

EXAMPLES
       Read a file twice:

              set f [open file.txt]
              set data1 [read $f]
              seek $f 0
              set data2 [read $f]
              close $f
              # $data1 eq $data2 if the file wasn't updated

       Read the last 10 bytes from a file:

              set f [open file.data]
              # This is guaranteed to work with binary data but
              # may fail with other encodings...
              fconfigure $f -translation binary
              seek $f -10 end
              set data [read $f 10]
              close $f

SEE ALSO
       file(n), open(n), close(n), gets(n), tell(n), Tcl_StandardChannels(3)

KEYWORDS
       access position, file, seek

Tcl                                   8.1                              seek(n)