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.
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() 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
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
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.
[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.
SEE ALSO
dup(2), open(2), pathconf(2)
STANDARDS
The lseek() function call is expected to conform to IEEE Std 1003.1-1990
("POSIX.1").
The SEEK_HOLE and SEEK_DATA directives, along with the ENXIO error, are
extensions to that specification.
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 6.5-DEVELOPMENT April 19, 1994 DragonFly 6.5-DEVELOPMENT
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)