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LSEEK(2)                 DragonFly System Calls Manual                LSEEK(2)


lseek -- reposition read/write file offset


Standard C Library (libc, -lc)


#include <unistd.h> off_t lseek(int fildes, off_t offset, int whence);


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.


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.


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.


dup(2), open(2)


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


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


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) ______________________________________________________________________________


seek - Change the access position for an open channel


seek channelId offset ?origin? ______________________________________________________________________________


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.


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


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


access position, file, seek Tcl 8.1 seek(n)

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