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LSEEK(2) System Calls Manual LSEEK(2)


lseekreposition read/write file offset


Standard C Library (libc, -lc)


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


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 of the start of the next hole greater than or equal to the supplied offset is returned. 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. The existence of a hole at the end of every data region allows for easy programming and implies that a virtual hole exists at the end of the file. 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.


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.


The lseek() system call will fail and the file position pointer will remain unchanged if:
The fildes argument is not an open file descriptor.
The whence argument is not a proper value or the resulting file offset would be negative for a non-character special file.
For SEEK_DATA, there are no more data regions past the supplied offset. For SEEK_HOLE, there are no more holes past the supplied offset.
The resulting file offset would be a value which cannot be represented correctly in an object of type off_t.
The fildes argument is associated with a pipe, socket, or FIFO.


dup(2), open(2), pathconf(2)


The lseek() system call is expected to conform to IEEE Std 1003.1-1990 (“POSIX.1”).


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


This document's use of whence is incorrect English, but is maintained for historical reasons.
April 5, 2007 Debian