---
name: "lseek"
section: "2"
platform: "macos"
package: "xnu"
version: "12377.121.6"
release: "macOS 26.5"
platforms: ["linux", "freebsd", "macos"]
url: "https://new.carta.tech/lseek"
---

# lseek(2)

reposition read/write file offset

## NAME

**lseek** – reposition read/write file offset

## 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 pointer *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)](/pathconf/2.md).

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 pointer will remain unchanged if:

- \[EBADF\]

  *Fildes* is not an open file descriptor.

- \[EINVAL\]

  *Whence* is not a proper value.

- \[EINVAL\]

  The seek location (calculated from *offset* and *whence*) is negative.

- \[ENXIO\]

  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 seek location is too large to be stored in an object of type off\_t.

- \[ESPIPE\]

  *Fildes* is associated with a pipe, socket, or FIFO.

## SEE ALSO

[dup(2)](/dup/2.md), [open(2)](/open/2.md)

## BUGS

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

## STANDARDS

The **lseek**() function conforms to  IEEE Std 1003.1-1988 (“POSIX.1”).