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LSEEK(2) System Calls Manual LSEEK(2)

lseek, seekreposition read/write file offset

library “libc”

#include <unistd.h>

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 pointer fildes as follows:

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.

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 pointer will remain unchanged if:

fildes is not an open file descriptor.
whence is not a proper value, or the resulting file offset would be invalid.
fildes is associated with a pipe, socket, or FIFO.

dup(2), open(2)

The lseek() function conforms to IEEE Std 1003.1-1990 (“POSIX.1”).

A seek() function appeared in Version 2 AT&T UNIX, later renamed into lseek() for “long seek” due to a larger offset argument type.

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

April 3, 2010 NetBSD-7.0.1