NAME

lseek - reposition read/write file offset

SYNOPSIS

1. include <sys/types.h>
2. include <unistd.h>

off_t lseek(int fildes, off_t offset, int whence);

DESCRIPTION

The lseek(2) function repositions the offset of the file descriptor fildes to the argument offset according to the directive whence as follows:

SEEK_SET?

The offset is set to offset bytes.

SEEK_CUR?

The offset is set to its current location plus offset bytes.

SEEK_END?

The offset is set to the size of the file plus offset bytes.

The lseek(2) 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).

RETURN VALUE

Upon successful completion, lseek returns the resulting offset location as measured in bytes from the beginning of the file. Otherwise, a value of (off_t)-1 is returned and errno is set to indicate the error.

ERRORS

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.

EOVERFLOW

file is larger than can fit in off_t

CONFORMING TO

SVr4, POSIX, BSD 4.3

RESTRICTIONS

Some devices are incapable of seeking and POSIX does not specify which devices must support it.

Linux specific restrictions: using lseek(2) on a tty device returns ESPIPE. Other systems return the number of written characters, using SEEK_SET to set the counter. Some devices, e.g. /dev/null do not cause the error ESPIPE, but return a pointer which value is undefined.

NOTES

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

When converting old code, substitute values for whence with the following macros:

SVR1-3 returns long instead of off_t, BSD returns int.

SEE ALSO

dup(2), open(2), fseek(3)