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.TH LSEEK64 3 2017-09-15 "Linux" "Linux Programmer's Manual"
.SH NAME
lseek64 \- reposition 64-bit read/write file offset
.SH SYNOPSIS
.BR "#define _LARGEFILE64_SOURCE" "     /* See feature_test_macros(7) */"
.br
.B #include <sys/types.h>
.br
.B #include <unistd.h>
.PP
.BI "off64_t lseek64(int " fd ", off64_t " offset ", int " whence );
.SH DESCRIPTION
The
.BR lseek (2)
family of functions reposition the offset of the open file associated
with the file descriptor
.I fd
to
.I offset
bytes relative to the start, current position, or end of the file,
when
.I whence
has the value
.BR SEEK_SET ,
.BR SEEK_CUR ,
or
.BR SEEK_END ,
respectively.
.PP
For more details, return value, and errors, see
.BR lseek (2).
.PP
Four interfaces are available:
.BR lseek (2),
.BR lseek64 (),
.BR llseek (2),
and
.BR _llseek (2).
.SS lseek()
Prototype:
.PP
.in +4n
.EX
.BI "off_t lseek(int " fd ", off_t " offset ", int " whence );
.EE
.in
.PP
.BR lseek (2)
uses the type
.IR off_t .
This is a 32-bit signed type on 32-bit architectures, unless one
compiles with
.PP
.in +4n
.EX
#define _FILE_OFFSET_BITS 64
.EE
.in
.PP
in which case it is a 64-bit signed type.
.SS lseek64()
Prototype:
.PP
.in +4n
.EX
.BI "off64_t lseek64(int " fd ", off64_t " offset ", int " whence );
.EE
.in
.PP
The library routine
.BR lseek64 ()
uses a 64-bit type even when
.I off_t
is a 32-bit type.
Its prototype (and the type
.IR off64_t )
is available only when one compiles with
.PP
.in +4n
.EX
#define _LARGEFILE64_SOURCE
.EE
.in
.PP
The function
.BR lseek64 ()
.\" in glibc 2.0.94, not in 2.0.6
is available since glibc 2.1, and is defined to be an alias for
.BR llseek ().
.SS llseek()
Prototype:
.PP
.in +4n
.EX
.BI "loff_t llseek(int " fd ", loff_t " offset ", int " whence );
.EE
.in
.PP
The type
.I loff_t
is a 64-bit signed type.
The library routine
.BR llseek ()
.\" in libc 5.0.9, not in 4.7.6
is available in glibc and works without special defines.
However, the glibc headers do not provide a prototype.
Users should add
the above prototype, or something equivalent, to their own source.
When users complained about data loss caused by a miscompilation of
.BR e2fsck (8),
glibc 2.1.3 added the link-time warning
.PP
.in +4n
"the \`llseek\' function may be dangerous; use \`lseek64\' instead."
.in
.PP
This makes this function unusable if one desires a warning-free
compilation.
.SS _llseek()
On 32-bit architectures,
this is the system call that is used to implement all of the above functions.
The prototype is:
.PP
.in +4n
.EX
.BI "int _llseek(int " fd ", off_t " offset_hi ", off_t " offset_lo ,
.BI "            loff_t *" result ", int " whence );
.EE
.in
.PP
For more details, see
.BR llseek (2).
.PP
64-bit systems don't need an
.BR _llseek ()
system call.
Instead, they have an
.BR lseek (2)
system call that supports 64-bit file offsets.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lb lb lb
l l l.
Interface	Attribute	Value
T{
.BR lseek64 ()
T}	Thread safety	MT-Safe
.TE
.SH SEE ALSO
.BR llseek (2),
.BR lseek (2)
.SH COLOPHON
This page is part of release 5.04 of the Linux
.I man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
\%https://www.kernel.org/doc/man\-pages/.