man3/lseek64.3

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.TH LSEEK64 3 2021-03-22 "Linux" "Linux Programmer's Manual"
.SH NAME
lseek64 \- reposition 64-bit read/write file offset
.SH SYNOPSIS
.nf
.BR "#define _LARGEFILE64_SOURCE" "     /* See feature_test_macros(7) */"
.B #include <sys/types.h>
.B #include <unistd.h>
.PP
.BI "off64_t lseek64(int " fd ", off64_t " offset ", int " whence );
.fi
.SH DESCRIPTION
The
.BR lseek ()
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 (),
.BR lseek64 (),
.BR llseek (),
and
.BR _llseek ().
.\"
.\" For some background details, see:
.\" https://lore.kernel.org/linux-man/CAKgNAkhNSWR3uYhYYaxx74fZfJ3JrpfAAPVrK0AFk_cAOUsbDg@mail.gmail.com/
.\"
.SS lseek()
Prototype:
.PP
.in +4n
.EX
.BI "off_t lseek(int " fd ", off_t " offset ", int " whence );
.EE
.in
.PP
The C library's
.BR lseek ()
wrapper function 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
.BR lseek64 ()
library function 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.
.\"
.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
.BR llseek ()
library function 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.
.PP
Since glibc 2.28,
.\" glibc commit 5c5c0dd747070db624c8e2c43691cec854f114ef
this function symbol is no longer available to newly linked applications.
.\"
.SS _llseek()
On 32-bit architectures,
this is the system call that is used (by the C library wrapper functions)
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.
.\" In arch/x86/entry/syscalls/syscall_32.tbl,
.\" we see the following line:
.\"
.\"    140     i386    _llseek                 sys_llseek
.\"
.\" This is essentially telling us that 'sys_llseek' (the name generated
.\" by SYSCALL_DEFINE5(llseek...)) is exposed to user-space as system call
.\" number 140, and that system call number will (IIUC) be exposed in
.\" autogenerated headers with the name "__NR__llseek" (i.e., "_llseek").
.\" The "i386" is telling us that this happens in i386 (32-bit Intel).
.\" There is nothing equivalent on x86-64, because 64 bit systems don't
.\" need an _llseek system call.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR lseek64 ()
T}	Thread safety	MT-Safe
.TE
.hy
.ad
.sp 1
.SH NOTES
.BR lseek64 ()
is one of the functions that was specified in the Large File Summit (LFS)
specification that was completed in 1996.
The purpose of the specification was to provide transitional support
that allowed applications on 32-bit systems to access
files whose size exceeds that which can be represented with a 32-bit
.IR off_t
type.
As noted above, this symbol is exposed by header files if the
.B _LARGEFILE64_SOURCE
feature test macro is defined.
ALternatively, on a 32-bit system, the symbol
.I lseek
is aliased to
.I lseek64
if the macro
.B _FILE_OFFSET_BITS
is defined with the value 64.
.SH SEE ALSO
.BR llseek (2),
.BR lseek (2)