lseek64(3)		    Library Functions Manual		      lseek64(3)

NAME
     lseek64 - reposition 64-bit read/write file offset

LIBRARY
     Standard C library (libc, -lc)

SYNOPSIS
     #define _LARGEFILE64_SOURCE     /* See feature_test_macros(7) */
     #include <sys/types.h>
     #include <unistd.h>

     off64_t lseek64(int fd, off64_t offset, int whence);

DESCRIPTION
     The  lseek() family of functions reposition the offset of the open file as-
     sociated with the file descriptor fd to offset bytes relative to the start,
     current position, or end of the file, when whence has the	value  SEEK_SET,
     SEEK_CUR, or SEEK_END, respectively.

     For more details, return value, and errors, see lseek(2).

     Four interfaces are available: lseek(), lseek64(), llseek(), and _llseek().

   lseek()
     Prototype:

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

     The  C  library's	lseek() wrapper function uses the type off_t.  This is a
     32-bit signed type on 32-bit architectures, unless one compiles with

	 #define _FILE_OFFSET_BITS 64

     in which case it is a 64-bit signed type.

   lseek64()
     Prototype:

	 off64_t lseek64(int fd, off64_t offset, int whence);

     The lseek64() library function uses a 64-bit type	even  when  off_t  is  a
     32-bit  type.   Its prototype (and the type off64_t) is available only when
     one compiles with

	 #define _LARGEFILE64_SOURCE

     The function lseek64() is available since glibc 2.1.

   llseek()
     Prototype:

	 loff_t llseek(int fd, loff_t offset, int whence);

     The type loff_t is a 64-bit signed type.  The 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 e2fsck(8), glibc  2.1.3  added  the
     link-time warning

	 "the `llseek' function may be dangerous; use `lseek64' instead."

     This  makes  this	function unusable if one desires a warning-free compila-
     tion.

     Since glibc 2.28, this function symbol is	no  longer  available  to  newly
     linked applications.

   _llseek()
     On 32-bit architectures, this is the system call that is used (by the C li-
     brary wrapper functions) to implement all of the above functions.	The pro-
     totype is:

	 int _llseek(int fd, off_t offset_hi, off_t offset_lo,
		     loff_t *result, int whence);

     For more details, see llseek(2).

     64-bit  systems don't need an _llseek() system call.  Instead, they have an
     lseek(2) system call that supports 64-bit file offsets.

ATTRIBUTES
     For an explanation of the terms used in this  section,  see  attributes(7).
     allbox; lbx lb lb l l l.  Interface Attribute Value T{ lseek64()
     T}	  Thread safety	  MT-Safe

NOTES
     lseek64() is one of the functions that was specified in the Large File Sum-
     mit  (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 rep-
     resented with a 32-bit off_t type.	 As noted above, this symbol is	 exposed
     by	 header	 files if the _LARGEFILE64_SOURCE feature test macro is defined.
     ALternatively, on a 32-bit system, the symbol lseek is aliased  to	 lseek64
     if the macro _FILE_OFFSET_BITS is defined with the value 64.

SEE ALSO
     llseek(2), lseek(2)

Linux man-pages 6.18		   2026-02-08			      lseek64(3)