lseek64 — reposition 64-bit read/write file offset


#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);


The lseek(2) family of functions reposition the offset of the open file associated 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(2), lseek64(), llseek(2), and the raw system call _llseek(2).



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

lseek(2) 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.



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

The library routine lseek64() 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, and is defined to be an alias for llseek().



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

The type loff_t is a 64-bit signed type. The library routine llseek() is available in libc5 and glibc and works without special defines. Its prototype was given in <unistd.h> with libc5, but glibc does not provide a prototype. This is bad, since a prototype is needed. 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 compilation.


All the above functions are implemented in terms of this system call. The prototype is:

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

For more details, see llseek(2).


Multithreading (see pthreads(7))

The lseek64() function is thread-safe.


llseek(2), lseek(2)


This page is part of release 3.54 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at−pages/.

  Copyright 2004 Andries Brouwer <>.

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