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 _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 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 compilation.


On 32-bit architectures, this is the system call that is used to implement all of the above functions. 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).

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


For an explanation of the terms used in this section, see attributes(7).

Interface Attribute Value
lseek64() Thread safety MT-Safe


llseek(2), lseek(2)


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  Copyright 2004 Andries Brouwer <>.

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