lseek64

Prototype:

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.

Prototype:

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().

Prototype:

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.