lseeki64

lseeki64 is a function from the Microsoft C runtime used to reposition the file offset of an open file descriptor using a 64-bit offset. It provides large-file support on 32-bit Windows and is analogous to the POSIX lseek with a 64-bit offset.

The canonical name in the MSVC runtime is _lseeki64; documentation and code sometimes refer to lseeki64 without the leading underscore. The function is declared in io.h and is used with standard origin constants such as SEEK_SET, SEEK_CUR, and SEEK_END.

Prototype and behavior typically resemble: long long _lseeki64(int fd, long long offset, int origin); The function moves the file pointer for the given descriptor to the specified offset relative to origin. On success it returns the new offset as a 64-bit value. On error it returns -1 and sets errno.

Usage examples include opening a file descriptor and seeking to a position using a 64-bit offset, for example to find the file end or to position for large-file reads. A common form is to call _open with appropriate flags, then _lseeki64(fd, offset, SEEK_SET or SEEK_END), checking the return value for -1 to detect errors.

Portability notes: _lseeki64 is not part of the C standard and is specific to the Windows/MSVC runtime. On POSIX systems, lseek supports large offsets, and some environments offer lseek64. For cross-platform code, developers may prefer fseeko/ftello or use the Windows SetFilePointerEx API as an alternative, depending on the target platform. Some toolchains (e.g., MinGW) provide compatible aliases or wrappers to bridge between Windows and POSIX-like interfaces.