LSEEK(P) POSIX Programmer's Manual LSEEK(P)
lseek - move the read/write file offset
off_t lseek(int fildes, off_t offset, int whence);
The lseek() function shall set the file offset for the open file description associated
with the file descriptor fildes, as follows:
* If whence is SEEK_SET, the file offset shall be set to offset bytes.
* If whence is SEEK_CUR, the file offset shall be set to its current location plus off-
* If whence is SEEK_END, the file offset shall be set to the size of the file plus off-
The symbolic constants SEEK_SET, SEEK_CUR, and SEEK_END are defined in <unistd.h>.
The behavior of lseek() on devices which are incapable of seeking is implementation-
defined. The value of the file offset associated with such a device is undefined.
The lseek() function shall allow the file offset to be set beyond the end of the existing
data in the file. If data is later written at this point, subsequent reads of data in the
gap shall return bytes with the value 0 until data is actually written into the gap.
The lseek() function shall not, by itself, extend the size of a file.
If fildes refers to a shared memory object, the result of the lseek() function is unspeci-
If fildes refers to a typed memory object, the result of the lseek() function is unspeci-
Upon successful completion, the resulting offset, as measured in bytes from the beginning
of the file, shall be returned. Otherwise, (off_t)-1 shall be returned, errno shall be
set to indicate the error, and the file offset shall remain unchanged.
The lseek() function shall fail if:
EBADF The fildes argument is not an open file descriptor.
EINVAL The whence argument is not a proper value, or the resulting file offset would be
negative for a regular file, block special file, or directory.
The resulting file offset would be a value which cannot be represented correctly in
an object of type off_t.
ESPIPE The fildes argument is associated with a pipe, FIFO, or socket.
The following sections are informative.
The ISO C standard includes the functions fgetpos() and fsetpos(), which work on very
large files by use of a special positioning type.
Although lseek() may position the file offset beyond the end of the file, this function
does not itself extend the size of the file. While the only function in
IEEE Std 1003.1-2001 that may directly extend the size of the file is write(), truncate(),
and ftruncate(), several functions originally derived from the ISO C standard, such as
fwrite(), fprintf(), and so on, may do so (by causing calls on write()).
An invalid file offset that would cause [EINVAL] to be returned may be both implementa-
tion-defined and device-dependent (for example, memory may have few invalid values). A
negative file offset may be valid for some devices in some implementations.
The POSIX.1-1990 standard did not specifically prohibit lseek() from returning a negative
offset. Therefore, an application was required to clear errno prior to the call and check
errno upon return to determine whether a return value of ( off_t)-1 is a negative offset
or an indication of an error condition. The standard developers did not wish to require
this action on the part of a conforming application, and chose to require that errno be
set to [EINVAL] when the resulting file offset would be negative for a regular file, block
special file, or directory.
open() , the Base Definitions volume of IEEE Std 1003.1-2001, <sys/types.h>, <unistd.h>
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System
Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by
the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and The Open Group
Standard, the original IEEE and The Open Group Standard is the referee document. The orig-
inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .
IEEE/The Open Group 2003 LSEEK(P)