lseek(2) System Calls lseek(2)
lseek - move read/write file pointer
off_t lseek(int fildes, off_t offset, int whence);
The lseek() function sets the file pointer associated with the open
file descriptor specified by fildes as follows:
o If whence is SEEK_SET, the pointer is set to offset bytes.
o If whence is SEEK_CUR, the pointer is set to its current location
o If whence is SEEK_END, the pointer is set to the size of the file
The symbolic constants SEEK_SET, SEEK_CUR, and SEEK_END are defined in
the header <<unistd.h>>.
Some devices are incapable of seeking. The value of the file pointer
associated with such a device is undefined.
The lseek() function allows the file pointer to be set beyond the
existing data in the file. If data are later written at this point,
subsequent reads in the gap between the previous end of data and the
newly written data will return bytes of value 0 until data are written
into the gap.
If fildes is a remote file descriptor and offset is negative, lseek()
returns the file pointer even if it is negative. The lseek() function
will not, by itself, extend the size of a file.
If fildes refers to a shared memory object, lseek() behaves as if
fildes referred to a regular file.
Upon successful completion, the resulting offset, as measured in bytes
from the beginning of the file, is returned. Otherwise, (off_t)-1 is
returned, the file offset remains unchanged, and errno is set to indi-
cate the error.
The lseek() function will fail if:
EBADF The fildes argument is not an open file descriptor.
EINVAL The whence argument is not SEEK_SET, SEEK_CUR, or
SEEK_END; or the fildes argument is not a remote file
descriptor and the resulting file pointer would be neg-
EOVERFLOW The resulting file offset would be a value which cannot
be represented correctly in an object of type off_t for
ESPIPE The fildes argument is associated with a pipe, a FIFO,
or a socket.
The lseek() function has a transitional interface for 64-bit file off-
sets. See lf64(5).
In multithreaded applications, using lseek() in conjunction with a
read(2) or write(2) call on a file descriptor shared by more than one
thread is not an atomic operation. To ensure atomicity, use pread() or
See attributes(5) for descriptions of the following attributes:
tab() allbox; cw(2.750000i)| cw(2.750000i) lw(2.750000i)|
lw(2.750000i). ATTRIBUTE TYPEATTRIBUTE VALUE Interface StabilityStan-
creat(2), dup(2), fcntl(2), open(2), read(2), write(2), attributes(5),
SunOS 5.10 17 Apr 2002 lseek(2)