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
o If whence is SEEK_CUR, the pointer is set to its
current location plus offset.
o If whence is SEEK_END, the pointer is set to the size
of the file plus offset.
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 nega-
tive, lseek() returns the file pointer even if it is nega-
tive. The lseek() function will not, by itself, extend the
size of a file.
Upon successful completion, the resulting offset, as meas-
ured 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 indicate the error.
The lseek() function will fail if:
EBADF The fildes argument is not an open file descriptor.
The whence argument is not SEEK_SET, SEEK_CUR, or
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System Calls lseek(2)
SEEK_END; or the fildes argument is not a remote file
descriptor and the resulting file pointer would be
The resulting file offset would be a value which can-
not be represented correctly in an object of type
off_t for regular files.
The fildes argument is associated with a pipe, a FIFO,
or a socket.
The lseek() function has a transitional interface for 64-bit
file offsets. 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 pwrite().
See attributes(5) for descriptions of the following attri-
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
| MT-Level | Async-Signal-Safe |
creat(2), dup(2), fcntl(2), open(2), read(2), write(2),
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