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lseek() - Unix, Linux System Call
lseek - reposition read/write file offset
off_t lseek(int fildes, off_t offset, int whence);
lseek() function repositions the offset of the open file associated with the
fildes to the argument
offset according to the directive
whence as follows:
lseek() function allows the file offset to be set beyond the end
of the file (but this does not change the size of the file).
If data is later written at this point, subsequent reads of the data
in the gap (a "hole") return null bytes (\0) until
data is actually written into the gap.
The offset is set to
The offset is set to its current location plus
The offset is set to the size of the file plus
Upon successful completion,
lseek() returns the resulting offset location as measured in bytes from the
beginning of the file.
Otherwise, a value of (off_t)-1 is returned and
errno is set to indicate the error.
fildes is not an open file descriptor.
whence is not one of SEEK_SET, SEEK_CUR, SEEK_END;
or the resulting file offset would be negative,
or beyond the end of a seekable device.
The resulting file offset cannot be represented in an
fildes is associated with a pipe, socket, or FIFO.
SVr4, 4.3BSD, POSIX.1-2001.
Some devices are incapable of seeking and POSIX does not specify which
devices must support
Linux specific restrictions: using lseek() on a tty device returns
This documents use of
whence is incorrect English, but maintained for historical reasons.
When converting old code, substitute values for whence with the
SVr1-3 returns long instead of off_t, BSD returns int.
Note that file descriptors created by
share the current file position pointer, so seeking on such files may be
subject to race conditions.