lseek - move read/write pointer
#define SEEK_SET 0 /* offset is absolute */
#define SEEK_CUR 1 /* relative to current position */
#define SEEK_END 2 /* relative to end of file */
off_t lseek(int d, off_t offset, int whence)
The descriptor d refers to a file or device open for reading and/or
writing. Lseek sets the file pointer of d as follows:
If whence is SEEK_SET, the pointer is set to offset bytes.
If whence is SEEK_CUR, the pointer is set to its current location
If whence is SEEK_END, the pointer is set to the size of the file
Upon successful completion, the resulting pointer location as measured in
bytes from beginning of the file is returned. Some devices are incapable
of seeking. The value of the pointer associated with such a device is
Seeking far beyond the end of a file, then writing, creates a gap or
"hole", which occupies no physical space and reads as zeros.
Upon successful completion, the current file pointer value is returned.
Otherwise, a value of -1 is returned and errno is set to indicate the
Lseek will fail and the file pointer will remain unchanged if:
[EBADF] Fildes is not an open file descriptor.
[ESPIPE] Fildes is associated with a pipe or a socket.
[EINVAL] Whence is not a proper value.
This document's use of whence is incorrect English, but maintained for