LSEEK(2) System Calls Manual LSEEK(2)
lseek - move read/write pointer
#define L_SET 0 /* set the seek pointer */
#define L_INCR 1 /* increment the seek pointer */
#define L_XTND 2 /* extend the file size */
pos = lseek(d, offset, 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 L_SET, the pointer is set to offset bytes.
If whence is L_INCR, the pointer is set to its current location plus offset.
If whence is L_XTND, the pointer is set to the size of the file plus offset.
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 undefined.
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
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 historical reasons.
4th Berkeley Distribution February 24, 1986 LSEEK(2)