Positions a file to an arbitrary byte position and returns the new
position as an int.
Format
#include <unistd.h>
off_t lseek (int file_desc, off_t offset, int
direction);
Arguments
- file_desc
- An integer returned by open, creat, dup, or dup2.
- offset
- The offset, specified in bytes.
- direction
- An integer indicating whether the offset is to be measured
forward from the beginning of the file (direction=SEEK_SET), forward
from the current position (direction=SEEK_CUR), or backward from the
end of the file (direction=SEEK_END).
Description
This function can position fixed-length record-access file
with no carriage control or a stream-access file on any byte offset,
but can position all other files only on record boundaries.
The available Standard I/O functions position a record file at its
first byte, at the end-of-file, or on a record boundary. Therefore,
the arguments given to lseek must specify either the beginning or
end of the file, a 0 offset from the current position (an arbitrary
record boundary), or the position returned by a previous, valid
lseek call.
For a portable way to position an arbitrary byte location with any
type of file, see the fgetpos and fsetpos functions in this section.
- CAUTION
- If, while accessing a stream
file, you seek beyond the end-of-file and then write to the file,
the lseek function creates a hole by filling the skipped bytes with
zeros.
In general, for record files, lseek should only be directed to an
absolute position that was returned by a previous valid call to
lseek or to the beginning or end of a file. If a call to lseek does
not satisfy these conditions, the results are unpredictable.
See also open, creat, dup, dup2, and fseek in this section.
Return Values
| x | The new file position. |
| -1 | Indicates that the file descriptor
is undefined, or a seek was attempted before the beginning of the
file. |
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