Method Stdio.Fd()->seek()

Method seek

int seek(int offset)
int seek(int offset, string whence)

Description

The seek() function repositions the offset of the open file associated with the file descriptor fd to the argument offset according to the directive whence as follows:

Stdio.SEEK_SET	The offset is set to offset bytes.
Stdio.SEEK_CUR	The offset is set to its current location plus offset bytes.
Stdio.SEEK_END	The offset is set to the size of the file plus offset bytes.

If whence is not specified it is SEEK_SET if offset is positive, and if offset is negative SEEK_END.

The seek() function on most operating systems 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.

Seeking file data and holes

Stdio.SEEK_DATA and Stdio.SEEK_HOLE are nonstandard extensions present in Linux, Solaris, FreeBSD, and DragonFly BSD; they are proposed for inclusion in the next POSIX revision.

Stdio.SEEK_DATA	Adjust the file offset to the next location in the file greater than or equal to offset containing data. If offset points to data, then the file offset is set to offset.
Stdio.SEEK_HOLE	Adjust the file offset to the next hole in the file greater than or equal to offset. If offset points into the middle of a hole, then the file offset is set to offset. If there is no hole past offset, then the file offset is adjusted to the end of the file (i.e., there is an implicit hole at the end of any file).

In both of the above cases, seek() fails if offset points past the end of the file.

These operations allow applications to map holes in a sparsely allocated file. This can be useful for applications such as file backup tools, which can save space when creating backups and preserve holes, if they have a mechanism for discovering holes.

For the purposes of these operations, a hole is a sequence of zeros that (normally) has not been allocated in the underlying file storage. However, a filesystem is not obliged to report holes, so these operations are not a guaranteed mechanism for mapping the storage space actually allocated to a file. (Furthermore, a sequence of zeros that actually has been written to the underlying storage may or may not be reported as a hole.)

In the simplest implementation, a filesystem can support the operations by making SEEK_HOLE always return the offset of the end of the file, and making SEEK_DATA always return offset (i.e., even if the location referred to by offset is a hole, it can be considered to consist of data that is a sequence of zeros).

Returns

Upon successful completion, seek() returns the resulting offset location as measured in bytes from the beginning of the file. On error, the value (off_t) -1 is returned and errno is set to indicate the error.

See also

tell()