Manpage of LSEEK
LSEEKSection: Linux Programmer's Manual (2)
NAMElseek - reposition read/write file offset
DESCRIPTIONThe lseek() function repositions the file offset of the open file description associated with the file descriptor fdto the argument offsetaccording to the directive whenceas follows:
- The file offset is set to offsetbytes.
- The file offset is set to its current location plus offsetbytes.
- The file offset is set to the size of the file plus offsetbytes.
The 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 (aq\0aq) until data is actually written into the gap.
Seeking file data and holesSince version 3.1, Linux supports the following additional values for whence:
- Adjust the file offset to the location in the file greater than or equal to offsetcontaining data. If offsetpoints to data, then the file offset is set to offset.
- Adjust the file offset to the hole in the file greater than or equal to offset. If offsetpoints 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, lseek() fails if offsetpoints 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 not be reported as a hole.) In the simplest implementation, a filesystem can support the operations by making SEEK_HOLEalways return the offset of the end of the file, and making SEEK_DATAalways return offset(i.e., even if the location referred to by offsetis a hole, it can be considered to consist of data that is a sequence of zeros).
The _GNU_SOURCEfeature test macro must be defined in order to obtain the definitions of SEEK_DATAand SEEK_HOLEfrom <unistd.h>.
The SEEK_HOLEand SEEK_DATAoperations are supported for the following filesystems:
- Btrfs (since Linux 3.1)
- OCFS (since Linux 3.2)
- XFS (since Linux 3.5)
- ext4 (since Linux 3.8)
- tmpfs(5) (since Linux 3.8)
- NFS (since Linux 3.18)
- FUSE (since Linux 4.5)
RETURN VALUEUpon successful completion, lseek() 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 errnois set to indicate the error.
- fdis not an open file descriptor.
- whenceis not valid. Or: the resulting file offset would be negative, or beyond the end of a seekable device.
- whenceis SEEK_DATAor SEEK_HOLE, and the file offset is beyond the end of the file.
- The resulting file offset cannot be represented in an off_t.
- fdis associated with a pipe, socket, or FIFO.
CONFORMING TOPOSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
NOTESSee open(2) for a discussion of the relationship between file descriptors, open file descriptions, and files.
If the O_APPENDfile status flag is set on the open file description, then a write(2) alwaysmoves the file offset to the end of the file, regardless of the use of lseek().
The off_tdata type is a signed integer data type specified by POSIX.1.
Some devices are incapable of seeking and POSIX does not specify which devices must support lseek().
SEE ALSOdup(2), fork(2), open(2), fseek(3), lseek64(3), posix_fallocate(3)
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