| lseek(2) | System Calls Manual | lseek(2) |
lseek - reposition read/write file offset
Standard C library (libc, -lc)
#include <unistd.h>
off_t lseek(int fd, off_t offset, int whence);
lseek() repositions the file offset of the open file description associated with the file descriptor fd to the argument offset according to the directive whence as follows:
lseek() 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.
Since Linux 3.1, Linux supports the following additional values for whence:
In both of the above cases, lseek() 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 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).
The _GNU_SOURCE feature test macro must be defined in order to obtain the definitions of SEEK_DATA and SEEK_HOLE from <unistd.h>.
The SEEK_HOLE and SEEK_DATA operations are supported for the following filesystems:
Upon 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 errno is set to indicate the error.
On Linux, using lseek() on a terminal device fails with the error ESPIPE.
POSIX.1-2008.
POSIX.1-2001, SVr4, 4.3BSD.
SEEK_DATA and SEEK_HOLE are nonstandard extensions also present in Solaris, FreeBSD, and DragonFly BSD; they are proposed for inclusion in the next POSIX revision (Issue 8).
See open(2) for a discussion of the relationship between file descriptors, open file descriptions, and files.
If the O_APPEND file status flag is set on the open file description, then a write(2) always moves the file offset to the end of the file, regardless of the use of lseek().
Some devices are incapable of seeking and POSIX does not specify which devices must support lseek().
dup(2), fallocate(2), fork(2), open(2), fseek(3), lseek64(3), posix_fallocate(3)
| 2023-03-30 | Linux man-pages 6.05.01 |