NAME
lseek - reposition read/write file offset
SYNOPSIS
#include <sys/types.h>
#include <unistd.h>
off_t lseek(int fd, off_t offset, int whence);
fd：文件描述符；
offset: 移動文件位置的偏移數，以字節為單位；
whence為系統定義的常量，可能的值有SEEK_SET、SEEK_CUR、SEEK_END、SEEK_DATA、SEEK_HOLE。
DESCRIPTION
The lseek() 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:
SEEK_SET
The offset is set to offset bytes.
文件讀寫位置從文件開始往后移動offset個字節。
SEEK_CUR
The offset is set to its current location plus offset bytes.
文件讀寫位置從當前位置開始移動offset個字節。offset可以為負值。
SEEK_END
The offset is set to the size of the file plus offset bytes.
文件讀寫位置從文件結尾移動offset個字節。offset可以為負值。
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 ('\0') until
data is actually written into the gap.
Seeking file data and holes
Since version 3.1, Linux supports the following additional values for whence:
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.
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, 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>.
RETURN VALUE
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.
ERRORS
EBADF fd is not an open file descriptor. 傳入的參數不是一個已經打開的文件。
EINVAL whence is not valid. Or: the resulting file offset would be negative, or beyond the end of a seekable device.傳入的whence參數不合理。
EOVERFLOW
The resulting file offset cannot be represented in an off_t.
傳入的移動參數導致文件頭指針指向了文件頭以前，產生了溢出錯誤。
ESPIPE fd is associated with a pipe, socket, or FIFO.
ENXIO whence is SEEK_DATA or SEEK_HOLE, and the current file offset is beyond the end of the file.
NOTES
Some devices are incapable of seeking and POSIX does not specify which devices must support lseek().
On Linux, using lseek() on a terminal device returns ESPIPE.
When converting old code, substitute values for whence with the following macros:
old new
0 SEEK_SET
1 SEEK_CUR
2 SEEK_END
L_SET SEEK_SET
L_INCR SEEK_CUR
L_XTND SEEK_END
Note that file descriptors created by dup(2) or fork(2) share the current file position pointer, so seeking on such files may be subject to race conditions.
SEE ALSO
dup(2), fork(2), open(2), fseek(3), lseek64(3), posix_fallocate(3)