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LSEEK(2) | System Calls Manual | LSEEK(2) |
NAME¶
lseek — reposition read/write file offsetLIBRARY¶
Standard C Library (libc, -lc)SYNOPSIS¶
#include <unistd.h> off_tlseek(int fildes, off_t offset, int whence);
DESCRIPTION¶
The lseek() system call repositions the offset of the file descriptor fildes to the argument offset according to the directive whence. The argument fildes must be an open file descriptor. The lseek() system call repositions the file position pointer associated with the file descriptor fildes as follows:- If whence is
SEEK_SET
, the offset is set to offset bytes. - If whence is
SEEK_CUR
, the offset is set to its current location plus offset bytes. - If whence is
SEEK_END
, the offset is set to the size of the file plus offset bytes. - If whence is
SEEK_HOLE
, the offset of the start of the next hole greater than or equal to the supplied offset is returned. The definition of a hole is provided below. - If whence is
SEEK_DATA
, the offset is set to the start of the next non-hole file region greater than or equal to the supplied offset.
SEEK_HOLE
. File
systems are allowed to expose ranges of zeros with
SEEK_HOLE
, but not required to. Applications can use
SEEK_HOLE
to optimise their behavior for ranges of
zeros, but must not depend on it to find all such ranges in a file. The
existence of a hole at the end of every data region allows for easy
programming and implies that a virtual hole exists at the end of the file.
Applications should use
fpathconf(_PC_MIN_HOLE_SIZE) or
pathconf(_PC_MIN_HOLE_SIZE) to
determine if a file system supports SEEK_HOLE
. See
pathconf(2).
For file systems that do not supply information about holes, the file will be
represented as one entire data region.
RETURN VALUES¶
Upon successful completion, lseek() returns the resulting offset location as measured in bytes from the beginning of the file. Otherwise, a value of -1 is returned and errno is set to indicate the error.ERRORS¶
The lseek() system call will fail and the file position pointer will remain unchanged if:- [
EBADF
] - The fildes argument is not an open file descriptor.
- [
EINVAL
] - The whence argument is not a proper value or the resulting file offset would be negative for a non-character special file.
- [
ENXIO
] - For
SEEK_DATA
, there are no more data regions past the supplied offset. ForSEEK_HOLE
, there are no more holes past the supplied offset. - [
EOVERFLOW
] - The resulting file offset would be a value which cannot be represented correctly in an object of type off_t.
- [
ESPIPE
] - The fildes argument is associated with a pipe, socket, or FIFO.
SEE ALSO¶
dup(2), open(2), pathconf(2)STANDARDS¶
The lseek() system call is expected to conform to IEEE Std 1003.1-1990 (“POSIX.1”).HISTORY¶
The lseek() function appeared in Version 7 AT&T UNIX.BUGS¶
This document's use of whence is incorrect English, but is maintained for historical reasons.April 5, 2007 | Debian |