Originální popis anglicky:
lockf - record locking on files
Návod, kniha: POSIX Programmer's Manual
#include <unistd.h>
int lockf(int
fildes, int
function , off_t
size);
The
lockf() function shall lock sections of a file with advisory-mode
locks. Calls to
lockf() from other threads which attempt to lock the
locked file section shall either return an error value or block until the
section becomes unlocked. All the locks for a process are removed when the
process terminates. Record locking with
lockf() shall be supported for
regular files and may be supported for other files.
The
fildes argument is an open file descriptor. To establish a lock with
this function, the file descriptor shall be opened with write-only permission
(O_WRONLY) or with read/write permission (O_RDWR).
The
function argument is a control value which specifies the action to be
taken. The permissible values for
function are defined in
<unistd.h> as follows:
| Function |
Description |
| F_ULOCK |
Unlock locked sections. |
| F_LOCK |
Lock a section for exclusive use. |
| F_TLOCK |
Test and lock a section for exclusive use. |
| F_TEST |
Test a section for locks by other processes. |
F_TEST shall detect if a lock by another process is present on the specified
section.
F_LOCK and F_TLOCK shall both lock a section of a file if the section is
available.
F_ULOCK shall remove locks from a section of the file.
The
size argument is the number of contiguous bytes to be locked or
unlocked. The section to be locked or unlocked starts at the current offset in
the file and extends forward for a positive size or backward for a negative
size (the preceding bytes up to but not including the current offset). If
size is 0, the section from the current offset through the largest
possible file offset shall be locked (that is, from the current offset through
the present or any future end-of-file). An area need not be allocated to the
file to be locked because locks may exist past the end-of-file.
The sections locked with F_LOCK or F_TLOCK may, in whole or in part, contain or
be contained by a previously locked section for the same process. When this
occurs, or if adjacent locked sections would occur, the sections shall be
combined into a single locked section. If the request would cause the number
of locks to exceed a system-imposed limit, the request shall fail.
F_LOCK and F_TLOCK requests differ only by the action taken if the section is
not available. F_LOCK shall block the calling thread until the section is
available. F_TLOCK shall cause the function to fail if the section is already
locked by another process.
File locks shall be released on first close by the locking process of any file
descriptor for the file.
F_ULOCK requests may release (wholly or in part) one or more locked sections
controlled by the process. Locked sections shall be unlocked starting at the
current file offset through
size bytes or to the end-of-file if
size is (
off_t)0. When all of a locked section is not released
(that is, when the beginning or end of the area to be unlocked falls within a
locked section), the remaining portions of that section shall remain locked by
the process. Releasing the center portion of a locked section shall cause the
remaining locked beginning and end portions to become two separate locked
sections. If the request would cause the number of locks in the system to
exceed a system-imposed limit, the request shall fail.
A potential for deadlock occurs if the threads of a process controlling a locked
section are blocked by accessing another process' locked section. If the
system detects that deadlock would occur,
lockf() shall fail with an
[EDEADLK] error.
The interaction between
fcntl() and
lockf() locks is unspecified.
Blocking on a section shall be interrupted by any signal.
An F_ULOCK request in which
size is non-zero and the offset of the last
byte of the requested section is the maximum value for an object of type
off_t, when the process has an existing lock in which
size is 0
and which includes the last byte of the requested section, shall be treated as
a request to unlock from the start of the requested section with a size equal
to 0. Otherwise, an F_ULOCK request shall attempt to unlock only the requested
section.
Attempting to lock a section of a file that is associated with a buffered stream
produces unspecified results.
Upon successful completion,
lockf() shall return 0. Otherwise, it shall
return -1, set
errno to indicate an error, and existing locks shall not
be changed.
The
lockf() function shall fail if:
- EBADF
- The fildes argument is not a valid open file
descriptor; or function is F_LOCK or F_TLOCK and fildes is
not a valid file descriptor open for writing.
- EACCES or EAGAIN
-
The function argument is F_TLOCK or F_TEST and the section is already
locked by another process.
- EDEADLK
- The function argument is F_LOCK and a deadlock is
detected.
- EINTR
- A signal was caught during execution of the function.
- EINVAL
- The function argument is not one of F_LOCK, F_TLOCK,
F_TEST, or F_ULOCK; or size plus the current file offset is less
than 0.
- EOVERFLOW
- The offset of the first, or if size is not 0 then
the last, byte in the requested section cannot be represented correctly in
an object of type off_t.
The
lockf() function may fail if:
- EAGAIN
- The function argument is F_LOCK or F_TLOCK and the
file is mapped with mmap().
- EDEADLK or ENOLCK
-
The function argument is F_LOCK, F_TLOCK, or F_ULOCK, and the request
would cause the number of locks to exceed a system-imposed limit.
- EOPNOTSUPP or EINVAL
-
The implementation does not support the locking of files of the type
indicated by the fildes argument.
The following sections are informative.
In the following example, a file named
/home/cnd/mod1 is being modified.
Other processes that use locking are prevented from changing it during this
process. Only the first 10000 bytes are locked, and the lock call fails if
another process has any part of this area locked already.
#include <fcntl.h>
#include <unistd.h>
int fildes;
int status;
...
fildes = open("/home/cnd/mod1", O_RDWR);
status = lockf(fildes, F_TLOCK, (off_t)10000);
Record-locking should not be used in combination with the
fopen(),
fread(),
fwrite(), and other
stdio functions. Instead,
the more primitive, non-buffered functions (such as
open()) should be
used. Unexpected results may occur in processes that do buffering in the user
address space. The process may later read/write data which is/was locked. The
stdio functions are the most common source of unexpected buffering.
The
alarm() function may be used to provide a timeout facility in
applications requiring it.
None.
None.
alarm() ,
chmod() ,
close() ,
creat() ,
fcntl() ,
fopen() ,
mmap() ,
open() ,
read() ,
write() , the Base Definitions volume of
IEEE Std 1003.1-2001,
<unistd.h>
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard, the original
IEEE and The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.opengroup.org/unix/online.html
.
