Standard C Library Functions lockf(3C)
NAME
lockf - record locking on files
SYNOPSIS
#include
int lockf(int fildes, int function, off_t size);
DESCRIPTION
The lockf() function allows sections of a file to be locked;
advisory or mandatory write locks depending on the modebits of the file (see chmod(2)). Calls to lockf() from other
threads that attempt to lock the locked file section will either return an error value or be put to sleep until the resource becomes unlocked. All the locks for a process are removed when the process terminates. See fcntl(2) for more information about record locking. The fildes argument is an open file descriptor. The filedescriptor must have O_WRONLY or O_RDWR permission in order
to establish locks with this function call. The function argument is a control value that specifies the action to be taken. The permissible values for function are defined inas follows: #define F_ULOCK 0 /* unlock previously locked section */
#define F_LOCK 1 /* lock section for exclusive use */
#define F_TLOCK 2 /* test & lock section for exclusive use */
#define F_TEST 3 /* test section for other locks */
All other values of function are reserved for future exten-
sions and will result in an error if not implemented.F_TEST is used to detect if a lock by another process is
present on the specified section. F_LOCK and F_TLOCK both
lock a section of a file if the section is available.F_ULOCK removes locks from a section of the file.
The size argument is the number of contiguous bytes to be locked or unlocked. The resource to be locked or unlocked starts at the current offset in the file and extends forward for a positive size and backward for a negative size (the preceding bytes up to but not including the current offset). If size is zero, the section from the current offset throughSunOS 5.11 Last change: 10 Apr 2002 1
Standard C Library Functions lockf(3C)
the largest file offset is locked (that is, from the currentoffset through the present or any future end-of-file). An
area need not be allocated to the file in order to be lockedas such 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 sec-
tion for the same process. Locked sections will be unlocked starting at the point of the offset through size bytes or tothe end of file if size is (off_t) 0. When this situation
occurs, or if this situation occurs in adjacent sections, the sections are combined into a single section. If the request requires that a new element be added to the table of active locks and this table is already full, an error is returned, and the new section is not locked.F_LOCK and F_TLOCK requests differ only by the action taken
if the resource is not available. F_LOCK blocks the calling
thread until the resource is available. F_TLOCK causes the
function to return -1 and set errno to EAGAIN if the section
is already locked by another process.File locks are released on first close by the locking pro-
cess of any file descriptor for the file.F_ULOCK requests may, in whole or in part, release one or
more locked sections controlled by the process. When sec-
tions are not fully released, the remaining sections are still locked by the process. Releasing the center section of a locked section requires an additional element in the table of active locks. If this table is full, an errno is set to EDEADLK and the requested section is not released.An F_ULOCK request in which size is non-zero and the offset
of the last byte of the requested section is the maximumvalue 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, will be treated as a request to unlock from the start of the requested section with asize equal to 0. Otherwise, an F_ULOCK request will attempt
to unlock only the requested section. A potential for deadlock occurs if the threads of a process controlling a locked resource is put to sleep by requestinganother process's locked resource. Thus calls to lockf() or
fcntl(2) scan for a deadlock prior to sleeping on a lockedSunOS 5.11 Last change: 10 Apr 2002 2
Standard C Library Functions lockf(3C)
resource. An error return is made if sleeping on the locked resource would cause a deadlock. Sleeping on a resource is interrupted with any signal. The alarm(2) function may be used to provide a timeout facility in applications that require this facility.RETURN VALUES
Upon successful completion, 0 is returned. Otherwise, -1 is
returned and errno is set to indicate the error.ERRORS
The lockf() function will fail if:
EBADF The fildes argument is not a valid openfile 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. ECOMM The fildes argument is on a remote machine and the link to that machine is no longer active. EINVAL The function argument is not one ofF_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 representedcorrectly in an object of type off_t.
SunOS 5.11 Last change: 10 Apr 2002 3
Standard C Library Functions lockf(3C)
The lockf() function may fail if:
EAGAIN The function argument is F_LOCK or
F_TLOCK and the file is mapped with
mmap(2).EDEADLK or ENOLCK The function argument is F_LOCK,
F_TLOCK, or F_ULOCK and the request
would cause the number of locks toexceed a system-imposed limit.
EOPNOTSUPP or EINVAL The locking of files of the type indicated by the fildes argument is not supported.USAGE
Record-locking should not be used in combination with the
fopen(3C), fread(3C), fwrite(3C) and other stdio functions.Instead, the more primitive, non-buffered functions (such as
open(2)) 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(2) function may be used to provide a timeout facility in applications requiring it.The lockf() function has a transitional interface for 64-bit
file offsets. See lf64(5).ATTRIBUTES
See attributes(5) for descriptions of the following attri-
butes:SunOS 5.11 Last change: 10 Apr 2002 4
Standard C Library Functions lockf(3C)
____________________________________________________________
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
|_____________________________|_____________________________|
| Interface Stability | Committed ||_____________________________|_____________________________|
| MT-Level | MT-Safe |
|_____________________________|_____________________________|
| Standard | See standards(5). ||_____________________________|_____________________________|
SEE ALSO
Intro(2), alarm(2), chmod(2), close(2), creat(2), fcntl(2), mmap(2), open(2), read(2), write(2), attributes(5), lf64(5), standards(5)SunOS 5.11 Last change: 10 Apr 2002 5