Standard C Library Functions lockf(3C)
NAME
lockf - record locking on files
SYNOPSIS
#include
int lockf(int fildes, int function, offt size);
DESCRIPTION
The lockf() function allows sections of a file to be locked;
advisory or mandatory write locks depending on the mode
bits 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 file
descriptor must have OWRONLY or ORDWR 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 in as follows:
#define FULOCK 0 /* unlock previously locked section */
#define FLOCK 1 /* lock section for exclusive use */
#define FTLOCK 2 /* test & lock section for exclusive use */
#define FTEST 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.
FTEST is used to detect if a lock by another process is
present on the specified section. FLOCK and FTLOCK both
lock a section of a file if the section is available.
FULOCK 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 through
SunOS 5.11 Last change: 10 Apr 2002 1
Standard C Library Functions lockf(3C)
the largest file offset is 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 in order to be locked
as such locks may exist past the end-of-file.
The sections locked with FLOCK or FTLOCK 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 to
the end of file if size is (offt) 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.
FLOCK and FTLOCK requests differ only by the action taken
if the resource is not available. FLOCK blocks the calling
thread until the resource is available. FTLOCK 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.
FULOCK 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 FULOCK 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 offt, 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 a
size equal to 0. Otherwise, an FULOCK 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 requesting
another process's locked resource. Thus calls to lockf() or
fcntl(2) scan for a deadlock prior to sleeping on a locked
SunOS 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.
ERORS
The lockf() function will fail if:
EBADF The fildes argument is not a valid open
file descriptor; or function is FLOCK
or FTLOCK and fildes is not a valid
file descriptor open for writing.
EACES or EAGAIN The function argument is FTLOCK or
FTEST and the section is already locked
by another process.
EDEADLK The function argument is FLOCK and a
deadlock is detected.
EINTR A signal was caught during execution of
the function.
ECOM 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 of
FLOCK, FTLOCK, FTEST, or FULOCK; 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 offt.
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 FLOCK or
FTLOCK and the file is mapped with
mmap(2).
EDEADLK or ENOLCK The function argument is FLOCK,
FTLOCK, or FULOCK and the request
would cause the number of locks to
exceed a system-imposed limit.
EOPNOTSUP 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).
ATRIBUTES
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)
ATRIBUTE TYPE ATRIBUTE VALUE
Interface Stability Standard
MT-Level MT-Safe
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
|
