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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)). Locking calls from other processes 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 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 in <unistd.h> as follows:
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#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 */
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All other values of function are reserved for future extensions 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 through 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 F_LOCK or F_TLOCK may, in whole or in part, contain or be contained by a previously locked
section for the same process. Locked sections will be unlocked starting at the the point of the offset through size bytes or to the 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 will cause the calling process to sleep until the resource is available. F_TLOCK will cause the function to return a -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 process 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 sections 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 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, will 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 will attempt to unlock only the requested section.
A potential for deadlock occurs if 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 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.
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