lockf(3C)

lockf, lockf64 -- record locking on files

Synopsis

   #include <unistd.h>
   
   int lockf (int fildes, int function, off_t size);
   

   int lockf64 (int fildes, int function, off64_t size);

Description

lockf locks sections of a file. Advisory or mandatory write locks depend on the mode bits of the file; see chmod(2). Other processes that try to lock the locked file section either get an error or go 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.

fildes is an open file descriptor. The file descriptor must have O_WRONLY or O_RDWR permission to establish locks with this function call.

function is a control value that specifies the action to be taken. The permissible values for function are defined in unistd.h as 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 extensions and will result in an error return 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.

size 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 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 point of the offset through size bytes or to the end of file if size is (off_t) 0. When this occurs, or if this 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 EACCES if the section is already locked by another process.

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.

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 scan for a deadlock before 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 system call may be used to provide a timeout facility in applications that require this facility.

Return values

On success, lockf and lockf64 return 0. On failure, lockf and lockf64 return -1 and set errno to indicate the error.

Errors

lockf will fail if one or more of the following are true:

EOVERFLOW: The offset of the first byte, or if the size is not 0 (zero), then the last byte, in the requested section cannot be represented correctly in an object of type off_t.

lockf and lockf64 will fail if one or more of the following are true:

EBADF: fildes is not a valid open descriptor.
EAGAIN: cmd is F_TLOCK or F_TEST and the section is already locked by another process.
EDEADLK: cmd is F_LOCK and a deadlock would occur.
EDEADLK: cmd is F_LOCK, F_TLOCK, or F_ULOCK and the number of entries in the lock table would exceed the number allocated on the system.
ECOMM: fildes is on a remote machine and the link to that machine is no longer active.
EINVAL: function is not one of F_LOCK, F_TLOCK, F_TEST, or F_UNLOCK, or size plus the current file offset is less than 0 (zero).

References

alarm(2), chmod(2), close(2), creat(2), fcntl(2), intro(2), intro(2), open(2), read(2), write(2)

Notices

Unexpected results may occur in processes that do buffering in the user address space. The process may later read/write data that is/was locked. The standard I/O package is the most common source of unexpected buffering.

Because in the future the variable errno will be set to EAGAIN rather than EACCES when a section of a file is already locked by another process, portable application programs should expect and test for either value.

Considerations for large file support

lockf64 supports large files, but is otherwise identical to lockf. For details on programming for large file capable applications, see ``Large File Support'' on intro(2).

#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 /