NAME

lockf - record locking on files

 SYNOPSIS

#include <unistd.h>

int lockf(int fildes, int function, off_t size);

 DESCRIPTION

The lockf() function allows sections of a file to be locked with advisory-mode locks. Calls to lockf() from other threads which attempt to lock the locked file section will 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() is supported for regular files and may be supported for other files.

The fildes argument is an open file descriptor. The file descriptor must have been opened with write-only permission (O_WRONLY) or with read/write permission (O_RDWR) to establish a lock with this function.

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 detects 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 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 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 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 are combined into a single locked section. If the request would cause the number of locks to exceed a system-imposed limit, the request will fail.

F_LOCK and F_TLOCK requests differ only by the action taken if the section is not available. F_LOCK blocks the calling thread until the section is available. F_TLOCK makes the function fail 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 release (wholly or in part) one or more locked sections controlled by the process. Locked sections will 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 are still locked by the process. Releasing the center portion of a locked section will 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 will 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() will fail with an [EDEADLK] error.

The interaction between fcntl() and lockf() locks is unspecified.

Blocking on a section is 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, 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.

Attempting to lock a section of a file that is associated with a buffered stream produces unspecified results.

 RETURN VALUE

Upon successful completion, lockf() returns 0. Otherwise, it returns -1, sets errno to indicate an error, and existing locks are not changed.

 ERRORS

The lockf() function will 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.

 EXAMPLES

None.

 APPLICATION USAGE

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.

 FUTURE DIRECTIONS

None.

 SEE ALSO

alarm(), chmod(), close(), creat(), fcntl(), fopen(), mmap(), open(), read(), write(), <unistd.h>.