LOCKF(3C) Standard C Library Functions LOCKF(3C)

lockf - POSIX-style record locking on files

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

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 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:

#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 if not implemented.

F_TEST is used to detect if a lock by another process or open file handle 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 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 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 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 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 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.

Upon successful completion, 0 is returned. Otherwise, −1 is returned and errno is set to indicate the error.

The lockf() function will fail if:


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.


The function argument is F_TLOCK or F_TEST and the section is already locked by another process.


The function argument is F_LOCK and a deadlock is detected.


A signal was caught during execution of the function.


The fildes argument is on a remote machine and the link to that machine is no longer active.


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.


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:


The function argument is F_LOCK or F_TLOCK and the file is mapped with mmap(2).


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.


The locking of files of the type indicated by the fildes argument is not supported.

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(7).

See attributes(7) for descriptions of the following attributes:

Interface Stability Standard
MT-Level MT-Safe

Intro(2), alarm(2), chmod(2), close(2), creat(2), fcntl(2), mmap(2), open(2), read(2), write(2), attributes(7), lf64(7), standards(7)

February 16, 2015 OmniOS