--- name: "lockf" section: "3" platform: "macos" package: "Libc" version: "1752.120.2" release: "macOS 26.5" platforms: ["linux", "freebsd", "macos"] url: "https://new.carta.tech/lockf/3" --- # lockf(3) record locking on files ## NAME **lockf** – record locking on files ## LIBRARY Standard C Library (libc, -lc) ## SYNOPSIS **#include \** *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 processes which attempt to lock the locked file section will either return an error value or block until the section becomes unlocked. All of the locks for a process are removed when the process terminates. The argument *fildes* is an open file descriptor. The file descriptor must have been opened either for write-only (O\_WRONLY) or read/write (O\_RDWR) operation. The *function* argument is a control value which specifies the action to be taken. The permissible values for *function* are 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\_ULOCK removes locks from a section of the file; F\_LOCK and F\_TLOCK both lock a section of a file if the section is available; F\_TEST detects if a lock by another process is present on the specified section. 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). However, it is not permitted to lock a section that starts or extends before the beginning of the file. 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). 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 process 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 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 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. 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 region is put to sleep by attempting to lock the locked region of another process. This implementation detects that sleeping until a locked region is unlocked would cause a deadlock and fails with an EDEADLK error. The **lockf**(), [fcntl(2)](/fcntl/2.md), and [flock(2)](/flock/2.md) locks are compatible. Processes using different locking interfaces can cooperate over the same file safely. However, only one of such interfaces should be used within the same process. If a file is locked by a process through [flock(2)](/flock/2.md), any record within the file will be seen as locked from the viewpoint of another process using [fcntl(2)](/fcntl/2.md) or **lockf**(), and vice versa. Blocking on a section is interrupted by any signal. ## RETURN VALUES The **lockf()** function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error. In the case of a failure, existing locks are not changed. ## ERRORS The **lockf**() function will fail if: - \[EAGAIN\] The argument *function* is F\_TLOCK or F\_TEST and the section is already locked by another process. - \[EBADF\] The argument *fildes* is not a valid open file descriptor. The argument *function* is F\_LOCK or F\_TLOCK, and *fildes* is not a valid file descriptor open for writing. - \[EDEADLK\] The argument *function* is F\_LOCK and a deadlock is detected. - \[EINTR\] The argument *function* is F\_LOCK and **lockf**() was interrupted by the delivery of a signal. - \[EINVAL\] The argument *function* is not one of F\_LOCK, F\_TEST, F\_TLOCK, or F\_ULOCK. The argument *fildes* refers to a file that does not support advisory locking. - \[ENOLCK\] The argument *function* is F\_LOCK, F\_TLOCK, or F\_ULOCK and satisfying the lock or unlock request would result in the number of locked regions in the system exceeding a system-imposed limit. - \[EOPNOTSUPP\] The argument *fildes* refers to a socket; these do not support advisory locking. ## SEE ALSO [fcntl(2)](/fcntl/2.md), [flock(2)](/flock/2.md) ## STANDARDS The **lockf**() function conforms to X/Open Portability Guide Issue 4, Version 2 (“XPG4.2”).