| Standards / Extensions | C or C++ | Dependencies |
|---|---|---|
XPG4.2 |
both |
#define _XOPEN_SOURCE_EXTENDED 1
#include <unistd.h>
int lockf(int filedes, int function, off_t size);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 the locks for a process are removed when the process terminates. Record locking with lockf() is supported for regular files.
The filedes argument is an open file descriptor. The file descriptor must have been opened with a write-only permission (O_WRONLY) or with read/write permission (O_RDWR) to establish a lock with this function.
Function Description
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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 processesF_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 (EOF)). 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 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 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 (EOF) 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 a process controlling a locked section is blocked by accessing another process's locked section. If the system detects that a deadlock would occur, lockf() will fail with an EDEADLK error.
Locks obtained by lockf() are controlled by the same facility controlling locks obtained by fcntl().
The interaction between fcntl() and lockf() locks is unspecified.
Blocking on a section is interrupted by any signal.
Large file support for z/OS UNIX files: Large z/OS UNIX files are supported automatically for AMODE 64 C/C++ applications. AMODE 31 C/C++ applications must be compiled with the option LANGLVL(LONGLONG) and define the _LARGE_FILES feature test macro before any headers are included to enable this function to operate on z/OS UNIX files that are larger than 2 GB in size. File size and offset fields are enlarged to 63 bits in width. Therefore, any other function operating on the file is required to define the _LARGE_FILES feature test macro as well.
If successful, lockf() returns 0.