#include <unistd.h> int lockf(int fildes, int function, off_t size);
The fildes argument is an open file descriptor. To establish a lock with this function, the file descriptor shall be opened with write-only permission (O_WRONLY) or with read/write permission (O_RDWR).
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:
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F_TEST shall detect if a lock by another process is present on the specified section.
F_LOCK and F_TLOCK shall both lock a section of a file if the section is available.
F_ULOCK shall remove 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 shall be 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 shall be combined into a single locked section. If the request would cause the number of locks to exceed a system-imposed limit, the request shall fail.
F_LOCK and F_TLOCK requests differ only by the action taken if the section is not available. F_LOCK shall block the calling thread until the section is available. F_TLOCK shall cause the function to fail if the section is already locked by another process.
File locks shall be 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 shall 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 shall remain locked by the process. Releasing the center portion of a locked section shall 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 shall fail.
A potential for deadlock occurs if the threads of a process controlling a locked section are blocked by accessing a locked section of another process. If the system detects that deadlock would occur, lockf() shall fail with an [EDEADLK] error.
The interaction between fcntl() and lockf() locks is unspecified.
Blocking on a section shall be 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, shall 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 shall 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.
The lockf() function may fail if:
The following sections are informative.
In the following example, a file named /home/cnd/mod1 is being modified. Other processes that use locking are prevented from changing it during this process. Only the first 10000 bytes are locked, and the lock call fails if another process has any part of this area locked already.
#include <fcntl.h>
#include <unistd.h>
int fildes;
int status;
...
fildes = open("/home/cnd/mod1", O_RDWR);
status = lockf(fildes, F_TLOCK, (off_t)10000);
The alarm() function may be used to provide a timeout facility in applications requiring it.
The Base Definitions volume of POSIX.1-2017, <unistd.h>
Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .