# lockf

## PROLOG



           This manual page is part of the POSIX Programmer's Manual.  The
           Linux implementation of this interface may differ (consult the
           corresponding Linux manual page for details of Linux behavior), or
           the interface may not be implemented on Linux.
    


## NAME



           lockf — record locking on files
    


## SYNOPSIS



           #include <unistd.h>
    
           int lockf(int _fildes_ , int _function_ , off_t _size_);
    


## DESCRIPTION



           The _lockf_() function shall lock sections of a file with advisory-
           mode locks. Calls to _lockf_() from threads in other processes which
           attempt to lock the locked file section shall 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_() shall be supported for regular files
           and may be supported for other files.
    
           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:
              ┌──────────┬──────────────────────────────────────────────┐
              │ **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 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.
    


## RETURN VALUE



           Upon successful completion, _lockf_() shall return 0. Otherwise, it
           shall return -1, set _errno_ to indicate an error, and existing
           locks shall not be changed.
    


## ERRORS



           The _lockf_() function shall 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.
    
           _The following sections are informative._
    


## EXAMPLES



       **Locking a Portion of a File**
           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);
    


## 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.
    


## RATIONALE



           None.
    


## FUTURE DIRECTIONS



           None.
    


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