# lockf

## NAME



           lockf - apply, test or remove a POSIX lock on an open file
    


## LIBRARY



           Standard C library (_libc_ , _-lc_)
    


## SYNOPSIS



           **#include <unistd.h>**
    
           **int lockf(int** _fd_**, int** _op_**, off_t** _size_**);**
    
       Feature Test Macro Requirements for glibc (see
       feature_test_macros(7)):
    
           **lockf**():
               _XOPEN_SOURCE >= 500
                   || /* glibc >= 2.19: */ _DEFAULT_SOURCE
                   || /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
    


## DESCRIPTION



           Apply, test, or remove a POSIX lock on a section of an open file.
           The file is specified by _fd_ , a file descriptor open for writing,
           the action by _op_ , and the section consists of byte positions
           _pos_.._pos_ +_size_ -1 if _size_ is positive, and _pos_ -_size_.._pos_ -1 if _size_
           is negative, where _pos_ is the current file position, and if _size_
           is zero, the section extends from the current file position to
           infinity, encompassing the present and future end-of-file
           positions.  In all cases, the section may extend past current end-
           of-file.
    
           On Linux, **lockf**() is just an interface on top of fcntl(2) locking.
           Many other systems implement **lockf**() in this way, but note that
           POSIX.1 leaves the relationship between **lockf**() and fcntl(2) locks
           unspecified.  A portable application should probably avoid mixing
           calls to these interfaces.
    
           Valid operations are given below:
    
           **F_LOCK** Set an exclusive lock on the specified section of the file.
                  If (part of) this section is already locked, the call
                  blocks until the previous lock is released.  If this
                  section overlaps an earlier locked section, both are
                  merged.  File locks are released as soon as the process
                  holding the locks closes some file descriptor for the file.
                  A child process does not inherit these locks.
    
           **F_TLOCK**
                  Same as **F_LOCK** but the call never blocks and returns an
                  error instead if the file is already locked.
    
           **F_ULOCK**
                  Unlock the indicated section of the file.  This may cause a
                  locked section to be split into two locked sections.
    
           **F_TEST** Test the lock: return 0 if the specified section is
                  unlocked or locked by this process; return -1, set _errno_ to
                  **EAGAIN**(**EACCES** on some other systems), if another process
                  holds a lock.
    


## RETURN VALUE



           On success, zero is returned.  On error, -1 is returned, and _errno_
           is set to indicate the error.
    


## ERRORS



           **EACCES** or **EAGAIN**
                  The file is locked and **F_TLOCK** or **F_TEST** was specified, or
                  the operation is prohibited because the file has been
                  memory-mapped by another process.
    
           **EBADF** _fd_ is not an open file descriptor; or _op_ is **F_LOCK** or
                  **F_TLOCK** and _fd_ is not a writable file descriptor.
    
           **EDEADLK**
                  _op_ was **F_LOCK** and this lock operation would cause a
                  deadlock.
    
           **EINTR** While waiting to acquire a lock, the call was interrupted
                  by delivery of a signal caught by a handler; see signal(7).
    
           **EINVAL** An invalid operation was specified in _op_.
    
           **ENOLCK** Too many segment locks open, lock table is full.
    


## ATTRIBUTES



           For an explanation of the terms used in this section, see
           attributes(7).
           ┌──────────────────────────────────────┬───────────────┬─────────┐
           │ **Interface** │ **Attribute** │ **Value** │
           ├──────────────────────────────────────┼───────────────┼─────────┤
           │ **lockf**()                              │ Thread safety │ MT-Safe │
           └──────────────────────────────────────┴───────────────┴─────────┘
    


## STANDARDS



           POSIX.1-2008.
    


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