LOCKF(3C)                                                            LOCKF(3C)


NAME
     lockf - record locking on files

C SYNOPSIS
     #include <unistd.h>

     int lockf (int fildes, int function, off_t size);

DESCRIPTION
     The lockf command will allow sections of a file to be locked; advisory or
     mandatory write locks depending on the mode bits of the file [see
     chmod(2)].  Locking calls from other processes which attempt to lock the
     locked file section will either return an error value or be put to sleep
     until the resource becomes unlocked.  All the locks for a process are
     removed when the process terminates.  [See fcntl(2) for more information
     about record locking.]

     Fildes is an open file descriptor.  The file descriptor must have
     O_WRONLY or O_RDWR permission in order to establish lock with this
     function call.

     Function is a control value which specifies the action to be taken.  The
     permissible values for function are defined in <unistd.h> as follows:

     #define   F_ULOCK   0   /* Unlock a previously locked section */
     #define   F_LOCK    1   /* Lock a section for exclusive use */
     #define   F_TLOCK   2   /* Test and lock a section for exclusive use */
     #define   F_TEST    3   /* Test section for other processes locks */


     All other values of function are reserved for future extensions and will
     result in an error return if not implemented.

     F_TEST is used to detect 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.

     Size is the number of contiguous bytes to be locked or unlocked.  The
     resource to be locked starts at the current offset in the file and
     extends forward for a positive size and backward for a negative size (the
     preceding bytes up to but not including the current offset).  If size is
     zero, the section from the current offset through the largest file offset
     is locked (i.e., from the current offset through the present or any
     future end-of-file).  An area need not be allocated to the file in order
     to be locked as such 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 sections occur, the sections
     are combined into a single section.  If the request requires that a new
     element be added to the table of active locks and this table is already


     full, an error is returned, and the new section is not locked.

     F_LOCK and F_TLOCK requests differ only by the action taken if the
     resource is not available.  F_LOCK will cause the calling process to
     sleep until the resource is available.  F_TLOCK will cause the function
     to return a -1 and set errno to [EACCES] error if the section is already
     locked by another process.

     F_ULOCK requests may, in whole or in part, release one or more locked
     sections controlled by the process.  When sections are not fully
     released, the remaining sections are still locked by the process.
     Releasing the center section of a locked section requires an additional
     element in the table of active locks.  If this table is full, an
     [EDEADLK] error is returned and the requested section is not released.

     A potential for deadlock occurs if a process controlling a locked
     resource is put to sleep by accessing another process's locked resource.
     Thus calls to lockf or fcntl scan for a deadlock prior to sleeping on a
     locked resource.  An error return is made if sleeping on the locked
     resource would cause a deadlock.

     Sleeping on a resource is interrupted with any signal.  The alarm(2)
     command may be used to provide a timeout facility in applications which
     require this facility.

     The lockf utility will fail if one or more of the following are true:


     [EBADF]
          Fildes is not a valid open file.

     [EACCES]
          Cmd is F_TLOCK or F_TEST and the section is already locked by
          another process.

     [EDEADLK]
          Cmd is F_LOCK and a deadlock would occur.  Also the cmd is either
          F_LOCK, F_TLOCK, or F_ULOCK and the number of entries in the lock
          table would exceed the number allocated on the system.

NOTES
     Locks are on files, not file descriptors.  That is, file descriptors
     duplicated through dup(3C) do not result in multiple instances of locks,
     but rather multiple references to the same locks.  Thus if any of the
     descriptors associated with the same file are closed, the locks
     associated with the file are lost.

SEE ALSO
     chmod(2), close(2), creat(2), fcntl(2), intro(2), open(2), read(2),
     write(2).


DIAGNOSTICS
     Upon successful completion, a value of 0 is returned.  Otherwise, a value
     of -1 is returned and errno is set to indicate the error.

WARNINGS
     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 standard I/O package is the most common source of unexpected
     buffering.

     Because in the future the variable errno will be set to EAGAIN rather
     than EACCES when a section of a file is already locked by another
     process, portable application programs should expect and test for either
     value.


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