LOCKF(3C-SysV)      RISC/os Reference Manual       LOCKF(3C-SysV)



NAME
     lockf - record locking on files

SYNOPSIS
     #include <unistd.h>

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

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 sec-
     tion 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 exten-
     sions 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.



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     The sections locked with F_LOCK or F_TLOCK may, in whole or
     in part, contain or be contained by a previously locked sec-
     tion for the same process.  When this occurs, or if adjacent
     sections occur, the sections are combined into a single sec-
     tion.  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 sec-
     tions 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 descriptor.

     [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.

     [ECOMM]             fildes is on a remote machine and the
                         link to that machine is no longer
                         active.



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LOCKF(3C-SysV)      RISC/os Reference Manual       LOCKF(3C-SysV)



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.  Oth-
     erwise, a value of -1 is returned and errno is set to indi-
     cate 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 pro-
     grams should expect and test for either value.

































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