LOCKF(3C-SVR4)      RISC/os Reference Manual       LOCKF(3C-SVR4)



NAME
     lockf - record locking on files

SYNOPSIS
     #include <unistd.h>

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

DESCRIPTION
     lockf allows 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 that
     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
     locks with this function call.

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

     #define   F_ULOCK   0   /* unlock previously locked section */
     #define   F_LOCK    1   /* lock section for exclusive use */
     #define   F_TLOCK   2   /* test & lock section for exclusive use */
     #define   F_TEST    3   /* test section for other 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 or unlocked starts at
     the current offset in the file and extends forward for a
     positive size and backward for a negative size (the preced-
     ing 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 sec-
     tion for the same process.  Locked sections will be unlocked



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LOCKF(3C-SVR4)      RISC/os Reference Manual       LOCKF(3C-SVR4)



     starting at the the point of the offset through size bytes
     or to the end of file if size is (off_t) 0.  When this
     situation occurs, or if this situation occurs in adjacent
     sections, 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 if the section is already locked by another pro-
     cess.

     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 errno is
     set to ENOLK and the requested section is not released.

     A potential for deadlock occurs if a process controlling a
     locked resource is put to sleep by requesting 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 system call may be used to provide a timeout facility
     in applications that require this facility.

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

     EBADF  fildes is not a valid open descriptor.

     EAGAIN cmd is F_TLOCK or F_TEST and the section is already
            locked by another process.

     EDEADLKcmd is F_LOCK and a deadlock would occur.

     ENOLK  cmd is 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.

SEE ALSO
     intro(2), alarm(2), chmod(2), close(2), creat(2), fcntl(2),



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LOCKF(3C-SVR4)      RISC/os Reference Manual       LOCKF(3C-SVR4)



     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.

NOTES
     Unexpected results may occur in processes that do buffering
     in the user address space.  The process may later read/write
     data that 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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