lockf(3C)              UNIX System V(C Development Set)               lockf(3C)


NAME
      lockf - record locking on files

SYNOPSIS
      #include <unistd.h>

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

DESCRIPTION
      lockf locks sections of a file.  Advisory or mandatory write locks depend
      on the mode bits of the file; see chmod(2).  Other processes that try to
      lock the locked file section either get an error or go 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
      OWRONLY or ORDWR 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 FULOCK  0  /* unlock previously locked section */
      #define FLOCK   1  /* lock section for exclusive use */
      #define FTLOCK  2  /* test & lock section for exclusive use */
      #define FTEST   3  /* test section for other locks */
      All other values of function are reserved for future extensions and will
      result in an error return if not implemented.

      FTEST is used to detect if a lock by another process is present on the
      specified section.  FLOCK and FTLOCK both lock a section of a file if
      the section is available.  FULOCK 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 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 (that is, 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 FLOCK or FTLOCK may, in whole or in part,
      contain or be contained by a previously locked section for the same
      process.  Locked sections will be unlocked starting at the the point of
      the offset through size bytes or to the end of file if size is (offt) 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


10/89                                                                    Page 1

lockf(3C)              UNIX System V(C Development Set)               lockf(3C)


      is not locked.

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

      FULOCK 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 errno is
      set to ENOLCK 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 FTLOCK or FTEST and the section is already locked by
             another process.

     EDEADLK
             cmd is FLOCK and a deadlock would occur.

     ENOLCK cmd is FLOCK, FTLOCK, or FULOCK 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), open(2),
      read(2), write(2)

DIAGNOSTICS
      On success, lockf returns 0.  On failure, lockf returns -1 and sets errno
      to indicate 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.


Page 2                                                                    10/89

lockf(3C)              UNIX System V(C Development Set)               lockf(3C)


      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.















































10/89                                                                    Page 3