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Standard C Library Functions                                         lockf(3C)

       lockf - record locking on files

       #include <unistd.h>

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

       The  lockf()  function allows sections of a file to be locked; advisory
       or mandatory write locks depending on the mode bits of  the  file  (see
       chmod(2)). Calls to lockf() from other threads 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 informa‐
       tion about record locking.

       The  fildes  argument  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.

       The  function  argument 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 extensions and
       will result in an error 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

       The  size  argument  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 F_LOCK or F_TLOCK 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 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 blocks the calling thread  until  the
       resource is available. F_TLOCK causes the function to return −1 and set
       errno to EAGAIN if the section is already locked by another process.

       File locks are released on first close by the locking  process  of  any
       file descriptor for the file.

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

       An F_ULOCK request in which size is non-zero and the offset of the last
       byte  of  the  requested  section is the maximum value for an object of
       type off_t, when the process has an existing lock in which  size  is  0
       and  which  includes  the  last  byte of the requested section, will be
       treated as a request to unlock from the start of the requested  section
       with  a  size equal to 0. Otherwise, an F_ULOCK request will attempt to
       unlock only the requested section.

       A potential for deadlock occurs if the threads of a process controlling
       a  locked  resource  is  put  to  sleep by requesting another process's
       locked resource. Thus calls to lockf() or fcntl(2) 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)
       function may be used to provide a timeout facility in applications that
       require this facility.

       Upon successful completion, 0 is returned. Otherwise,  −1  is  returned
       and errno is set to indicate the error.

       The lockf() function will fail if:

       EBADF               The  fildes  argument  is  not  a  valid  open file
                           descriptor; or function is F_LOCK  or  F_TLOCK  and
                           fildes  is  not  a  valid  file descriptor open for

       EACCES or EAGAIN    The function argument is F_TLOCK or F_TEST and  the
                           section is already locked by another process.

       EDEADLK             The  function  argument is F_LOCK and a deadlock is

       EINTR               A signal was caught during execution of  the  func‐

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

       EINVAL              The  function  argument  is  not  one  of   F_LOCK,
                           F_TLOCK,  F_TEST, or F_ULOCK; or size plus the cur‐
                           rent file offset is less than 0.

       EOVERFLOW           The offset of the first, or if size is not  0  then
                           the  last,  byte in the requested section cannot be
                           represented correctly in an object of type off_t.

       The lockf() function may fail if:

       EAGAIN                  The function argument is F_LOCK or F_TLOCK  and
                               the file is mapped with mmap(2).

       EDEADLK or ENOLCK       The  function  argument  is F_LOCK, F_TLOCK, or
                               F_ULOCK and the request would cause the  number
                               of locks to exceed a system-imposed limit.

       EOPNOTSUPP or EINVAL    The  locking  of files of the type indicated by
                               the fildes argument is not supported.

       Record-locking should not be used in combination  with  the  fopen(3C),
       fread(3C),  fwrite(3C)  and  other  stdio  functions. Instead, the more
       primitive, non-buffered functions (such as  open(2))  should  be  used.
       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  stdio  functions are the most common source of unexpected

       The alarm(2) function may be used to  provide  a  timeout  facility  in
       applications requiring it.

       The  lockf() function has a transitional interface for 64-bit file off‐
       sets. See lf64(7).

       See attributes(7) for descriptions of the following attributes:

       tab() box; cw(2.75i) |cw(2.75i) lw(2.75i) |lw(2.75i) ATTRIBUTE  TYPEAT‐
       TRIBUTE  VALUE _ Interface StabilityCommitted _ MT-LevelMT-Safe _ Stan‐
       dardSee standards(7).

       alarm(2), chmod(2), close(2), creat(2),  fcntl(2),  Intro(2),  mmap(2),
       open(2), read(2), write(2), attributes(7), lf64(7), standards(7)

Oracle Solaris 11.4               15 Jun 2012                        lockf(3C)
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