lockf man page on Ultrix

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lockf(3)							      lockf(3)

Name
       lockf - record locking on files

Syntax
       #include <unistd.h>

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

Description
       The subroutine allows sections of a file to be locked.  These are advi‐
       sory mode locks.	 Locking calls from other processes which  attempt  to
       lock the locked file section return either an error value or are put to
       sleep until the resource becomes unlocked.  All the locks for a process
       are  removed  when  the process terminates.  For more information about
       record locking, see

       The 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.

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

       #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
       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_UNLOCK removes locks from a section of the
       file.

       The 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.	 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.	 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 causes the calling process to sleep
       until the resource is available.	 F_TLOCK causes 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 or 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.  You can use the
       command to provide a timeout facility  in  applications	which  require
       this facility.

       File  region  locking is supported over NFS, if the NFS locking service
       has been enabled.

Restrictions
       Unexpected results may occur in processes that do buffering in the user
       address	space.	 The  process may later read or write data which is or
       was locked.  The standard I/O package is	 the  most  common  source  of
       unexpected buffering.

Return Values
       Upon successful completion, 0 is returned.  Otherwise, a -1 is returned
       and the global variable errno is set to indicate the error.

Diagnostics
       The subroutine fails if:

       [EBADF]	      The fildes is not a valid open descriptor.

       [EACCESS]      The cmd is F_TLOCK or F_TEST and the section is  already
		      locked  by  another  process.   Or, the file is remotely
		      mounted, and  the	 NFS  locking  service	has  not  been
		      enabled.

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

       [EINVAL]	      The value given for the request argument is invalid.

See Also
       close(2), creat(2), fcntl(2),  intro(2),	 open(2),  read(2),  write(2),
       lockd(8c)

								      lockf(3)
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