1lockf(3C)                Standard C Library Functions                lockf(3C)
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NAME

6       lockf - record locking on files
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SYNOPSIS

9       #include <unistd.h>
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11       int lockf(int fildes, int function, off_t size);
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DESCRIPTION

15       The  lockf()  function allows sections of a file to be locked; advisory
16       or mandatory write locks depending  on the mode bits of the  file  (see
17       chmod(2)). Calls to lockf() from other threads that attempt to lock the
18       locked file section will either return an error  value  or  be  put  to
19       sleep  until the resource becomes unlocked. All the locks for a process
20       are removed when the process terminates. See fcntl(2) for more informa‐
21       tion about record locking.
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24       The  fildes  argument  is  an open file descriptor. The file descriptor
25       must have O_WRONLY or O_RDWR permission in  order  to  establish  locks
26       with this function call.
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29       The  function  argument is a control value that specifies the action to
30       be taken. The permissible values for function are defined in <unistd.h>
31       as follows:
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33         #define   F_ULOCK   0   /* unlock previously locked section */
34         #define   F_LOCK    1   /* lock section for exclusive use */
35         #define   F_TLOCK   2   /* test & lock section for exclusive use */
36         #define   F_TEST    3   /* test section for other locks */
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40       All  other  values  of  function are reserved for future extensions and
41       will result in an error if not implemented.
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44       F_TEST is used to detect if a lock by another process is present on the
45       specified  section. F_LOCK and F_TLOCK both lock a section of a file if
46       the section is available. F_ULOCK removes locks from a section  of  the
47       file.
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50       The  size  argument  is  the number of contiguous bytes to be locked or
51       unlocked. The resource to be locked or unlocked starts at  the  current
52       offset in the file and extends forward for a positive size and backward
53       for a negative size (the preceding bytes up to but  not  including  the
54       current  offset).  If size is zero, the section from the current offset
55       through the largest file offset is locked (that is,  from  the  current
56       offset through the present or any future end-of-file). An area need not
57       be allocated to the file in order to be locked as such locks may  exist
58       past the end-of-file.
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61       The  sections  locked  with F_LOCK or F_TLOCK may, in whole or in part,
62       contain or be contained by a previously locked  section  for  the  same
63       process.  Locked sections will be unlocked starting at the point of the
64       offset through size bytes or to the end of file if size is  (off_t)  0.
65       When  this  situation  occurs,  or if this situation occurs in adjacent
66       sections, the sections are combined  into  a  single  section.  If  the
67       request  requires  that  a  new element be added to the table of active
68       locks and this table is already full, an error is returned, and the new
69       section is not locked.
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72       F_LOCK  and  F_TLOCK  requests  differ  only by the action taken if the
73       resource is not available. F_LOCK blocks the calling thread  until  the
74       resource is available. F_TLOCK causes the function to return −1 and set
75       errno to EAGAIN if the section is already locked by another process.
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78       File locks are released on first close by the locking  process  of  any
79       file descriptor for the file.
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82       F_ULOCK  requests  may, in whole or in part, release one or more locked
83       sections controlled  by  the  process.  When  sections  are  not  fully
84       released,  the  remaining  sections  are  still  locked by the process.
85       Releasing the center section of a locked section requires an additional
86       element  in  the table of active locks. If this table is full, an errno
87       is set to EDEADLK and the requested section is not released.
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90       An F_ULOCK request in which size is non-zero and the offset of the last
91       byte  of  the  requested  section is the maximum value for an object of
92       type off_t, when the process has an existing lock in which  size  is  0
93       and  which  includes  the  last  byte of the requested section, will be
94       treated as a request to unlock from the start of the requested  section
95       with  a  size equal to 0. Otherwise, an F_ULOCK request will attempt to
96       unlock only the requested section.
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99       A potential for deadlock occurs if the threads of a process controlling
100       a  locked  resource  is  put  to  sleep by requesting another process's
101       locked resource. Thus calls to lockf() or fcntl(2) scan for a  deadlock
102       prior  to  sleeping  on  a  locked resource. An error return is made if
103       sleeping on the locked resource would cause a deadlock.
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106       Sleeping on a resource is interrupted with  any  signal.  The  alarm(2)
107       function may be used to provide a timeout facility in applications that
108       require this facility.
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RETURN VALUES

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

115       The lockf() function will fail if:
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117       EBADF               The  fildes  argument  is  not  a  valid  open file
118                           descriptor; or function is F_LOCK  or  F_TLOCK  and
119                           fildes  is  not  a  valid  file descriptor open for
120                           writing.
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123       EACCES or EAGAIN    The function argument is F_TLOCK or F_TEST and  the
124                           section is already locked by another process.
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127       EDEADLK             The  function  argument is F_LOCK and a deadlock is
128                           detected.
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130
131       EINTR               A signal was caught during execution of  the  func‐
132                           tion.
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135       ECOMM               The  fildes argument is on a remote machine and the
136                           link to that machine is no longer active.
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139       EINVAL              The  function  argument  is  not  one  of   F_LOCK,
140                           F_TLOCK,  F_TEST, or F_ULOCK; or size plus the cur‐
141                           rent file offset is less than 0.
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144       EOVERFLOW           The offset of the first, or if size is not  0  then
145                           the  last,  byte in the requested section cannot be
146                           represented correctly in an object of type off_t.
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149
150       The lockf() function may fail if:
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152       EAGAIN                  The function argument is F_LOCK or F_TLOCK  and
153                               the file is mapped with mmap(2).
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156       EDEADLK or ENOLCK       The  function  argument  is F_LOCK, F_TLOCK, or
157                               F_ULOCK and the request would cause the  number
158                               of locks to exceed a system-imposed limit.
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161       EOPNOTSUPP or EINVAL    The  locking  of files of the type indicated by
162                               the fildes argument is not supported.
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164

USAGE

166       Record-locking should not be used in combination  with  the  fopen(3C),
167       fread(3C),  fwrite(3C)  and  other  stdio functions.  Instead, the more
168       primitive, non-buffered functions (such as  open(2))  should  be  used.
169       Unexpected results may occur in processes that do buffering in the user
170       address space.  The process may  later  read/write  data  which  is/was
171       locked.   The  stdio functions are the most common source of unexpected
172       buffering.
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175       The alarm(2) function may be used to  provide  a  timeout  facility  in
176       applications requiring it.
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179       The  lockf() function has a transitional interface for 64-bit file off‐
180       sets.  See lf64(5).
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ATTRIBUTES

183       See attributes(5) for descriptions of the following attributes:
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188       ┌─────────────────────────────┬─────────────────────────────┐
189       │      ATTRIBUTE TYPE         │      ATTRIBUTE VALUE        │
190       ├─────────────────────────────┼─────────────────────────────┤
191       │Interface Stability          │Standard                     │
192       ├─────────────────────────────┼─────────────────────────────┤
193       │MT-Level                     │MT-Safe                      │
194       └─────────────────────────────┴─────────────────────────────┘
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SEE ALSO

197       Intro(2), alarm(2), chmod(2), close(2),  creat(2),  fcntl(2),  mmap(2),
198       open(2), read(2), write(2), attributes(5), lf64(5), standards(5)
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202SunOS 5.11                        10 Apr 2002                        lockf(3C)
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