1LOCKF(3)                   Linux Programmer's Manual                  LOCKF(3)
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NAME

6       lockf - apply, test or remove a POSIX lock on an open file
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SYNOPSIS

9       #include <unistd.h>
10
11       int lockf(int fd, int cmd, off_t len);
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13   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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15       lockf():
16           _XOPEN_SOURCE >= 500
17               || /* Glibc since 2.19: */ _DEFAULT_SOURCE
18               || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
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DESCRIPTION

21       Apply,  test  or remove a POSIX lock on a section of an open file.  The
22       file is specified by fd, a file descriptor open for writing, the action
23       by  cmd,  and  the section consists of byte positions pos..pos+len-1 if
24       len is positive, and pos-len..pos-1 if len is negative,  where  pos  is
25       the current file position, and if len is zero, the section extends from
26       the current file position to infinity,  encompassing  the  present  and
27       future  end-of-file  positions.   In  all cases, the section may extend
28       past current end-of-file.
29
30       On Linux, lockf() is just an interface  on  top  of  fcntl(2)  locking.
31       Many other systems implement lockf() in this way, but note that POSIX.1
32       leaves the relationship between lockf() and fcntl(2) locks unspecified.
33       A  portable  application  should  probably  avoid mixing calls to these
34       interfaces.
35
36       Valid operations are given below:
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38       F_LOCK Set an exclusive lock on the specified section of the file.   If
39              (part  of) this section is already locked, the call blocks until
40              the previous lock is released.  If this section overlaps an ear‐
41              lier  locked  section, both are merged.  File locks are released
42              as soon as the  process  holding  the  locks  closes  some  file
43              descriptor for the file.  A child process does not inherit these
44              locks.
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46       F_TLOCK
47              Same as F_LOCK but the call never blocks and  returns  an  error
48              instead if the file is already locked.
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50       F_ULOCK
51              Unlock  the  indicated  section  of  the file.  This may cause a
52              locked section to be split into two locked sections.
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54       F_TEST Test the lock: return 0 if the specified section is unlocked  or
55              locked  by  this process; return -1, set errno to EAGAIN (EACCES
56              on some other systems), if another process holds a lock.
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RETURN VALUE

59       On success, zero is returned.  On error, -1 is returned, and  errno  is
60       set appropriately.
61

ERRORS

63       EACCES or EAGAIN
64              The  file  is locked and F_TLOCK or F_TEST was specified, or the
65              operation is prohibited because the file has been  memory-mapped
66              by another process.
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68       EBADF  fd  is  not an open file descriptor; or cmd is F_LOCK or F_TLOCK
69              and fd is not a writable file descriptor.
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71       EDEADLK
72              The command was F_LOCK and this lock  operation  would  cause  a
73              deadlock.
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75       EINVAL An invalid operation was specified in cmd.
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77       ENOLCK Too many segment locks open, lock table is full.
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ATTRIBUTES

80       For   an   explanation   of   the  terms  used  in  this  section,  see
81       attributes(7).
82
83       ┌──────────┬───────────────┬─────────┐
84Interface Attribute     Value   
85       ├──────────┼───────────────┼─────────┤
86lockf()   │ Thread safety │ MT-Safe │
87       └──────────┴───────────────┴─────────┘

CONFORMING TO

89       POSIX.1-2001, POSIX.1-2008, SVr4.
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SEE ALSO

92       fcntl(2), flock(2)
93
94       locks.txt and mandatory-locking.txt in the Linux kernel  source  direc‐
95       tory  Documentation/filesystems  (on  older  kernels,  these  files are
96       directly under the Documentation directory,  and  mandatory-locking.txt
97       is called mandatory.txt)
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COLOPHON

100       This  page  is  part of release 4.16 of the Linux man-pages project.  A
101       description of the project, information about reporting bugs,  and  the
102       latest     version     of     this    page,    can    be    found    at
103       https://www.kernel.org/doc/man-pages/.
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107GNU                               2017-09-15                          LOCKF(3)
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