1LOCKF(3) Linux Programmer's Manual LOCKF(3)
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6 lockf - apply, test or remove a POSIX lock on an open file
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9 #include <unistd.h>
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11 int lockf(int fd, int cmd, off_t len);
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14 Apply, test or remove a POSIX lock on a section of an open file. The
15 file is specified by fd, a file descriptor open for writing, the action
16 by cmd, and the section consists of byte positions pos..pos+len-1 if
17 len is positive, and pos-len..pos-1 if len is negative, where pos is
18 the current file position, and if len is zero, the section extends from
19 the current file position to infinity, encompassing the present and
20 future end-of-file positions. In all cases, the section may extend
21 past current end-of-file.
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23 On Linux, this call is just an interface for fcntl(2). (In general,
24 the relation between lockf() and fcntl() is unspecified.)
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26 Valid operations are given below:
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28 F_LOCK Set an exclusive lock on the specified section of the file. If
29 (part of) this section is already locked, the call blocks until
30 the previous lock is released. If this section overlaps an ear‐
31 lier locked section, both are merged. File locks are released
32 as soon as the process holding the locks closes some file
33 descriptor for the file. A child process does not inherit these
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36 F_TLOCK
37 Same as F_LOCK but the call never blocks and returns an error
38 instead if the file is already locked.
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40 F_ULOCK
41 Unlock the indicated section of the file. This may cause a
42 locked section to be split into two locked sections.
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44 F_TEST Test the lock: return 0 if the specified section is unlocked or
45 locked by this process; return -1, set errno to EAGAIN (EACCES
46 on some other systems), if another process holds a lock.
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49 On success, zero is returned. On error, -1 is returned, and errno is
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53 EACCES or EAGAIN
54 The file is locked and F_TLOCK or F_TEST was specified, or the
55 operation is prohibited because the file has been memory-mapped
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58 EBADF fd is not an open file descriptor.
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60 EDEADLK
61 The command was T_LOCK and this lock operation would cause a
62 deadlock.
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64 EINVAL An invalid operation was specified in fd.
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66 ENOLCK Too many segment locks open, lock table is full.
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69 SVr4, POSIX.1-2001
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72 fcntl(2), flock(2)
73 There are also locks.txt and mandatory.txt in /usr/src/linux/Documenta‐
74 tion.
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78Linux 2.0 2002-04-22 LOCKF(3)