projects / unix-history / blame
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1 | .\" Copyright (c) 1983 Regents of the University of California. |
| 2 | .\" All rights reserved. The Berkeley software License Agreement | |
| 3 | .\" specifies the terms and conditions for redistribution. | |
| 4 | .\" | |
| 19ecbc2b | 5 | .\" @(#)flock.2 6.3 (Berkeley) %G% |
| 619ee695 | 6 | .\" |
| 9b1f9fbd | 7 | .TH FLOCK 2 "" |
| 619ee695 KM |
8 | .UC 5 |
| 9 | .SH NAME | |
| 10 | flock \- apply or remove an advisory lock on an open file | |
| 11 | .SH SYNOPSIS | |
| 12 | .nf | |
| 13 | .ft B | |
| 14 | #include <sys/file.h> | |
| 15 | .PP | |
| 16 | .ft B | |
| 17 | .DT | |
| 18 | #define LOCK_SH 1 /* shared lock */ | |
| 19 | #define LOCK_EX 2 /* exclusive lock */ | |
| 20 | #define LOCK_NB 4 /* don't block when locking */ | |
| 21 | #define LOCK_UN 8 /* unlock */ | |
| 22 | .PP | |
| 23 | .ft B | |
| 24 | flock(fd, operation) | |
| 25 | int fd, operation; | |
| 26 | .fi | |
| 27 | .SH DESCRIPTION | |
| 28 | .I Flock | |
| 29 | applies or removes an | |
| 30 | .I advisory | |
| 31 | lock on the file associated with the file descriptor | |
| 32 | .IR fd . | |
| 33 | A lock is applied by specifying an | |
| 34 | .I operation | |
| 38edb05c | 35 | parameter that is the inclusive or of |
| 619ee695 KM |
36 | LOCK_SH or LOCK_EX and, possibly, LOCK_NB. To unlock |
| 37 | an existing lock | |
| 38 | .I operation | |
| 39 | should be LOCK_UN. | |
| 40 | .PP | |
| 41 | Advisory locks allow cooperating processes to perform | |
| 42 | consistent operations on files, but do not guarantee | |
| 43 | consistency (i.e. processes may still access files | |
| 44 | without using advisory locks possibly resulting in | |
| 45 | inconsistencies). | |
| 46 | .PP | |
| 47 | The locking mechanism allows two types of locks: | |
| 48 | .I shared | |
| 49 | locks and | |
| 50 | .I exclusive | |
| 51 | locks. | |
| 52 | At any time multiple shared locks may be applied to a file, | |
| 53 | but at no time are multiple exclusive, or both shared and exclusive, | |
| 54 | locks allowed simultaneously on a file. | |
| 55 | .PP | |
| 56 | A shared lock may be | |
| 57 | .I upgraded | |
| 58 | to an exclusive lock, and vice versa, simply by specifying | |
| 59 | the appropriate lock type; this results in the previous | |
| 60 | lock being released and the new lock applied (possibly | |
| 61 | after other processes have gained and released the lock). | |
| 62 | .PP | |
| 38edb05c | 63 | Requesting a lock on an object that is already locked |
| 19ecbc2b | 64 | normally causes the caller to be blocked until the lock may be |
| 619ee695 KM |
65 | acquired. If LOCK_NB is included in |
| 66 | .IR operation , | |
| 67 | then this will not happen; instead the call will fail and | |
| 68 | the error EWOULDBLOCK will be returned. | |
| 69 | .SH NOTES | |
| 70 | Locks are on files, not file descriptors. That is, file descriptors | |
| 71 | duplicated through | |
| 72 | .IR dup (2) | |
| 73 | or | |
| 74 | .IR fork (2) | |
| 75 | do not result in multiple instances of a lock, but rather multiple | |
| 76 | references to a single lock. If a process holding a lock on a file | |
| 77 | forks and the child explicitly unlocks the file, the parent will | |
| 78 | lose its lock. | |
| 79 | .PP | |
| 80 | Processes blocked awaiting a lock may be awakened by signals. | |
| 81 | .SH "RETURN VALUE | |
| 82 | Zero is returned if the operation was successful; | |
| 83 | on an error a \-1 is returned and an error code is left in | |
| 84 | the global location \fIerrno\fP. | |
| 85 | .SH "ERRORS | |
| 86 | The \fIflock\fP call fails if: | |
| 87 | .TP 20 | |
| 88 | [EWOULDBLOCK] | |
| 89 | The file is locked and the LOCK_NB option was specified. | |
| 90 | .TP 20 | |
| 91 | [EBADF] | |
| 92 | The argument \fIfd\fP is an invalid descriptor. | |
| 93 | .TP 20 | |
| 94 | [EINVAL] | |
| 95 | The argument \fIfd\fP refers to an object other than a file. | |
| 96 | .SH "SEE ALSO" | |
| 97 | open(2), close(2), dup(2), execve(2), fork(2) |