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Annotated edit history of flock(2) version 2, including all changes. View license author blame.
Rev Author # Line
1 perry 1 !!NAME
2 WikiAdmin 2 flock - apply or remove an advisory lock on an open file
1 perry 3
4 !!SYNOPSIS
2 WikiAdmin 5 __#include <sys/file.h>__
6 __int flock(int__ fd__, int__ operation__)__
1 perry 7
8 !!DESCRIPTION
2 WikiAdmin 9 Apply or remove an advisory lock on an open file. The file is specified by ''fd''. Valid operations are given below:
10 ;LOCK_SH: Shared lock. More than one process may hold a shared lock for a given file at a given time.
11 ;LOCK_EX: Exclusive lock. Only one process may hold an exclusive lock for a given file at a given time.
12 ;LOCK_UN: Unlock.
1 perry 13
2 WikiAdmin 14 ;LOCK_NB: Don't block when locking. May be specified (by ''or'''ing) along with one of the other operations.
15 ;:A single file may not simultaneously have both shared and exclusive locks.
16 ;:A file is locked (i.e., the inode), ''not'' the file descriptor. So, dup(2) and fork(2) do not create multiple instances of a lock.
1 perry 17
2 WikiAdmin 18 !!RETURN VALUE
19 On success, zero is returned. On error, -1 is returned, and ''errno'' is set appropriately.
1 perry 20
2 WikiAdmin 21 !!ERRORS
22 ;[EWOULDBLOCK]: The file is locked and the __LOCK_NB__ flag was selected.
1 perry 23
2 WikiAdmin 24 !!CONFORMING TO
25 4.4BSD (the flock(2) call first appeared in 4.2BSD).
1 perry 26
2 WikiAdmin 27 !!NOTES
28 flock(2) does not lock files over NFS. Use fcntl(2) instead: that does work over NFS, given a sufficiently recent version of Linux and a server which supports locking.
1 perry 29
2 WikiAdmin 30 flock(2) and fcntl(2) locks have different semantics with respect to forked processes and dup(2).
1 perry 31
2 WikiAdmin 32 !!EXAMPLE
33 /*
34 * This program creates and locks /tmp/flock.example, waiting for the user
35 * before unlocking the file again.
36 *
37 * By running this program twice simultaniously, locking can be demonstrated.
38 *
39 */
1 perry 40
2 WikiAdmin 41 #include <sys/file.h> /* for flock(2) */
42 #include <sys/stat.h> /* for S_* constants */
43 #include <string.h> /* for strerror(3) prototype */
44 #include <stdio.h> /* for fprintf(3),printf(3),stderr protype */
45 #include <errno.h> /* for errno prototype */
46 #include <unistd.h> /* for close(2) prototypes */
1 perry 47
2 WikiAdmin 48 #define FILENAME "/tmp/flock.example"
1 perry 49
2 WikiAdmin 50 int main(int argc,char **argv)
51 {
52 int fd;
53 char buf;
1 perry 54
2 WikiAdmin 55 fd = open(FILENAME,O_RDWR|O_CREAT,S_IRUSR|S_IWUSR);
56 if (fd==-1) {
57 fprintf(
58 stderr,
59 "open(\"%s\",O_RDWR,S_IRUSR|S_IWUSR): %s (%i)\n",
60 FILENAME,
61 strerror(errno),
62 errno);
63 return 1;
64 }
1 perry 65
2 WikiAdmin 66 printf("Aquiring lock...");
67 fflush(stdout);
1 perry 68
2 WikiAdmin 69 /* Aquire an exclusive lock */
70 if (flock(fd,LOCK_EX) == -1) {
71 fprintf(
72 stderr,
73 "flock(fd,LOCK_EX): %s (%i)\n",
74 strerror(errno),
75 errno);
76 return 1;
77 }
78 printf("Succeeded!\n");
1 perry 79
2 WikiAdmin 80 printf("Press enter to release the lock.\n");
1 perry 81
2 WikiAdmin 82 if (read(0,&buf,sizeof(buf)) == -1) {
83 printf("read(0,&buf,sizeof(buf)): %s (%i)\n",
84 strerror(errno),
85 errno
86 );
87 return 1;
88 }
1 perry 89
2 WikiAdmin 90 printf("Releasing lock...");
91 fflush(stdout);
1 perry 92
2 WikiAdmin 93 if (flock(fd,LOCK_UN)==-1) {
94 fprintf(
95 stderr,
96 "flock(fd,LOCK_UN): %s (%i)\n",
97 strerror(errno),
98 errno);
99 return 1;
100 }
1 perry 101
2 WikiAdmin 102 printf("Released!\n");
1 perry 103
2 WikiAdmin 104 if (close(fd)==-1) {
105 fprintf(
106 stderr,
107 "close(fd): %s (%i)\n",
108 strerror(errno),
109 errno);
110 return 1;
111 }
1 perry 112
2 WikiAdmin 113 return 0;
114 }
1 perry 115
116 !!SEE ALSO
2 WikiAdmin 117 open(2), close(2), dup(2), execve(2), fcntl(2), fork(2), lockf(3)
1 perry 118
2 WikiAdmin 119 There are also ''locks.txt'' and ''mandatory.txt'' in ''/usr/src/linux/Documentation''.
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