Differences between version 2 and revision by previous author of flock(2).
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| Newer page: | version 2 | Last edited on Sunday, November 3, 2002 10:23:04 pm | by WikiAdmin | Revert |
| Older page: | version 1 | Last edited on Tuesday, June 4, 2002 12:23:40 am | by perry | Revert |
@@ -1,107 +1,119 @@
-FLOCK
-!!!FLOCK
-NAME
-SYNOPSIS
-DESCRIPTION
-RETURN VALUE
-ERRORS
-CONFORMING TO
-NOTES
-SEE ALSO
-----
!!NAME
+flock - apply or remove an advisory lock on an open file
-
-flock - apply or remove an advisory lock on an open file
!!SYNOPSIS
+ __#include <sys/file.h>__
+ __int flock(int__ fd__, int__ operation__)__
-
-__#include __
-
-
-__int flock(int__ fd__, int__
-operation__)__
!!DESCRIPTION
+Apply or remove an advisory lock on an open file. The file is specified by ''fd''. Valid operations are given below:
+;LOCK_SH: Shared lock. More than one process may hold a shared lock for a given file at a given time.
+;LOCK_EX: Exclusive lock. Only one process may hold an exclusive lock for a given file at a given time.
+;LOCK_UN: Unlock.
+;LOCK_NB: Don't block when locking. May be specified (by ''or'''ing) along with one of the other operations.
+;:A single file may not simultaneously have both shared and exclusive locks.
+;: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.
-Apply or remove an advisory lock on an open file. The file
-is specified by
''fd
''. Valid operations are given
-below:
+!!RETURN VALUE
+On success, zero
is returned. On error, -1 is returned, and
''errno
'' is set appropriately
.
+!!ERRORS
+;[EWOULDBLOCK]: The file is locked and the __LOCK_NB__ flag was selected.
-LOCK_SH
+!!CONFORMING TO
+4.4BSD (the flock(2) call first appeared in 4.2BSD).
+!!NOTES
+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.
-Shared lock. More than one process may hold a shared lock
-for a given file at a given time
.
+flock(2) and fcntl(2) locks have different semantics with respect to forked processes and dup(2)
.
+!!EXAMPLE
+ /*
+ * This program creates and locks /tmp/flock.example, waiting for the user
+ * before unlocking the file again.
+ *
+ * By running this program twice simultaniously, locking can be demonstrated.
+ *
+ */
-LOCK
_EX
+ #include <sys/file.h> /* for flock(2) */
+ #include <sys/stat.h> /* for S
_* constants */
+ #include <string.h> /* for strerror(3) prototype */
+ #include <stdio.h> /* for fprintf(3),printf(3),stderr protype */
+ #include <errno.h> /* for errno prototype */
+ #include <unistd.h> /* for close(2) prototypes */
+ #define FILENAME "/tmp/flock.example"
-Exclusive lock. Only one process may hold an exclusive lock
-for a given file at a given time.
+ int main(int argc,char **argv)
+ {
+ int fd;
+ char buf;
+ fd = open(FILENAME,O_RDWR|O_CREAT,S_IRUSR|S_IWUSR);
+ if (fd==-1) {
+ fprintf(
+ stderr,
+ "open(\"%s\",O_RDWR,S_IRUSR|S_IWUSR): %s (%i)\n",
+ FILENAME,
+ strerror(errno),
+ errno);
+ return 1;
+ }
-LOCK_UN
+ printf("Aquiring lock...");
+ fflush(stdout);
+ /* Aquire an exclusive lock */
+ if (flock(fd,LOCK_EX) == -1) {
+ fprintf(
+ stderr,
+ "flock(fd,LOCK_EX): %s (%i)\n",
+ strerror(errno),
+ errno);
+ return 1;
+ }
+ printf("Succeeded!\n");
-Unlock
.
+ printf("Press enter to release the lock
.\n");
+ if (read(0,&buf,sizeof(buf)) == -1) {
+ printf("read(0,&buf,sizeof(buf)): %s (%i)\n",
+ strerror(errno),
+ errno
+ );
+ return 1;
+ }
-LOCK_NB
+ printf("Releasing lock...");
+ fflush(stdout);
+ if (flock(fd,LOCK_UN)==-1) {
+ fprintf(
+ stderr,
+ "flock(fd,LOCK_UN): %s (%i)\n",
+ strerror(errno),
+ errno);
+ return 1;
+ }
-Don't block when locking. May be specified
(by
-''or'''ing
) along with one of the other
-operations.
+ printf
("Released!\n"
);
+ if (close(fd)==-1) {
+ fprintf(
+ stderr,
+ "close(fd): %s (%i)\n",
+ strerror(errno),
+ errno);
+ return 1;
+ }
-A single file may not simultaneously have both shared and
-exclusive locks.
+ return ;
+ }
-
-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.
-!!RETURN VALUE
-
-
-On success, zero is returned. On error, -1 is returned, and
-''errno'' is set appropriately.
-!!ERRORS
-
-
-__EWOULDBLOCK__
-
-
-The file is locked and the __LOCK_NB__ flag was
-selected.
-!!CONFORMING TO
-
-
-4.4BSD (the flock(2) call first appeared in
-4.2BSD).
-!!NOTES
-
-
-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.
-
-
-flock(2) and fcntl(2) locks have different
-semantics with respect to forked processes and
-dup(2).
!!SEE ALSO
+open(2), close(2), dup(2), execve(2), fcntl(2), fork(2), lockf(3)
-
-open(2), close(2), dup(2),
-execve(2), fcntl(2), fork(2),
-lockf(3)
-
-
-
-
-----
+There are also ''locks.txt'' and ''mandatory.txt'' in ''/usr/src/linux/Documentation''.
