View Source: flock(2) - Waikato Linux Users Group

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!!NAME
flock - apply or remove an advisory lock on an open file

!!SYNOPSIS
 __#include <sys/file.h>__
 __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.

!!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).

!!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.
  *
  */

 #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"

 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;
   }

   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");

   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;
   }

   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;
   }

   printf("Released!\n");

   if (close(fd)==-1) {
     fprintf(
 	stderr,
 	"close(fd): %s (%i)\n",
 	strerror(errno),
 	errno);
     return 1;
   }

   return 0;
 }

!!SEE ALSO
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''.
15 pages link to flock(2):
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Last edited on Sunday, November 3, 2002 10:23:04 pm by WikiAdmin
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