Hi all ,
I am new to unix programming . I have written a sample to
achieve Exclusive file lock. Now if i invoke a.out&;a.out&;a.out&;
2& next instances doesnt print "Cannot Lock".

int main(){
FILE *Fd;
Fd=fopen("/tmp/TeSt","w");
if (!Fd)
printf ("cannot create file \n");
if(0==lockf(Fd,2,0L)){
printf("Cannot Lock \n");
exit(0);
}
sleep(10);
fclose(Fd);
}
Can somebody explain how to achieve it using lockf(),

Thanks,
SeemaRao

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On 7 Nov 2005 08:58:34 -0800, seema wrote:
> Hi all ,
>           I am new to unix programming . I have written a sample to
> achieve Exclusive file lock. Now if i invoke a.out&;a.out&;a.out&;
> 2& next instances doesnt print "Cannot Lock".

How do you know, remove the
exit(0);
and try again.

If I had to figure out what is going on, I would spin up debug for two
instances of a.out and single step each to see what happens.

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seema wrote:
> Hi all ,
>           I am new to unix programming . I have written a sample to
> achieve Exclusive file lock. Now if i invoke a.out&;a.out&;a.out&;
> 2& next instances doesnt print "Cannot Lock".
>
> int main(){
>         FILE *Fd;
>         Fd=fopen("/tmp/TeSt","w");
>         if (!Fd)
>                 printf ("cannot create file \n");
>         if(0==lockf(Fd,2,0L)){
>                 printf("Cannot Lock \n");
>                 exit(0);
>         }
>         sleep(10);
>         fclose(Fd);
> }
> Can somebody explain how to achieve it using lockf(),
>
> Thanks,
> SeemaRao
>

The sense of your test is wrong.  According to the man page, lockf
returns zero if the lock succeeded.

--

Fletcher Glenn

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>          I am new to unix programming . I have written a sample to
>achieve Exclusive file lock. Now if i invoke a.out&;a.out&;a.out&;
>2& next instances doesnt print "Cannot Lock".
>
>int main(){
>        FILE *Fd;
>        Fd=fopen("/tmp/TeSt","w");
>        if (!Fd)
>                printf ("cannot create file \n");
>        if(0==lockf(Fd,2,0L)){
>                printf("Cannot Lock \n");
>                exit(0);
>        }
>        sleep(10);
>        fclose(Fd);
>}
>Can somebody explain how to achieve it using lockf(),

Read the lockf() manual page.  What does lockf() take as a first
argument? (int)  What are you giving it?  (FILE *)  (fileno() might
be useful here.) What does lockf() return when it succeeds?

Note that you destroy the contents of the file *BEFORE* you get
the lock.  If you're using the file only as a lock file and
don't care about the contents, that's fine.

Gordon L. Burditt

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seema wrote:

>Hi all ,
>          I am new to unix programming . I have written a sample to
>achieve Exclusive file lock. Now if i invoke a.out&;a.out&;a.out&;
>2& next instances doesnt print "Cannot Lock".
>
>int main(){
>        FILE *Fd;
>        Fd=fopen("/tmp/TeSt","w");
>        if (!Fd)
>                printf ("cannot create file \n");
>        if(0==lockf(Fd,2,0L)){
>                printf("Cannot Lock \n");
>                exit(0);
>        }
>        sleep(10);
>        fclose(Fd);
>}
>Can somebody explain how to achieve it using lockf(),
>
>Thanks,
>SeemaRao
>
>
>
Hello,

If you will execute the following program then you can see the results :
#include <stdio.h>
#include <unistd.h>
#include <sys/file.h>
#include <fcntl.h>

int main(){
FILE *Fd;
int c;
Fd=fopen("/tmp/TeSt","w");
if (!Fd)
printf ("cannot create file \n");
c=lockf((int)Fd,1,0);
printf ("lock = %d\n",c);
if(lockf((int)Fd,1,0L)==0){
printf("Cannot Lock \n");
}
sleep(10);
fclose(Fd);
}

Create the file TeSt in /tmp directory
cd /tmp
touch TeSt
chmod 044 TeSt

After this execute the program then you can see the return value of
lockf is 0.
In other cases the return value is -1.

Regards,
Thobias
--
Thobias Vakayil
Alcatel Development India (ADI)
PH: 2349961/72/86 EXTN :7018

[ Post a follow-up to this message ]

Thobias Vakayil <Vakayil.Thobias@alcatel.com> writes:

> seema wrote:

> If you will execute the following program then you can see the results :
> #include <stdio.h>
> #include <unistd.h>
> #include <sys/file.h>
> #include <fcntl.h>
>
> int main(){
>         FILE *Fd;
>         int c;
>         Fd=fopen("/tmp/TeSt","w");
>         if (!Fd)
>                 printf ("cannot create file \n");
>         c=lockf((int)Fd,1,0);
>         printf ("lock = %d\n",c);
>         if(lockf((int)Fd,1,0L)==0){
>                 printf("Cannot Lock \n");
>         }
>         sleep(10);
>         fclose(Fd);
> }
>
> Create the file TeSt in /tmp directory
> cd /tmp
> touch TeSt
> chmod 044 TeSt
>
> After this execute the program then you can see the return value of
> lockf is 0.
> In other cases the return value is -1.

Actually it prints

lock = -1

This is because you can't get the file descriptor associated with Fd
simply by casting it to an int. That only gives you the pointer value
(possibly truncated or otherwise mangled) as an integer.

As someone else suggested, fileno() is what's needed.

It would also be better to use the #defines rather than integer
constants. F_LOCK isn't guaranteed to always be 1. The OP probably
also wants LOCK_EX, given the title of this thread.


joe
--
Gort, klatu barada nikto

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