Avoiding Duplicate Copy

[logic4fun]

I have a C program with embedded SQL in it. All it does is it kicks off from a shellScript with a particular sleep. I wanted to add a logic to the program where it doesnot allow a duplicate copy of my C program to run. to be specific on a box only one copy should run. and if someone tries to run another It should freak out with an error. Any Ideas In C coz i wanted to add that logic to the C program.

thanks in advance
logic4fun

 

[Salem]

One simple way is to create a lock file

FILE *fp = fopen( "/tmp/lock", "w" );
if ( fp != NULL ) {
  // do your thing
  // ...
  fclose( fp );  // release the lock
} else {
  printf("someone else running\n");
}

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[logic4fun]

lock file technique is good. but we dont want to rely on file handles and files as this is a critical application and in somecases the file system may put us..in trouble.

thanks
logic4fun

 

[Salem]

Ok, so which OS are you running on then?
Perhaps the same idea using some other kind of OS resource (temporary files, shared memory, semaphores) will work better for you.



--

 

[logic4fun]

I am on RedHat Linux 8.1 .

thanks
logic4fun

 

[itsgsd]

If you don't want to use a lock/pid file (which would be my first choice), I would suggest using semaphores. This is the type of thing that semaphores do well: controlling access to resources (in this case your program). Of course, semaphores are more complex than using a lock file and will require more code.

If you want some code I could post some.

Good Luck.

 

[logic4fun]

I dont have any experience with semaphores. can u please get me a start with an example

thanks
logic4fun

 

[itsgsd]

I don't have the code right here. Take a look at:

http://www.tldp.org/LDP/lpg/node63.html

You will probably need to define the union semun before its use (see man semctl):

union semun {
int val;
struct semid_ds *buf;
unsigned short int *array;
struct seminfo *__buf;
};

Also, pay special attention to the ftok() function to insure that you are generating a unique key.

Hope this helps.