How to stop all pthreads when one has completed its work? - c++

Question

I'm trying to create a code to brute-force a random string but running it on one thread makes it take too long (as expected). I'm fiddling around with pthreads and this is what i've come up with:
void*
bruteForce ( void* ARGS )
{
args *arg = ( args * ) ARGS;
string STRING= arg->STRING;
string charSet = arg->charSet;
string guess = arg->guess;
char c;
int size;
int pos;
int lenght;
int j = 0;
char CHAR[STRING.length ( )];
size = charSet.length ( );
do
{
for ( j = 0; j < STRING.length ( ); j++ )
{
pos = rand ( ) % size;
CHAR[j] = charSet[pos];
guess = string ( CHAR );
//cout << guess[j];
}
//cout << guess << endl;
}
while ( STRING!= guess );
}
int
main ( int argc, char** argv )
{
srand ( ( unsigned ) ( time ( NULL ) ) );
const int NUMBER_OF_THREADS = 10;
args arg;
ifstream myFile;
string STRING;
string charSet = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
string guess;
pthread_t threads[NUMBER_OF_THREADS];
void* status;
arg.charSet = charSet;
arg.STRING= STRING;
char c;
int size;
int pos;
int lenght;
int j = 0;
myFile.open ( "string.txt" );
getline ( myFile, STRING);
size = charSet.length ( );
int rc;
//Creating threads for cracking the string
for ( int i = 0; i < NUMBER_OF_THREADS; i++ )
{
rc = pthread_create ( &threads[i], NULL, bruteForce, ( void* ) &arg );
if ( rc )
{
cout << "Couldnt create thread";
exit ( 1 );
}
}
//Joining threads
for ( int i = 0; i < NUMBER_OF_THREADS; i++ )
{
rc = pthread_join ( threads[i], &status );
if ( rc )
{
cout << "thread number " << i << " was unable to join: " << rc << endl;
exit ( 1 );
}
}
}
Now, I need someway of signaling that one of the threads has already guessed the string correctly and terminate the others. I read some of the documentation for pthread library and couldn't find anything. Any help is appreciated.
PS: I know the brute-force algorithm is by far not the best.

Answer 1

As long as you don't want your program to run any longer after the answer is found, you can just call exit(0) from the thread which found the answer.
do
{
// ...
}
while ( STRING!= guess );
std::cout << guess << std::endl;
std::exit(0);

Answer 2

Clumsy but workable in your case:
Add a DONE flag in global scope. Set it when a result is found by any thread.
Make each thread's loop be dependent on the flag.
bool DONE=false; // set to true to stop other threads
void*bruteForce ( void* ARGS )
{ ...
do
{ <try a string>
}
while ( !DONE && STRING!= guess );
DONE=true; // set redundantly but that doesn't hurt
}
Your main program can still do the join to collect finished pthreads, and then continue on with any work it might want to do on the guessed answer.