typedef struct {
    int N, M;
    char *posns.
} POSNS;

POSNS *initGenerator( int N, int M ) {
     POSNS *p = malloc( sizeof( POSNS ) );

     p->N = N; p->M = M; p->posns = malloc( M );

     for( i = 0; i < M; ++i ) p->posns[ i ] = N - M + i;
     return posns;
}

char *iterGenerator( POSNS *p ) {
    if( done) {
        free( p->posns );
        free( p );
        return NULL;
    }
    // modify p->posns;
    ...
    return p->posns;
}

POSNS p = initGenerator( n, m );

while( c = iterGenerator( p ) ) {
    // use c[].
}