Global variable in fork() child process - c++

Question

I have encountered an issue when trying to make a duplicate check function.
The main objective is to store data from child process (based on the check() function result that returns true or false) to parent process. I already tried global variables but didn't work.
This is my code :
...
for(j=0; j<indIP; j++)
{
fflush(stdout);
if (!(fork()))
{
char* this_ip = strdup(ip[j]);
if(is_duplicate_check(this_ip,"file1"))
{
if(debugLevel >= 2) printf("Duplicate IP %s\n", this_ip);
}
else if( is_duplicate_check(this_ip,"file2"))
{
if(debugLevel >= 2) printf("Duplicate IP %s\n",this_ip);
}
else
{
if(debugLevel >= 2) printf("Checking IP [%d of %d] -> [%s]\n",current_combo,possible_combinations,ip[j]);
checkauth(this_ip);
}
exit(0);
}
else
{
numforks++;
current_combo += trys;
if (current_combo > possible_combinations)
{
break;
}
if (numforks >= maxf)
{
wait(NULL);
numforks--;
}
}
indInterface++;
if(indInterface>=countInterface) indInterface=0;
}
puts("Finalizing...");
while(numforks>0)
{
printf("Waiting for the child processes [%d] are finished ....\n", numforks);
wait(NULL);
numforks--;
}
puts("Script completed!");
return 0;
...
In short the program reads ips from a text file and checks them if they are a certain GEO CODE location. The is_duplicate_check checks if the ip was not already checked, but it is working by storing ips to file1 and file2, and when it arrives on 100.000 records it is making a lot of load.
Now all i want is to store ips from child process in an array and check before calling check() function or not.
I already tried with
static int *glob_var;
glob_var = (int *) mmap(NULL, sizeof *glob_var, PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, -1, 0);
glob_var++;
printf("glob_var = [%d]\n",glob_var);
allways gives me something like (-1232557773525 ...), no use.
How can i solve this?

Answer 1

A fork duplicates the memory. So when you store the variable it is different memory from the parent process.
If you can use threads you can have the behavior where you share a global variable (because threads share memory).
Or if you have to fork you can look at
How to use shared memory with Linux in C
for shared memory usage or have a look at the boost version:
http://www.boost.org/doc/libs/1_60_0/doc/html/interprocess/sharedmemorybetweenprocesses.html#interprocess.sharedmemorybetweenprocesses.sharedmemory