MPI_Gather prime number arrays into one array - c

Question

I am trying to split up a program that calculates whether or not a number is a prime. So, I spread out into each process a range of numbers and find the primes in that range. This works, each process takes the range and finds the correct primes. Then when I want to Gather all of the arrays from each process into one is where I am running into problems.
Either it prints out the proper number of array values, but has extra 0's so it will print out something like 2,3,5,7,0,11,13,0,0,0,17,19... or I get the assertion failed error where the memcpy argument memory ranges overlap.
Here is the relevant code in my main -
thesePrimes = (int*)malloc(sizeof(int) * maxNumberOfTotalPrimes);
//findPrimes returns k which is the number of primes found and
//puts all of the prime numbers from this process into thesePrimes.
//n is the highest number to check if prime (ie n = 100 check for primes
//less than 100)
k = findPrimes(thesePrimes, start, end, n);
//Reduce k to get the total number of primes within the input
MPI_Reduce(&k, &numberOfPrimes, 1, MPI_INT, MPI_SUM, 0, MPI_COMM_WORLD);
//Allocate just enough space to hold all of the primes based on the reduced
//number of primes
allPrimes = (int*)malloc(sizeof(int) * numberOfPrimes);
//Gather each process's thesePrimes into allPrimes using k as the buffer
//size since k is the number of primes for the process, just send k numbers
MPI_Gather(thesePrimes, k, MPI_INT, allPrimes, k, MPI_INT, 0, MPI_COMM_WORLD);
if(myRank == 0) {
printf("Attempting to print...\n");
for(i = 0; i < numberOfPrimes; i++)
printf("allPrimes[%d]=%d\n", i, allPrimes[i]);
printf("There are %d prime numbers in the range 0 to %d\n", numberOfPrimes, n);
}
Here is my function to find the number of primes -
int findPrimes(int primes[], int start, int end, int n){
//k is used to count the number of primes
int i, j, maxJ, k = 0;
int isPrime = 1;
printf("Finding primes from %d to %d\n", start, end);
if(end > n) end = n;
if(start == 0) start = 2;
for(i = start; i <= end; i++) {
maxJ = sqrt(i);
for(j = 2; j <= maxJ; j++) {
if(i%j == 0) {
isPrime = 0;
break;
}
}
printf("Checking if %d is prime...\n", i);
if(isPrime) {
primes[k++] = i;
printf("%d is a prime number.\n", primes[k-1]);
}
else isPrime = 1;
// printf("Prime check complete.\n");
}
printf("k = %d\n", k);
return k;
}

Answer 1

You need to MPI_Gather() the number of prime numbers on each rank, and then you will be able to MPI_Gatherv() the prime numbers.