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I am trying to complete a tutorial on running a parallel programme in C using fork() and execl commands.

The user enters the number of inputs(N). The next N lines contain a number <= 10 digits. My programme will calculate the number of prime factors for each digit.

I am trying to make the execution of the lines parallel using fork and execl.

Parallel.c 

int main() {
    int userInput, childPid, childResult, number;
    //Since largest number is 10 digits, a 12 characters string is more
    //than enough
    char cStringExample[12];

    scanf("%d", &userInput);

    for (int i = 0; i < userInput; i++) {
        scanf("%d",&number);
        childPid = fork();
        if (childPid == 0) {
            sprintf(cStringExample,"%d",number);
            execl("./PF","PF",cStringExample,NULL);
        }
    }

    while (wait(&childResult) != -1) {
         printf("%d has %d prime factors\n",number,childResult >> 8);
     }
}

PrimeFactorization.c

int main( int argc, char* argv[]) {


 int nFactor = 0, userInput, factor;

    //Convert string to number
    userInput = atoi( argv[1] ); 

    nFactor = 0;
    factor = 2;

    //quick hack to get the number of prime factors
    // only for positive integer
    while (userInput > 1){
        if (userInput % factor == 0){
            userInput /= factor;
            nFactor++;
        } else {
            factor++;
        }
    }
   // printf("%d has %d prime factors\n",userInput,nFactor);
    return nFactor;
}

I want to be able to print for each forked() process the number that was inputted into the Prime Factorisation programme as well as the number of prime factors for it. For example if I input 

5
44721359
99999989
9
111113111
118689518

the output is 

118689518 has 2 prime factors
118689518 has 3 prime factors
118689518 has 1 prime factors
118689518 has 1 prime factors
118689518 has 1 prime factors

The prime factors are correct just that it does not correspond to the number that has been inputted. How am I able to do that? I tried to insert the wait command inside the for loop but I don't think that produces parallelism. I understand why it gives me the output but I can't think of a solution for it.

calveeen
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  • The value given to *wait()* is a status... While that may work, you want two values returned (input and result). Currently only the last input is displayed. You could keep the order of input and based on that assume the order of results, but unfortunately might lead to race conditions (process B started *after* A completes *before* A). See possible duplicate of [get return value from child process](https://stackoverflow.com/questions/49581349/how-to-get-return-value-from-child-process-to-parent). You could also run and wait for *each* process, one by one, probably not what you want. – Déjà vu Sep 01 '18 at 06:32
  • all right i see i think pipe is the only way ? jus that we havent gotten to piping yet so i thought it would be possible to do it without using that. – calveeen Sep 01 '18 at 09:29
  • Well you could write the `(input,output)` values into a file (child) read by the parent. If bits(in)+bits(out) is less than sizeof(int)*8 you could cram (for instance) `(out<<32)|in` in the result (that's a hack...). Shared memory, ... messages, ... mail ... – Déjà vu Sep 01 '18 at 09:54

0 Answers0