How to pass value to a Grandchild thread under a Child thread from Parent thread or Child thread in C++

Question

I have a file.txt with the following values: 1234 567 8910

I want to create a program in c++, that creates x number of child threads with x being the number of lines in "file.txt". The child thread receives the line so it splits the line into digits and stores them in an array.

Then I want to create y number of grandchild threads under each child thread with y being the number of digits in that child thread array or received line value and pass each grandchild thread a single digit.

For example, for the above file.txt, my parent thread in main() will create 3 child threads. The first child thread will receive "1234", second "567", third "8910". Then the first child will create 4 grandchild threads and pass the first grandchild thread "1", second "2", third "3", fourth "4". Similarly, the second child will create 3 grandchild threads and pass the "5", "6", "7" digits. And lastly, the third child will create 4 grandchild threads and pass the "8", "9", "1", "0" digits.

I want to pass all these values using multi-threading in parallel. I am able to get everything working fine till the child thread, and I am able to create the right amount of grandchild threads, but I am unable to pass them the values. Any help and guidance are much appreciated.

Here is my code: (please guide me to fix it)

#include <iostream>
#include <pthread.h>
#include <math.h>
#include <fstream>
#include <sys/wait.h>
#include <cstdlib>
#include <unistd.h>
using namespace std;

struct info {
    int* digit = new int;
    int* totalDigits = new int;
};

struct grandchildThreadData {
    int GCTDdigit;
    int GCTDgrandchildIndex;
    int GCTDchildIndex;
    info* GCTDinfo = new info;
    info* GCTDparentInfo = new info;
};

struct childThreadData {
    long int CTDlineValue;
    int CTDchildIndex;
    info* CTDinfo = new info;
};

// thread declaration
void* childThread(void*);
void* grandchildThread(void*);

// function to convert and store line to digits
int* digitSeparator(long int);

// MAIN PROGRAM
int main() {
    // FILE OPENING
    int totalLines{0};
    ifstream file1("file.txt"), file2("file.txt");
    string temp;
    while (getline(file1, temp)) {
        totalLines++;
    }
    file1.close();
    long int* valueOnLine = new long int[totalLines];
    int i =0;
    while (!file2.eof()) {
        file2 >> valueOnLine[i];
        i++;
    }
    file2.close();
    // FILE CLOSING

    // THREAD START

    static struct info* mainInfo = new info[totalLines];
    pthread_t* child = new pthread_t[totalLines];
    // will be used to pass values to child
    static struct childThreadData* Carg = new childThreadData[totalLines];

    // Creating Childthreads
    for (int i = 0; i < totalLines; i++) {
        Carg[i].CTDinfo[i] = mainInfo[i];
        Carg[i].CTDchildIndex = i;
        Carg[i].CTDlineValue = valueOnLine[i];

        if (pthread_create(&child[i], nullptr, childThread, &Carg[i])) {
            fprintf(stderr, "Error creating thread\n");
            return 1;
        }
    }

    // Joining Childthreads
    for (int i = 0; i < totalLines; i++) {
        if (pthread_join(child[i], nullptr)) {
            fprintf(stderr, "Error joining thread\n");
            return 2;
        }
    }

    delete[] valueOnLine;
    delete[] child;
    delete[] Carg;

    return 0;
}

void* childThread(void* i) {
    struct childThreadData* CTptr = (struct childThreadData*)i;

    int totalDigits = log10((float)CTptr->CTDlineValue) + 1;
    int* numberArray = digitSeparator(CTptr->CTDlineValue);
 
    // THIS LINE WILL PUT TOTAL DIGITS INTO mainInfo
    CTptr->CTDinfo[CTptr->CTDchildIndex].totalDigits[0] = totalDigits;

    static struct info* childInfo = new info[totalDigits];  // This can be used to print modified info in grandchild
    pthread_t* grandchild = new pthread_t[totalDigits];
    static struct grandchildThreadData* GCarg = new grandchildThreadData[totalDigits];

    // THIS LINE WILL PUT EACH DIGIT ON CORRECT LOCATION of mainInfo
    for (int i=0; i< totalDigits; i++) {
        CTptr->CTDinfo[CTptr->CTDchildIndex].digit[i] = numberArray[i];
    }

    // GRANDCHILD THREAD
    for (int i = 0; i < totalDigits; i++) {
        GCarg[i].GCTDinfo[i] = childInfo[i];    // grandchild to child communication but does not work
        // or
        GCarg[i].GCTDparentInfo[i] = CTptr->CTDinfo[i]; // grandchild to parent communication but does not work
        GCarg[i].GCTDgrandchildIndex = CTptr->CTDchildIndex;    // Here CTptr->CTDchildIndex should pass 0, 1, 2 to grandchild but I get different values
        GCarg[i].GCTDchildIndex = i; // This line works fine for some reason
        GCarg[i].GCTDdigit = CTptr->CTDinfo[CTptr->CTDchildIndex].digit[i]; // This line should pass the correct digit, but again, I am getting different results in grandchild
     
        if (pthread_create(&grandchild[i], nullptr, grandchildThread, &GCarg[i])) {
            fprintf(stderr, "Error creating thread\n");
        }
    }

    //Joining GrandChildthreads
    for (int i = 0; i < totalDigits; i++) {
        if (pthread_join(grandchild[i], nullptr)) {
            fprintf(stderr, "Error joining thread\n");
        }
    }

    return nullptr;
}

void* grandchildThread(void* i) {
    struct grandchildThreadData* GCTptr = (struct grandchildThreadData*)i;

    // THIS LINE SHOULD PRINT THE DIGIT
    cout << GCTptr->GCTDdigit;

    return nullptr;
}

int* digitSeparator(long int number) {
    int totalDigits = log10((float)number) + 1;
    int* separatorPtr = new int[totalDigits];
    int j = 0;

    for (int i = totalDigits - 1; i >= 0; i--) {
        long int divisor = pow((float)10, i);
        long int digit = number / divisor;
        number -= digit * divisor;
        separatorPtr[j] = digit;
        j++;
    }

    return separatorPtr;
}

Can I assume this is an assignment requirement for a course? If it's not, you a greatly overcomplicating things for what will likely be a performance loss. — user4581301, Nov 02 '21 at 18:57
This seems to me to be an utterly nonsensical application of threading. — Mike Robinson, Nov 02 '21 at 19:56
So, you are learning C++ but you are required to use a platform specific C API do to it? That sounds terrible :-( — Ted Lyngmo, Nov 02 '21 at 21:41

score 0 · Answer 1 · answered Nov 02 '21 at 21:29

0

The issue was caused by using static structs:

So, if we replace:

static struct childThreadData* Carg = new childThreadData[totalLines];
static struct grandchildThreadData* GCarg = new grandchildThreadData[totalLines];

with:

childThreadData* Carg = new childThreadData[totalLines];
grandchildThreadData* GCarg = new grandchildThreadData[totalLines];

It will work fine.

answered Nov 02 '21 at 21:29

MWaleed Malik

21
2

No, that's not nearly enough. There are so many out of bounds accesses and other bugs that I couldn't even finalize writing a proper answer. – Ted Lyngmo Nov 02 '21 at 21:39

score 0 · Answer 2 · answered Nov 02 '21 at 22:06

This is not an exhaustive list of the problems in the program but it deals some fundamental problems.

Your program does not check if file.txt is successfully opened. If it's not, the program will continue to run until it gets a signed integer overflow - which means that the program has undefined behavior.
If the file is successfully opened, you'll run into heap-buffer-overflow
```
// Creating Childthreads
for(int i = 0; i < totalLines; i++) {
     Carg[i].CTDinfo[i] = mainInfo[i];        // <- here
```
That's because CTDinfo[i] is out of bounds when i > 0. CTDinfo is a pointer to one single info - not totalLines of infos.

Same thing below. CTDinfo can't be treated as an array since it's a pointer to a single info - and digit is a pointer to a single int - not an array of int.

// THIS LINE WILL PUT EACH DIGIT ON CORRECT LOCATION of mainInfo
for(int i = 0; i < totalDigits; i++) {
     CTptr->CTDinfo[CTptr->CTDchildIndex].digit[i] = numberArray[i];
}

Suggestions:

Write a much smaller program to get the hang of threading.
Write a much smaller program to learn how to build up very complicated structures (if you are really required to do so). Use C++ classes, like std::vector. I doubt this program would need a single new/new[] if you replaced all the manual memory management with standard C++ containers. Things like info looks like there's been a misunderstanding. There is no reason why pointers should be used here:
```
struct info {
     int* digit = new int;
     int* totalDigits = new int;
};
```
Use std::thread and its support functions and classes instead of the platform specific C API in pthread.

How to pass value to a Grandchild thread under a Child thread from Parent thread or Child thread in C++

2 Answers2