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#Error while creating 2 child processes from one single parent
crude elkBOT
high venture
!f
crude elkBOT
I want to create a program where it creates 2 child processes from one parent and then with the given array its adds the even position to child A and odd position to child B. I have got the output but forks() keep pr```cpp
inting multiple statements !!
Here's my code:
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
int
main(int argc, char argv[]) {
int n = atoi(argv[1]);
int A[n];
int i = 0;
int sumA = 0;
int sumB = 0;
int sumTotal = 0;
int statusA;
int statusB;
pid_t pidA;
pid_t pidB;
int num = 0;
for (i = 0; i < n; i++) {
A[i] = num++;
printf("%d ", A[i]);
}
pidA = fork();
if (pidA == 0) {
// child A
for (i = 1; i < n; i += 2) {
sumA += A[i];
}
printf("subtotal of child A: %d\n", sumA);
}
pidB = fork();
if (pidB == 0) {
// child B
for (i = 0; i < n; i += 2) {
sumB += A[i];
}
printf("subtotal of child B: %d\n", sumB);
}
// parent
wait(pidA /, &statusA, 0 /);
wait(pidB /, &statusB, 0 * /);
sumTotal = sumA + sumB;
printf("total sum: %d\n", sumTotal);
return 0;
}
55555
Formatted by ohmyginger
high venture
both the parent and child A are calling fork so you're actually making 3 children
also all three children are doing child B's work
oh wait
sharp veldt
how can I correct this?
sharp veldt
can you run it on your side and check ?
cause I need one parent and 2 child ! and no multiple statements
high venture
i ran it in my head, basically you need to have child A return once it's done its job
and child B
sharp veldt
can you format my code and tell me ?
cause i cant seem to get it how the nesting of child parent works?
high venture
when you call fork, everything below that gets run by each process, the existing ones and the newly created ones. you can branch off of the return value from fork to specify "only the child should run this" or "only the parent should run this", but it doesn't mean execution stops once you reach the end of the if. you can stop it at the end of the if though if you return or exit
sharp veldt
i have some errors as well
i tried it but i am getting errors thats why i'm telling you if you can format and correct my mistakes
high venture
you mean like the random slashes? and you're missing an asterisk for the argv declaration
sharp veldt
i mean if you run on your laptop pc that would be easy for me
to rectify my mistake so we can format it together
high venture
and you're calling wait instead of waitpid
sharp veldt
you wont know if you cant run it sir
high venture
well i just fixed all those things and ran it
$ ./tst 5
0 1 2 3 4 subtotal of child A: 4
0 1 2 3 4 subtotal of child B: 6
0 1 2 3 4 total sum: 0
also you're using a variable length array (vla) which is problematic. vlas are actually slated to be removed from C in upcoming releases because they cause a lot of problems. when you don't know how much space you'll need ahead of time, you should use malloc
oh wow
just realized that the total sum was wrong, sumA and sumB are local variables, and each fork has its own copy of the local variables
if you make them global then they will persist after the subprocesses end (and the waitpids have returned)
i should probably also say that you could instead allocate them on the heap in order to avoid using global variables
sharp veldt
yah total sum is wrong
can you send me the rectify code you had????
@high venture ??
can you send me the rectified code ?
high venture
I want to create a program where it creates 2 child processes from one parent an...
i changed argv to char *argv[], added returns inside of the if statements after the print statements, fixed wait to waitpid, removed the slashes and random asterisk, and if you pull sumA and sumB outside of main then that'll fix the total sum
i'm not going to just hand you the code and prevent you from using your brain
sharp veldt
which sum A and sum B you are talking?
high venture
the variables, sumA and sumB
because they're inside of a function definition, they're local variables that use the stack, and each thread or process gets its own stack on creation, so to fix the sumTotal which depends on the values from sumA and sumB, sumA and sumB have to be moved off the stack, either to the heap by using malloc, or as part of the .bss section by making it either static or global
sharp veldt
bro i figured out everything except the total sum part
should i send you my fixed code?
high venture
yeah
sharp veldt
alright
here you go
i have sent my code in pastebin
the only thing i cant figure it out is my total sum ??
idk why
high venture
the link that space hazart sent?
from that link, the first thing that strikes me is that you're still writing the sums to local variables. childA is a local variable on the stack, which means childA.sumA is still a local variable on the stack
which means that the child is updating its own copy, whereas the parent's sits unchanged
if you did
int main()
{
static int sumA;
static int sumB;
...
sumTotal = sumA + sumB;
...
}```
then it should work just fine!solved