Can anyone please explain how this C code works? How many processes will be spawned after its completion?
int main(void)
{
int i;
for(i=1; i<=42; i++)
{
fork();
}
return 0;
}
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3How many do you think get created?Oliver Charlesworth– Oliver Charlesworth2014-08-30 11:19:45 +00:00Commented Aug 30, 2014 at 11:19
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2 Answers
At each iteration of the for loop, fork is called exactly once. For each call of fork a new process is created; how many processes are spawned at the end depends on the length of the loop.
If the loop had length 1, only one new process would be created, for a total of 2 processes.
If the loop had length 2, at the first iteration a new process would be created; each process would then spawn another one at the next iteration, for a total of 3 calls to fork and 4 total processes.
As you might suppose, at each iteration the number of processes doubles; if n is the length of the loop, we're saying that the total number of processes would be f(n) = 2^n (with 2^n - 1 processes spawned). Let's prove it's true by induction. We have already seen that the formula holds for n=1 and n=2. Let's suppose it's valid for n and prove it for n+1.
If the lenght is n+1, at the first iteration a new process is spawned, for a total of 2 processes each still having n iterations to do, which means (by inductive assumption) that each process will spawn 2^n - 1 processes, which means that a total of 2^(n+1) - 2 processes will be spawned in the last n iterations; we already had 2 processes, which means that the total created processes (counting the main one) would be 2^(n+1) - 2 + 2 = 2^(n+1).
In conclusion, for a loop of lenght n we have 2^n - 1 calls to fork, which means that in your case we have 2^42 - 1 new processes spawned by the main one and its descendants.
Comments
Use this program to get the answer:
#include<stdio.h>
#include<math.h>
int main()
{
float sum=0;
int i;
for(i=1;i<=42;i++)
{
sum = sum + pow(2,i);
printf("fork call:%d, processes:%1.f\n",i,sum);
}
return 0;
}
//When fork call is 42 the processes will be 8796093022208
3 Comments
Muhammad Awais Khalid
Do the Child process also call the fork statement when it get spawned.? As we know that child process gets the complete copy of parent. So while getting a copy of parent the child process will get the fork statement. Do Child process call this fork statement?
ani627
Yes child process will also call the fork statement.
gcali
Shouldn't it be 2^42, half of what you wrote?