Reputation: 4284

System Programming - Fork() Method

I am studying about system programming system calls. I have a code block in my assignment (given below). The question asks me to how many A,B or C will be printed. My question is what is the meaning of if(pid == 0)? I guess if(pid == 0) means false, so I analyse that 2 x A and 2 x B will be printed. Am I write or? Second question is does pid2 = fork() executes main again?

int main()
{
  int pid,pid2;
  int i;
  pid = fork();
  printf("A\n");
  if (pid == 0)
    pid2=fork();
  if (pid2)
    printf("B\n");
  printf("C\n");
  return 0;
}

Upvotes: 2

Answers (5)

jlliagre

Reputation: 30803

pid2 is not initialized in the parent process case. How much B will be printed is undefined behavior.

pid=fork() doesn't execute main() again, hopefully ...

Upvotes: 1

Erwald

Reputation: 2206

he return value of the Fork call returns a different value depending in which proccess you currently are.

Let's say you want some code to be executed in the parent process you would pu that part of the code in this condition block :

p = fork();
if (p > 0)
{
    // We're the parent process
}

And if you want to execute some code in the child process the same would apply :

p = fork();
if (0 == p)
{
    // We're the child process
}

And the rest (executed by both parents process and child) in a else block.

Upvotes: 0

gulyan

Reputation: 662

Fork return 2 values:

0 in the child process and a positive value in the parent process.
After the fork() call you will have 2 processes (if no error occurs, in which case -1 is returned).

In your example, you create 3 processes and will output 2A, 1B and 3C

Upvotes: 2