fork and printf

Intimidator · 12-16-2006, 04:40 AM

#include<stdio.h>
main()
{
fork();
fork();
printf("Hello world\n");

}

The code prints Hello world 4 times..why is that?

qwijibow · 12-16-2006, 05:27 AM

read man fork

your program forks into 2 threads, then those threads fork again.
And 2x2=4.

jim mcnamara · 12-16-2006, 08:00 AM

Your code should have generated a lot of complaints when you compiled it - unless this is just for show.

When you compile use gcc -Wall and fix all the complaints. Otherwise your life in programming will be hellish.

Also try man waitpid

matthewg42 · 12-16-2006, 08:26 AM

After a fork() you have two processes - the original and a child process - which are both about to execute the instruction after the fork(). You should test the return value of form fork() to work out which one you are in.
So, after the first fork() in your process, you have two processes which both execute the second fork(). After that you have four processes which then execute the printf().

Intimidator · 12-16-2006, 10:04 AM

Quote:

Originally Posted by jim mcnamara

Your code should have generated a lot of complaints when you compiled it - unless this is just for show.

When you compile use gcc -Wall and fix all the complaints. Otherwise your life in programming will be hellish.

Also try man waitpid

My Code didn't generate any complaints

btw,It has nothing to do with waitpid

Intimidator · 12-16-2006, 10:24 AM

Code:

main()
{
        fork();
        printf("Hello world");
        fork();
        fork();
        printf("Hello world 2\n");
}

Code:

main()
{
        fork();
        printf("Hello world\n");
        fork();
        fork();
        printf("Hello world 2\n");
}

The above two pieces of code produces different output.
The only difference is the '\n' in the first printf function.

The output of the first code is

Code:

Hello world
Hello world 2
Hello world 2
Hello world 2
Hello world 2
Hello world
Hello world 2
Hello world 2
Hello world 2
Hello world 2

for second piece of code..

Code:

Hello worldHello world 2
Hello worldHello world 2
Hello worldHello world 2
Hello worldHello world 2
Hello worldHello world 2
Hello worldHello world 2
Hello worldHello world 2
Hello worldHello world 2

This has something to-do with the buffer property of printf function..

matthewg42 · 12-16-2006, 02:52 PM

There's no guarantee what order the forked processes will execute in. Context switches are not guaranteed in a particular order or within any specific time.

BTW, put code in [CODE] tags to format it properly in the forum port.

Have a look at this:

Code:

#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>

char* level_desc[25] = {
        "Original process",
        "Child of original",
        "Grandchild of original",
};

int main(int argc, char** argv)
{
        pid_t my_pid;
        pid_t moms_pid;
        int   level = 0;

        my_pid   = getpid();
        moms_pid = getppid();
        printf("at start:            process id = %5d, parent = %5d (%s)\n",
                my_pid, moms_pid, level_desc[level]);

        my_pid = fork();
        if ( my_pid == 0 ) {
                // if 0, this is the child process, so lets set the PID using getpid()
                // and the PPID as well.
                my_pid   = getpid();
                moms_pid = getppid();
                level++;
        }
        else {
                // this is the parent process, PID is 0, and the PPID hasn't changed
                // we'll get the PID again.
                my_pid   = getpid();
        }
        printf("after first fork():  process id = %5d, parent = %5d (%s)\n",
                        my_pid, moms_pid, level_desc[level]);

        my_pid = fork();
        if ( my_pid == 0 ) {
                my_pid   = getpid();
                moms_pid = getppid();
                level++;
        }
        else {
                my_pid   = getpid();
        }
        printf("after second fork(): process id = %5d, parent = %5d (%s)\n",
                        my_pid, moms_pid, level_desc[level]);

        return 0;
}

The output is clearer than your original program:

Code:

at start:            process id =  9524, parent =  5270 (Original process)
after first fork():  process id =  9525, parent =  9524 (Child of original)
after first fork():  process id =  9524, parent =  5270 (Original process)
after second fork(): process id =  9527, parent =  9524 (Child of original)
after second fork(): process id =  9524, parent =  5270 (Original process)
after second fork(): process id =  9526, parent =  9525 (Grandchild of original)
after second fork(): process id =  9525, parent =  9524 (Child of original)

jim mcnamara · 12-17-2006, 08:21 AM

zombies have everything to do with waitpid.