什么时候调用fork（）？

Question

I have two code samples here

#include<stdio.h>
int main()
{
        int i = 0;
        i++;
        fork();
        printf("i - %d, pid - %d, addr -%p\n",i,getpid(),&i);
        return 0;
}

user@Ubuntu ~/Arena/c $ ./a
i - 1, pid - 6765, addr -0x7fffd892950c
i - 1, pid - 6766, addr -0x7fffd892950c

with my second program being

#include<stdio.h>
int main()
{
        int i = 0;
        i++;
        printf("i - %d, pid - %d, addr -%p\n",i,getpid(),&i);
        fork();
        return 0;
}

user@Ubuntu ~/Arena/c $ ./b
i - 1, pid - 6772, addr -0x7fff39120f2c

Well as far as I know fork should create a COMPLETE copy of the parent program from top to bottom and execute it, if it is that way why is the position of fork() call making such a big difference ? Could some one explain why is the printf omitted in my second program ?

Answer 1

fork() creates a copy of the process, and continues executing both processes at the point you call fork().

So in your second example, your printf is executed before the fork when there is only one process.

Answer 2

Fork creates a complete copy of your program but execution continues from the point in which fork is called. Put printf after the fork and see what happens.

Usually a fork call will be followed by a check if fork returned the pid of the child or not. If it did, then your current running process is the parent which received the child's pid in order to be able to manage the child, if it didn't then your current running process is the child.

Answer 3

For further enlightenment, try:

#include <stdio.h>
#include <unistd.h>

int main(void)
{
    int i = 0;
    i++;
    printf("A: i - %d, pid - %d, addr -%p", i, getpid(), &i);
    fork();
    printf("\nB: i - %d, pid - %d, addr -%p\n", i, getpid(), &i);
    return 0;
}

The first printf() doesn't include a newline, so the output is held in memory. The second printf() includes newlines, so the output appears after the program has forked. You should see the same information in the two lines tagged A ; you should see different information in the two lines tagged B .

In a single-threaded application, the parent and child processes are almost identical except for the PID, Parent PID, and the value returned by fork() . For the full details, see the POSIX specification of fork() .