如何找出代码何时读取父进程或子进程以及它们如何在C中进行通信

Question

I have a shell program where a fork is created and then used two if else statements to separate the parent and the child process.

My sample program is here and i got a few questions

while(true)
{
    /* read command line input */

    x = fork();

    if( x > 0)
        {
         wait(&status);      
        }
    else
        {
         /* run the exec() command  */
        }
}

from the above code which statement would it execute first after fork and how would parent know that child exec command was executed successfully or unsuccessfully and when would the parent stop to wait and in which condition and how. And also how would the wait(&status) code work.

Help would be much appreciated.

Answer 1

This should give you a better idea of how the fork() system call works.

Answer 2

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We can't decide which process (parent or child) should execute first. If a child has already terminated and is a zombie, wait returns immediately with that child's status. vfork() guarantees that the child runs first, until the child calls exec or exit

Answer 3

According to the POSIX specification of fork , there is no priority to run the parent or the child process. So, in fact, you cannot expect one to be prior to the other one.

But, if you are using Linux, the child is always scheduled first after the fork. Mainly, because of implementation reasons. Thus, the parent is started after. But, you also have to understand that they run concurrently once they are started. See the fork manpage .