output of forking process
开发者_开发技巧hello every one I want to ask a question about forking
here is the code
#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#define MAX_COUNT 5
#define BUF_SIZE 100
void main(void)
{
pid_t pid;
int i;
char buf[BUF_SIZE];
fork();
pid = getpid();
for (i = 1; i <= MAX_COUNT; i++) {
sprintf(buf, "This line is from pid %d, value = %d\n", pid, i);
write(1, buf, strlen(buf));
}
printf("child id :%d\n" , pid);
printf("parent id :%d\n" , getppid());
}
and when i run it here is the output
This line is from pid 31130, value = 1
This line is from pid 31130, value = 2
This line is from pid 31130, value = 3
This line is from pid 31130, value = 4
This line is from pid 31130, value = 5
child id :31130
This line is from pid 31131, value = 1
parent id :31052
This line is from pid 31131, value = 2
This line is from pid 31131, value = 3
This line is from pid 31131, value = 4
This line is from pid 31131, value = 5
child id :31131
parent id:31130
it really confused me why
the line parent id and child id printed two times
why child id value is different both time when i have called for only once
why the second time parent id value is equal to first one child id value
what is actually the parent and chile Id thanks alot
The function fork
is called once and returns twice. So both the child and the parent do the printf
s.
printf("child id :%d\n" , pid); /* Both child and parent print this. */
printf("parent id :%d\n" , getppid()); /* Both child and parent print this. */
The fact that you say "child" and "parent" doesn't really change anything so that's why you get misleading results.
A common pattern is to test the value returned by fork
. It returnss 0
in the child and the PID of the child in the parent.
pid_t pid = fork();
if (pid)
printf("I am the parent, pid: %d. The child has pid: %d\n", getpid(), pid);
else
printf("I am the child, pid: %d\n", getpid());
/* Common code. */
for...
following is the answers of your questions:
When you create a process fork it actually starts one another process hence after forking everything is executed by the main process as well as the child process so the line parent id and child id printed twice and it given different values both the time because both are executed by different processes so both have different id and different parent id now the answer of your third question is at that time of execution the second process executing the statement so its parent is the previous process so the second time parent id value is equal to first one child id value.
I hope you would be able to understand it.
After fork call parent and child executing same code
that is the reason for getting 4 ids printed
31131 is your child process
31130 its parent
31052 is parent's parent
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