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Full Discussion: fork(), parent and child processes???
Top Forums Programming fork(), parent and child processes??? Post 302572993 by gabam on Saturday 12th of November 2011 09:29:59 AM
Old 11-12-2011
fork(), parent and child processes???
Hi friends,
I have a small question regarding unix system call fork, I hope you will solve my problem. Here is the small program

Code:
 
$ cat fork1.c
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
int main()
{
int pid;
int x = 0;
x = x + 1;
pid = fork();
if(pid < 0)
{
printf("Fork failed\n");
return -1;
}
if(pid == 0)
printf("I am the child, my x is %d and my PID is %d\n",x,getpid());
else
printf("I am the parent, my x is %d and my PID is %d\n",x,getpid());
return 0;
}

And here is the output
Code:
$ ./fork1.exe
I am the child, my x is 1 and my PID is 1282
I am the parent, my x is 1 and my PID is 1281

Ok, when we call fork(), the following things happen
1. The parent process passes it's memory image, including the variables
Code:
pid

and
Code:
x

, to the child process that is created.
2. fork returns zero to the child, and the child's PID, a positive integer, to the parent.
3. If fork returns a negative value, it means there was some error, and no process was created.
Ok, I am going all right till now, here comes the question that has been bothering me.
How can the single variable
Code:
pid

contain two values at the same time? zero for the child process and a positive integer for the parent process? We have two values saved in the same memory location identified by
Code:
pid

, how does that make sense. Why the values are not overwritten???
I hope you guys will help me with this mystery thing, and explain it to me.
Thanks a lot, looking forward to your nice replies!
 

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LEARN ABOUT OSF1

bind_to_cpu

bind_to_cpu(3)						     Library Functions Manual						    bind_to_cpu(3)

NAME

       bind_to_cpu - Bind execution to a specific CPU.

LIBRARY

       Mach Library (libmach.a)

SYNOPSIS

       #include <sys/types.h> #include <sys/resource.h>

       int bind_to_cpu(
	       pid_t pid,
	       unsigned long cpu_mask ,
	       unsigned long flag );

PARAMETERS

       Specifies the target pid.  You must have access rights to the pid.  Specifies the CPU on which the thread should run. The target CPU is the
       bit index in the mask.  If you set more than one bit, an error is generated.  A cpu_mask of zero clears any  previously	set  CPU  binding.
       Specifies  options  to  CPU binding.  Currently only the option BIND_NO_INHERIT is supported.  When set, this option causes child processes
       and threads to not inherit the CPU binding.

DESCRIPTION

       Upon return from bind_to_cpu, all threads of the target pid are running on the target CPU.  Bound threads are not eligible for execution on
       any other CPU. You release CPU binding by invoking bind_to_cpu with a  cpu_mask of zero.

EXAMPLES

       /*
	* Fork child process and force it to run on cpu number 3.
	* Processes created by the forked child will not inherit bindings.
	*/ #include <sys/resource.h> #include <sys/sysinfo.h> #include <sys/signal.h> #include <sys/types.h>

       #define CPU_3   0x8	       /* Bit 3 set */

       main() {
	       pid_t pid;

	       if (pid = fork()) {     /* parent */
		       if (bind_to_cpu(pid, CPU_3, BIND_NO_INHERIT)) {
			       kill(pid, SIGKILL);
			       exit(1); /* bind_to_cpu() will print error msg */
		       }
		       sleep(2);       /*      wait for child to print CPU */
	       }
	       else {		       /* child */
		       long cpu_num;
		       sleep(1);       /*      wait for parent to bind CPU */
		       getsysinfo(GSI_CURRENT_CPU, &cpu_num, 0L, 0L, 0L);
		       printf("child running on CPU %d
", cpu_num);
	       } }

       In this example, the CPU_3 symbol is defined so that bit three in the bit mask is set.  When the pid returned from the fork routine identi-
       fies the parent routine, the bind_to_cpu routine is called.  This routine binds the child process to CPU number three, as specified in  the
       CPU_3 symbol. When the pid returned from the fork routine identifies the child routine, the child routine sleeps to give the parent routine
       time to set its CPU binding.  Then it uses the getsysinfo call to determine its CPU and displays its CPU with the printf routine.

       If the return value from the bind_to_cpu routine indicates an error, the parent process kills the child process and  exits  with  an  error
       status.

RETURN VALUES

       Upon successful completion, bind_to_cpu returns zero.  Upon error, a -1 is returned.

RELATED INFORMATION

       Commands: runon(1)

       Functions: getsysinfo(2) delim off

																    bind_to_cpu(3)
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