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Full Discussion: fork(), parent and child processes???
Top Forums Programming fork(), parent and child processes??? Post 302573016 by Corona688 on Saturday 12th of November 2011 11:15:58 AM
Old 11-12-2011
Quote:
Originally Posted by gabam
How can the single variable
Code:
pid

contain two values at the same time?
The address of, say, 0xbfffffffff can hold its own, separate value in each and every individual process. That's really the point of having processes -- each one gets its own flat memory space, as if it was the only thing running on the computer, but it's actually quite secure and controlled.

This virtual memory space works by dividing real memory into 4096-byte chunks, and keeping a big table of which process gets what real memory at what virtual address. This table is checked in hardware by the processor itself, and configured by the kernel. If a process tries to access a memory location where no real pages have been assigned to it, you get the familiar error "segmentation fault".

What fork() does is it makes an exact copy of the process at the time of the fork(), but gives it its own independent memory space, then twiddles the value of 'pid' so it's different in the child. It uses some tricks like copy-on-write to avoid duplicating too much memory, but that's mostly safe to ignore.
These 2 Users Gave Thanks to Corona688 For This Post:
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LEARN ABOUT SUNOS

mem

mem(7D) 							      Devices								   mem(7D)

NAME

       mem, kmem, allkmem - physical or virtual memory access

SYNOPSIS

       /dev/mem

       /dev/kmem

       /dev/allkmem

DESCRIPTION

       The file /dev/mem is a special file that provides access to the physical memory of the computer.

       The  file  /dev/kmem  is  a special file that provides access to the virtual address space of the operating system kernel, excluding memory
       that is associated with an I/O device.

       The file /dev/allkmem is a special file that provides access to the virtual address space of the operating system kernel, including  memory
       that is associated with an I/O device.  You can use any of these devices to examine and modify the system.

       Byte  addresses	in  /dev/mem are interpreted as physical memory addresses. Byte addresses in /dev/kmem and /dev/allkmem are interpreted as
       kernel virtual memory addresses.  A reference to a non-existent location returns an error. See ERRORS for more information.

       The file /dev/mem accesses physical memory; the size of the file is equal to the amount of physical memory in the computer.  This size  may
       be  larger  than 4GB on a system running the 32-bit operating environment. In this case, you can access memory beyond 4GB using a series of
       read(2) and write(2) calls, a pread64() or pwrite64() call, or a combination of llseek(2) and read(2) or write(2).

ERRORS

       EFAULT	       Occurs when trying to  write(2) a read-only location (allkmem),	read(2) a write-only location  (allkmem),  or  read(2)	or
		       write(2) a non-existent or unimplemented location (mem, kmem, allkmem).

       EIO	       Occurs  when trying to read(2) or write(2) a memory location that is associated with an I/O device using the /dev/kmem spe-
		       cial file.

       ENXIO	       Results from attempting to mmap(2) a non-existent physical (mem) or virtual (kmem, allkmem) memory address.

FILES

       /dev/mem        Provides access to the computer's physical memory.

       /dev/kmem       Provides access to the virtual address space of the operating system kernel, excluding memory that is  associated  with	an
		       I/O device.

       /dev/allkmem    Provides  access  to  the virtual address space of the operating system kernel, including memory that is associated with an
		       I/O device.

SEE ALSO

       llseek(2), mmap(2), read(2), write(2)

WARNINGS

       Using these devices to modify (that is, write to) the address space of a live running operating system or to modify  the  state	of	 a
       hardware  device  is  extremely	dangerous  and	may  result  in a system panic if kernel data structures are damaged or if device state is
       changed.

SunOS 5.10							    18 Feb 2002 							   mem(7D)
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