Advanced Programming in the UNIX® Environment

ptg10805159

576 Interprocess Communication Chapter 15

stack

shared memory

heap

uninitialized data

(bss)

initialized data

text

command-line arguments

and environment variables

high address

low address

0x7fff957b146c

0x00000060bd00

0x0000006020c0

array[]of 40,000 bytes

0x0000009fb6b0

0x0000009e3010

mallocof 100,000 bytes

0x7fba578c36a0

0x7fba578ab000

shared memory of 100,000 bytes

Figure 15.32Memory layout on an Intel-based Linux system

Recall that themmapfunction (Section 14.8) can be used to map portions of a file

into the address space of a process. This is conceptually similar to attaching a shared

memory segment using theshmatXSI IPC function. The main difference is that the

memory segment mapped withmmapis backed by a file, whereas no file is associated

with an XSI shared memory segment.

Example — MemoryMapping of/dev/zero

Shared memory can be used between unrelated processes. But if the processes are

related, some implementations provide a different technique.

The following technique works on FreeBSD 8.0, Linux 3.2.0, and Solaris 10. Mac OS X 10.6.8

currently doesn’t support the mapping of character devices into the address space of a process.

The device/dev/zerois an infinite source of 0 bytes when read. This device also

accepts any data that is written to it, ignoring the data. Our interest in this device for

IPC arises from its special properties when it is memory mapped.

•Anunnamed memory region is created whose size is the second argument to

mmap,rounded up to the nearest page size on the system.

•The memory region is initialized to 0.

•Multiple processes can sharethis region if a common ancestor specifies the

MAP_SHAREDflag tommap.

The program in Figure15.33 is an example that uses this special device.