27_ipc_shmem.md

name: main

.aim[Systems: Sharing Is Caring]

<style> .aim { font-size: .75em; border-bottom: 1px solid lightgray; margin: 1px; } .remark-inline-code { background-color: lightgray; border-radius: 3px; padding-left: 2px; padding-right: 2px; } h4 { font-size: 1.5em; margin: 1px; } </style>

---

template: main

Inter-Process Communication (IPC)

There are many kinds of Inter-Process Communication, but there is a specific set of IPC features that have competing standardized implementations, these are:

• Shared Memory
• Semaphores
• Message Queues

--

For these features, there are 2 standards:

• Portable Operating System Interface (POSIX)
• System V
• Both of these cover more than IPC.

--

POSIX and System V provide the same features, just via a different set of function calls. We will use System V.

??? POSIX: A cross-platform specification supported by UNIX operating systems and those considered UNIX-like, such as Linux. The X in the name originally denoted that the interface was “based on UNIX.”

System V: The specification that defines the requirements for an operating system to be considered UNIX.

ipcs command line utility to see active IPC structures.

---

template: main

Shared Memory

A segment of heap-like memory that can be accessed by multiple processes.

Shared memory is accessed via a key that is known by any process that needs to access it.

Shared memory does not get released when a program exits.

--

• 5 Shared memory operations
  • Create the segment (happens once) - shmget
  • Access the segment (happens once per process) - shmget
  • Attach the segment to a variable (once per process) - shmat
  • Detach the segment from a variable (once per process) - shmdt
  • Remove the segment (happens once) - shmctl

---

template: main

System V Shared Memory Operations

Headers: <sys/shm.h> <sys/ipc.h> <sys/types.h>

int *data;
int shmd;
shmd = shmget(KEY, sizeof(int), IPC_CREAT | 0640);
printf("shmd: %d\n", shmd);
data = shmat(shmd, 0, 0);
printf("data: %p\n", data);
printf("*data: %d\n", *data);
*data = * data + 10;
printf("*data: %d\n", *data);
shmdt(data);