Lab 4 — System V IPC: Shared Memory & Message Queues

Overview

This lab introduces two System V IPC mechanisms: shared memory and message queues. You will write a parent/child program where the parent sends an integer via shared memory and a string via a message queue.

Resources

• Linux Kernel IPC Mechanisms
• Kernel Data Types
• System V IPC Suite
• Linux Programmer's Guide
• Message Queue Info
• Week 4 Code Examples

Step 1 — Setup

$ cd ~
$ cd cs3600
$ ./lab-start.sh
$ cd 4
$ cp /home/fac/dfanucchi/public_html/cs3600/examples/4/*.c .
$ cp /home/fac/dfanucchi/public_html/cs3600/examples/4/Makefile .
$ cp /home/fac/dfanucchi/public_html/cs3600/examples/4/cleanipc.sh .

Read each example, compile, and run it. When comfortable, begin dflab4.c.

$ ./cleanipc.sh <username>	#this is a script that should do the clean up for you

# to view and remove your IPC objects manually, use the following commands:
$ ipcs -m    # shared memory
$ ipcs -q    # message queues
$ ipcrm -m <shmid>
$ ipcrm -q <msqid>

Step 2 — Assignment (dflab4.c)

Create the dflab4.c file, it is not included this week.

# this is not an exhaustive list, you need to list the remaining headers

#include <sys/ipc.h>
#include <sys/msg.h>
#include <sys/shm.h>

Before fork()

1. Generate IPC key from a sentinel file foo:

   $ cd ~/cs3600/4
   $ touch foo

   Use ftok("foo", 17).
2. Open a log file for writing (child will write; open before fork()).
3. Create shared memory, attach, initialize to 0.
4. Create message queue using the same key; define a message struct and type.

key_t ipckey = ftok("foo", 17);
int shmid = shmget(ipckey, sizeof(int), IPC_CREAT | 0666);
int *shared = (int*)shmat(shmid, NULL, 0);
*shared = 0;

struct msg { long mtype; char text[128]; };
int msqid = msgget(ipckey, IPC_CREAT | 0666);

Call fork()

Child

1. Busy‑wait until *shared != 0 (with a small sleep):

   while (*shared == 0) {
     usleep(4250);   /* 4.25ms */
   }

2. Write the integer to log.
3. Block on msgrcv() for a word from parent; write it to log.
4. Close log, detach from shared memory, exit(0).
   Parent performs IPC cleanup after wait().

Parent

1. Prompt and read a two‑digit integer; convert and store in shared memory:

   write(1, "Enter a 2-digit number: ", 24);
   char buf[128]; memset(buf, 0, sizeof(buf));
   read(0, buf, 4);            // read up to 4 chars
   int num = atoi(buf);
   *shared = num;

Note on buffers: You always need a buffer when reading input. A buffer is a block of memory where the characters typed are stored before being processed. Without it, you can’t control how many characters are accepted, you risk overwriting memory (buffer overflow), and you lose the ability to clear/reset input between reads. For this lab, define a buffer large enough (e.g., #define BUFSIZE 256) so it can safely hold both the number and the word you’ll read from the user. Always memset() the buffer before reuse.
2. Prompt and read a word; send via msgsnd() to the queue.
3. wait(&status) to reap the child; print exit status.
4. Cleanup: shmdt(shared), shmctl(shmid, IPC_RMID, 0), msgctl(msqid, IPC_RMID, 0).

Build & Run

$ make
$ ./dflab4
Enter a two-digit number: 75
Enter a word: Linuxize
Child exited with status code 0

$ cat log
75
Linuxize

Run with an input file (myinput)

# myinput
83
System-V

$ ./dflab4 < myinput
$ cat log
83
System-V

Trace with strace (IPC only)

$ strace -f -e trace=ipc ./dflab4 < myinput

shmget(0x1102af98, 4, IPC_CREAT|0666)   = 911605760
shmat(911605760, 0, 0)                  = 0x7fac33f01000
msgget(0x1102af98, IPC_CREAT|0666)      = 297172992
Process 9356 attached
[pid  9355] msgsnd(297172992, {1, "System-V\n..."}, 112, 0) = 0
[pid  9356] msgrcv(297172992, {1, "System-V\n..."}, 112, 0, 0) = 112
[pid  9356] shmdt(0x7fac33f01000)       = 0
--- SIGCHLD (Child exited) @ 0 (0) ---
shmdt(0x7fac33f01000)                   = 0
msgctl(297172992, IPC_RMID, 0)          = 0
shmctl(911605760, IPC_RMID, 0)          = 0

Files to be collected:

• dflab4.c
• Makefile
• foo
• myinput