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Practical Case Study C Operating Systems Programming – COMP 3015 1 Introduction In this case study you will implement a simple logging service built on top of a message queue. 2 Specification The task is broken into two parts, a message logging server, and a library to log messages. The message server listens to a message queue and extracts the log messages from it. The library provides a more convenient way access the message queue. You have been provided with a program to test that your library communicates with your server. You will need to review the lecture notes, and the documentation supplied in Section 4 to implement these programs. 2.1 Message Logging Server The message logging server should attempt to create the message queue, if this fails then it should terminate with an error message, it should not run if the message queue actually exists (IPC_EXCL will help here). Once connected to the message queue, the program should sit in a loop, receiving a mes- sage, and printing it to the stdout. Messages should be formatted: id: message where id is the type from the message structure and message is the message field. The server should shutdown cleanly (i.e. delete the message queue) on receipt of aSIGINT (generated by pressing control and C keys at the same time). The sample code files logservice.h and logserver.c should form the basis of your solution for this part. There are a number of comments in this file to help you structure your code. 2.2 Messaging library The messaging library consists of two functions, both defined in logservice.h: int initLogService() This function should initialise the message queue to log messages to, returning an id if successful, and -1 on error. This function should not attempt to create the message queue, only attach it to the process. 1 int logMessage(int id, char *message) This function logs the message passed as the string message to the log service id. It should return 0 on success and -1 on error. When sending a message, the function should encode the processes pid into the type field of the message, and the string into the message field. It is your choice what to do if the message is too long (i.e. longer than MSGCHARS), sample behaviours include breaking the message up into smaller pieces or simply rejecting it. Whatever the choice, the documentation in the header file should reflect this choice. The sample code files logservice.h and logservice.c should form the basis of your solution for this part. 3 Sample Code In addition to the sample code files, two additional files have been provided, a makefile that contains build rules, and a server launch script. The make utility simplifies the build process for large projects, introductory documen- tation for make is included in the documentation section (Sec. 4). To use make to auto- mate compile process simply type “make” at the terminal (in the same directory as the other files), it will use the rules defined in the makefile to build both the logserver and logclient executables from the source files, and it will also ensure that the launch server.sh script is executable. If none of the source files have changed since the last build (based on their timestamps) the make utility will not rebuild the executables. There should be no need to modify the makefile, its format is a bit fussy so it is safer to download the file from vUWS than type it up. The launch server.sh script will open the logserver program in a new termi- nal window for you. This script detects the host operating system and performs an equivalent action after this detection. There is nop need to understand how this file achieves its goal. 2 3.1 logservice.h /* logservice.h -- definitions for the log service * Dr Evan Crawford (
[email protected]) * COMP 30015 Operating Systems Programming * Practical Case Study C * This is the sample file */ #ifndef LOGSERVICE_H /* prevent multiple inclusion */ #define LOGSERVICE_H #include
#include #include #include #include #include #include /* key for the queue */ #define KEY ftok("logservice.h", ’a’) /* message structure */ #define MSGCHARS 255 /* MSGCHARS is the number of characters in the message!*/ struct message { long type; char message[MSGCHARS+1]; /* allow it to be a string! */ }; /* function prototypes */ int logServiceInit(); /* initialises the log service client, returns a service id */ int logMessage(int serviceId, char* message); /* logs the message message to the log service serviceID */ #endif /* ifndef LOGSERVICE_H */ 3 3.2 logservice.c /* logservice.c -- implementation of the log service * Dr Evan Crawford ([email protected]) * COMP 30015 Operating Systems Programming * Practical Case Study C * This is the sample file */ #include "logservice.h" int logServiceInit() { int id; /* TODO: connect to message queue here */ return id; } int logMessage(int serviceId,char *message) { int rv; printf("The message is \"%s\"\n", message); /* TODO: Validate message length here */ /* TODO: Send the message */ return rv; } 4 3.3 logclient.c /* logclient.c -- implements a simple log service client * Dr Evan Crawford ([email protected]) * COMP 30015 Operating Systems Programming * Practical Case Study C * This is the sample file */ #include "logservice.h" int main(int argc,char **argv) { int id; /* Check if arguments are present on command line */ if (argc < 2)="" {="" fprintf(stderr,="" "usage:="" %s="" message",="" argv[0]);="" exit(1);="" }="" *="" connect="" to="" the="" log="" service="" */="" if(-1="=" (id="logServiceInit()))" {="" perror("connecting="" to="" queue");="" exit(1);="" }="" *="" log="" the="" message="" supplied="" on="" the="" command="" line="" */="" if(-1="=" logmessage(id,="" argv[1]))="" {="" perror("sending="" message");="" exit(1);="" }="" return="" 0;="" }="" 5="" 3.4="" logserver.c="" *="" logserver.c="" --="" implementation="" of="" the="" log="" server="" *="" dr="" evan="" crawford="" ([email protected])="" *="" comp="" 30015="" operating="" systems="" programming="" *="" practical="" case="" study="" c="" *="" this="" is="" the="" sample="" file="" */="" #include=""> #include "logservice.h" int queue_id; /* stores queue_id for use in signal handler */ void handler(int sig); int main() { printf("Please make me useful!\n"); exit(0); /* delete this line once you start */ /* TODO: initialise the message queue here */ /* install signal handler to clean up queue * do this after you have created the queue * then you dont need to check if it is valid! */ signal(SIGINT, handler); while (1) { /* TODO: receive a message */ /* TODO: display the message */ } return 0; } void handler(int sig) { /* TODO: clean up the queue here */ exit(0); } 6 3.5 makefile # makefile -- rules to build OSP workshop C # Dr Evan Crawford ([email protected]) # COMP 30015 Operating Systems Programming # Practical Case Study C # This is the sample file # # to use simply type "make" # this will build the server and client and launcher script # note, this is a configuration file for the MAKE utility # do not try to run it directly # if typing up the file, the indented lines need to be indented # with TABS not spaces. all: logserver logclient chmod +x launch_server.sh clean: rm -f *.o logserver logclient logclient: logclient.o logservice.o logservice.o: logservice.c logservice.h logserver: logserver.o logserver.o: logserver.c logservice.h 3.6 launch server.sh #!/bin/bash # Dr Evan Crawford ([email protected]) # COMP 30015 Operating Systems Programming # Practical Case Study C # This is the sample file ### This script launches the logserver process in a new window. ### Magic is needed for OSX as I can’t rely on xterm being installed! ### Only works when logged in via the console, not Putty/SSH ### It is not necessary to understand this script! if [ $(uname) == "Darwin" ] then osascript # include # include # include # include DESCRIPTION This manual page refers to the Linux implementation of the System V interprocess communication mecha- nisms: message queues, semaphore sets, and shared memory segments. In the following, the word resource means an instantiation of one among such mechanisms. Resource Access Permissions For each resource, the system uses a common structure of type struct ipc_perm to store information needed in determining permissions to perform an ipc operation. The ipc_perm structure, defined by the system header file, includes the following members: ushort cuid; /* creator user ID */ ushort cgid; /* creator group ID */ ushort uid; /* owner user ID */ ushort gid; /* owner group ID */ ushort mode; /* r/w permissions */ The mode member of the ipc_perm structure defines, with its lower 9 bits, the access permissions to the resource for a process executing an ipc system call. The permissions are interpreted as follows: 0400 Read by user. 0200 Write by user. 0040 Read by group. 0020 Write by group. 0004 Read by others. 0002 Write by others. Bits 0100, 0010, and 0001 (the execute bits) are unused by the system. Furthermore, "write" effectively means "alter" for a semaphore set. The same system header file also defines the following symbolic constants: IPC_CREAT Create entry if key doesn’t exist. IPC_EXCL Fail if key exists. IPC_NOWAIT Error if request must wait. IPC_PRIVATE Private key. IPC_RMID Remove resource. IPC_SET Set resource options. IPC_STAT Get resource options. Note that IPC_PRIVATE is a key_t type, while all the other symbolic constants are flag fields and can be OR’ed into an int type variable. Message Queues A message queue is uniquely identified by a positive integer (its msqid) and has an associated data structure of type struct msqid_ds, defined in , containing the following members: Linux 0.99.13 1993-11-01 1 SVIPC(7) Linux Programmer’s Manual SVIPC(7) struct ipc_perm msg_perm; ushort msg_qnum; /* no of messages on queue */ ushort msg_qbytes; /* bytes max on a queue */ ushort msg_lspid; /* PID of last msgsnd() call */ ushort msg_lrpid; /* PID of last msgrcv() call */ time_t msg_stime; /* last msgsnd() time */ time_t msg_rtime; /* last msgrcv() time */ time_t msg_ctime; /* last change time */ msg_perm ipc_perm structure that specifies the access permissions on the message queue. msg_qnum Number of messages currently on the message queue.