Question

shared memory writing and reading

Asked by: dpd3891

following is the part of my code. I am using shared memory between two programs. when one write other will see read after some delay and vice versa.
for reading i.e. displaying it works proper.........but after that it delays long for printing Enter message to BS1 and accepting value.
For the first time it skips accepting data from standard input and second time it works properly. (for this line       printf("\nEnter message to BS1 :");)

I don't understand what is exact problem?



      while(keep_going)
      {                 
      sleep(9);
//checking time for debug
curnt = time(NULL);
dtime = ctime(&curnt);
printf("\n      Current time: %s", dtime);
      printf("\nMessage received from MS2 : %s", mptr->msg);
      printf("\nEnter message to BS1 :");
      fgets(msg,STR_SIZE,stdin);
      len = strlen(msg);
      newstring = (char *)malloc(len-1);
      for(i = 0; i < strlen(msg)-1; i++)
      {
       newstring[i] = msg[i];
      }
      strcpy(mptr->who,"MS1");
      strcpy(mptr->whom,"BS1");
      strcpy(mptr->msgType,"CONV");
      strncpy(mptr->msg, newstring, STR_SIZE);
      if((strcmp(newstring,"bye")==0))
      {
      currentState1= 13;  
      strcpy(mptr -> msgType,"bye");
      }
      sleep(4);
//curnt = time(NULL);
//dtime = ctime(&curnt);
//printf("\n      Current time: %s", dtime);
      //printf("\nMessage received from BS1 : %s", mptr->msg);
      }




for creating shared memory, I have used following code..........here I have given some part of my code
key = 1234;
     
      if ((shmid = shmget(key, SHMSZ , IPC_CREAT | 0666)) < 0) {
      perror("shmget");
        exit(1);
          }
          if ((mptr = shmat(shmid,NULL, 0)) == (char *) -1) {
        perror("shmat");
        exit(1);


struct messageFormat *mptr;

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Asked On
2009-11-01 at 18:29:35ID24862636
Topic

C Programming Language

Participating Experts
1
Points
500
Comments
9

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Answers

 

by: mnhPosted on 2009-11-01 at 22:16:21ID: 25717364

Hi,

I see two sleep() calls in your code: sleep(9) and sleep(4).
If the delay is about 13 or 14 seconds, then obviously this is the cause.

Additionally, it's not clear why you need the malloc statement, and the loop

for(i = 0; i < strlen(msg)-1; i++)
{
    newstring[i] = msg[i];
}

Couldn't you simply use msg, as follows:
    strncpy(mptr->msg, msg, STR_SIZE);
    if((strcmp(msg,"bye")==0))

etc...

 

by: dpd3891Posted on 2009-11-02 at 06:49:08ID: 25719794

There is other program which write and read in same memory so I have added two sleep to match there both of them reading and writing.

also I have used
for(i = 0; i < strlen(msg)-1; i++)
{
newstring[i] = msg[i];
}
because when we take input by fgets, it also take '\n' so need to remove that to compare the 'bye' input.



Here is my ouptput


Communicating with BS1....!!
Current time: Mon Nov 2 08:41:33 2009

Message received from MS2 : 1@@
Enter message to BS1 :
Current time: Mon Nov 2 08:41:44 2009

Message received from MS2 : @@¨
Enter message to BS1 :2

Measuring signal strength to BS1 and BS2......
Signal strength of BS1 is greater than signal strength of BS2

Communicating with BS1....!!
Current time: Mon Nov 2 08:42:06 2009

Message received from MS2 : 3@@
Enter message to BS1 :

Bold part what I was talking about

"for reading i.e. displaying it works proper.........but after that it delays long for printing Enter message to BS1 and accepting value.
For the first time it skips accepting data from standard input and second time it works properly. (for this line       printf("\nEnter message to BS1 :");)"

and here is my other program code


printf("\nYou are in Conversation state");
sleep(4);
curnt = time(NULL);
dtime = ctime(&curnt);
printf("\n Current time: %s", dtime);
printf("\nEnter message to MS1: ");
fgets(msg_m2,STR_SIZE, stdin);
len = strlen(msg_m2);
newstring = (char *)malloc(len-1);
for(i = 0; i < strlen(msg_m2)-1; i++)
{
newstring[i] = msg_m2[i];
}
strcpy(mptrm2->who,"MS2");
strcpy(mptrm2->whom,"BS1");
strcpy(mptrm2->msgType,"CONV");
strncpy(mptrm2->msg, newstring, STR_SIZE);

if((strcmp(newstring,"bye")==0))
{
currentState2= 13;
strcpy(mptrm2 -> msgType,"bye");
}
else
{
sleep(10);
curnt = time(NULL);
dtime = ctime(&curnt);
printf("\n Current time: %s", dtime);
printf("\nMessage received from MS1 : %s", mptrm2->msg);
}
break;

Thank you.

dpd.

 

 

by: mnhPosted on 2009-11-02 at 07:13:24ID: 25720058

Hi,

Regarding the newline, here is a much simpler way to get rid of it:

    char *newline = NULL;

    newline = strrchr( msg, '\n' );
    if (newline != NULL)
    {
          *newline = '\0';
    }

You can replace the malloc operation, and the unnecessary loop, with the above simple code.

Please note also that as a rule checking

     i < strlen(msg)-1

in the for loop

  for(i = 0; i < strlen(msg)-1; i++)
  {
        newstring[i] = msg[i];
   }

is VERY inefficient, since the length of msg gets evaluated anew in every iteration, but the length of the string does not change!
So in these cases you should store the string length in a variable before the loop, and use that instead. For example:

length = strlen(msg)-1;
for(i = 0; i < length; i++)
{
     newstring[i] = msg[i];
}

Regarding the delay, it is caused by the various sleep statements.
In cases such as this you should use a synchronization mechanism, such as semaphores, to coordinate access between readers and writers of the common shared memory.

Regards

 

by: dpd3891Posted on 2009-11-02 at 07:22:24ID: 25720153

Can you please help me in synchronization mechanism, such as semaphores. i don't know how to use. also these sharing will not be limited to 2 program, there will be other process such as bs2 and bs1 also share the same memory for handoff purpose. so will it be difficult to manage semaphores for all processes?
Thank you.

 

by: mnhPosted on 2009-11-02 at 08:07:08ID: 25720606

Sure.

Semaphores are not very simple, so I suggest first to get acquainted with them.
Here are some overview articles about semaphores on Unix:

http://www.exforsys.com/tutorials/programming-concepts/semaphore-in-unix-an-overview.html
http://www.classes.cs.uchicago.edu/archive/2008/fall/51081-1/LabFAQ/lab5/Semaphores.html
http://www.ibm.com/developerworks/aix/library/au-ipc/index.html#ibm-pcon
http://snap.nlc.dcccd.edu/learn/fuller3/chap12/chap12.html

And here is a complete example about coordinating access to shared memory with semaphores:
http://publib.boulder.ibm.com/infocenter/iseries/v5r3/index.jsp?topic=/apis/apiexusmem.htm

There is a bit of new stuff to learn, but this is the correct way to solve the problem.
Let me know if you have any questions an the above.

Regarding the first time skip of
    printf("\nEnter message to BS1 :");

Maybe the first time mptr->msg, in the following statement
    printf("\nMessage received from MS2 : %s", mptr->msg);
is empty and contains garbage, which might cause problems. Try to initialize it to an empty string.

Regards,
-- mnh

 

by: dpd3891Posted on 2009-11-02 at 09:23:18ID: 25721375

ok I will study these links and let you know if I have any difficulty because I have to finish in 1 and 1/2 days. If you want I will email my code to you after some modification so i that you can guide me on that. let me know can I send you my code.

Thank You.

 

by: mnhPosted on 2009-11-03 at 00:53:25ID: 25727055

I'll be glad to assist with any problems you will face in your assignment.

 

by: dpd3891Posted on 2009-11-03 at 08:58:35ID: 25730982

Hey hi There,

I got one week extension for the assignment so I will study it and let you know if I need any help.

Thanks,
dpd.

 

by: dpd3891Posted on 2009-11-12 at 20:09:29ID: 25811310

Hi,
I have few questions regarding semaphore.
Can we use semaphore with mulltiple processes? for example if process1 using shared memory then other two can't use that shared memory. If process1 releases lock then process2 can use that but we don't want process3 to access it till process2 finish it and then after finishing process2, we want process3 to use but not process1 and process2.

I tried to study above links, but don't understand the union semun part from http://snap.nlc.dcccd.edu/learn/fuller3/chap12/chap12.html link.

Can anyone please help me to start using semaphore in shared memory?

Hey mnh, I understand semaphore concept, but don't know how to start it?

Thank you,
dpd.

20120131-EE-VQP-002

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