EmbLogic's Blog

MESSAGE QUEUES
message queues are like pipes, only difference is that in pipes data go byte by byte. but in message queues ,message go block by block.
message queues use user free space.
message have 2 parts—
1. contol
2.data

BLOCK is define as-
struct_msg
{
unsigned long type;
char *payload;
};
where type is process pid
payload is message.
In message queues, both sender and receiver are independent of each other.
MSGMAX——-maximum size of payload
MSGMNB——-maximum number of blocks

1. msgget–
int msgget(key_t key,int msgflg);
2. msgctl—-
int msgctl(int msqid,
int command,
struct msqid_ds *buf);
commands are——
IPC_STAT—structure initialized
IPC_SET—-assigne value to message queqe
IPC_RMID—delete message queue
3. msgsend
int msgsend( int msqid,
const void *msg_ptr,
size_t msg_sz,
int msgflg,);

please tell how rand & srand function work.

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RCS file: mes_rc.c,v
Working file: mes_rc.c
head:
branch:
locks: strict
access list:
symbolic names:
keyword substitution: kv
total revisions: 0
description:
message queues reciver ….recieve data in string..
=============================================================================

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RCS file: mes_que.c,v
Working file: mes_que.c
head: 1.1
branch:
locks: strict
root: 1.1
access list:
symbolic names:
keyword substitution: kv
total revisions: 1; selected revisions: 1
description:
implement message queues for learn it’s working
—————————-
revision 1.1 locked by: root;
date: 2014/03/26 06:06:15; author: root; state: Exp;
Initial revision
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Allocate available memory efficiently to multiple processes
Main functions

• Allocate memory to processes when needed
• Protect one process’s memory from another
• Keep track of what memory is used and what is free.

Memory Allocation

• Contiguous Allocation:Each process allocated a single contiguous chunk of memory

• Non-contiguous Allocation
• Parts of a process can be allocated non-
• contiguous chunks of memory

Process:- Process is execution of set of statements in sequential way.

Fork():-It is a system call that create a duplicate process of the process which is evoking the system call(fork). After the fork statement the remaining part of a process is executed by two process if fork system call is used once. It creates one duplicate process with a new process id and and with parent process id same as that of process that is calling this system call. And program counter is given from where it is called. we can create as much process as we want in our project.As shown in Figure.

Pipe:- Pipe is a virtual link between the two process that are related in past for communicating between them. Pipe is a size of ram that is maximum 64kb but at a time you can just write 4kb in it. Pipe is form of circular queue which create array of least file descriptors available. At 0 index of that array you will find a file descriptor for reading from the pipe and at 1 index file descriptor of writing data into the pipe is available you can use any file descriptor at a time.And once the data is read from the pipe it is deleted automatically. and if you are reading from the pipe you need to write from another process otherwise if writer is not available then pipe will go in read on block condition and it will wait till something is written in the pipe and similarly the case is for write on block if their is no one to read from pipe.

Execl():- It is a statement which takes the execution process to other program given in its argument. And the current program is hijacked by other program.

RCS (revision control system) is a project management tool. RCS uses a number of commands to manage source file. It works by tracking source file as its changed by maintaining a single file with list of changes in sufficient detail to recreate the previous version. It also allows you to store comment with every change , which can be very useful to look back in the history of changes that has made to the files. To RCS a file following commands are used.

1.   rcs -i filename

this creates a rcs file with name filenamec,v.

2. ci filename

3 . co -l filename

this locks the previous version of file and allow you to do further changes into it.

RCS (revision control system) is a project management tool. RCS uses a number of commands to manage source file. It works by tracking source file as its changed by maintaining a single file with list of changes in sufficient detail to recreate the previous version. It also allows you to store comment with every change , which can be very useful to look back in the history of changes that has made to the files. To RCS a file following commands are used.

1. rcs -i filename

this creates a rcs file with name filenamec,v.

2. ci filename

3 . co -l filename

this locks the previous version of file and allow you to do further changes into it.

we can use pipe between two related process only, related means both process has same pid in their past. after that we can read and write the data from any end.
To make a related process between two unrelated process, we have to do fork.
Now question is why we are using fork to make related process.
We are using fork to create a child process for our current process.
In this case child has its own pid but its ppid is same as current process pid.
It means both has a relation, but now problem is how we relate two different process.
we can create a relation between two different process with the help of execl().
We are using execl() in child only. besause if we are using execl() in parent thn it is not execute the whole process, it is terminated after the execl(). So it is compulsary to use execl() in child .
pipe has two descriptor one and zero.
zero for read abd One for write.
if pipe return zero it means it is created else it is not created.we have to pass the file descriptor to the other process through execl () in child.
Execl() takes string only and it is terminated by null.
and to make a string to pipe file descriptor, we are using sprintf function.
To take all the arrguments from child to other process we have to make the command line function to that funcion.
Now we can use pipe between two different process.