T-Kernel Specification (1.B0.02)

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96 CHAPTER 4. T-KERNEL/OS FUNCTIONS 4.4.3 Mailbox A mailbox is an object used to achieve synchronization and communication by passing messages in system (shared) memory space. Functions are provided for creating and deleting a mailbox, sending and receiving messages in a mailbox, and referencing the mailbox status. A mailbox is an object identified by an ID number called a mailbox ID. A mailbox has a message queue for sent messages, and a task queue for tasks waiting to receive messages. At the message sending end (making event notification), messages to be sent go in the message queue. On the message receiving end (waiting for event notification), a task fetches one message from the message queue. If there are no queued messages, the task goes to a state of waiting for receipt from the mailbox until the next message is sent. Tasks waiting for message receipt from a mailbox are put in the task queue of that mailbox. Since the contents of messages using this function are in memory shared by the sending and receiving sides, only the start address of a message located in this shared space is actually sent and received. The contents of the messages themselves are not copied. T-Kernel manages messages in the message queue by means of a link list. An application program must allocate space at the beginning of a message to be sent, for link list use by T-Kernel. This area is called the message header. The message header and the message body together are called a message packet. When a system call sends a message to a mailbox, the start address of the message packet (pk msg) is passed in a parameter. When a system call receives a message from a mailbox, the start address of the message packet is passed in a return parameter. If messages are assigned a priority in the message queue, the message priority (msgpri) of each message must be designated in the message header (see Figure 4.2). The user puts the message contents not at the beginning of the packet but after the header part (msgcont in the figure). pk msg → Message header ∗ May include message priority (msgpri) Message content (msgcont) Figure 4.2: Format of Messages Using a Mailbox T-Kernel overwrites the contents of the header when a message is put in the message queue (except for the message priority area). An application, on the other hand, must not overwrite the header of a message in the queue (including the message priority area). The behavior if an application overwrites the message header is not defined. This restriction applies not only to the direct writing of a message header by an application program, but also to the passing of a header address to T-Kernel and having T-Kernel overwrite the message header with the contents. Accordingly, the behavior when a message already in the message queue is again sent to a mailbox is undefined. [Additional Notes] Since the application program allocates the message header space for this mailbox function, there is no limit on the number of messages that can be queued. A system call sending a message does not enter Copyright c○ 2002, 2003 by T-Engine Forum T-Kernel 1.B0.02
4.4. SYNCHRONIZATION AND COMMUNICATION FUNCTIONS 97 WAIT state. Memory blocks allocated dynamically from a fixed-size memory pool or variable-size memory pool, or else a statically allocated area can be used for message packets; but these must not be located in task space. Generally, a sending task allocates a memory block from a memory pool, sending that as a message packet. After a task on the receiving end fetches the message, it returns the memory block directly to its memory pool. Copyright c○ 2002, 2003 by T-Engine Forum T-Kernel 1.B0.02
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T-Kernel Specification T-Kernel 1.B
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Contents 1 T-Kernel Overview 1 1.1
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CONTENTS v tk set flg (Set Event Fl
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CONTENTS vii 5.2.2 Address Space Ch
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CONTENTS ix td cal que (Reference Q
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List of Figures 1.1 Position of T-K
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List of Tables 2.1 State Transition
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System Call Notation In the parts o
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Index of T-Kernel/OS System Calls T
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INDEX OF T-KERNEL/OS SYSTEM CALLS x
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Index of T-Kernel/SM Extended SVC L
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Index of T-Kernel/DS System Calls T
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Chapter 1 T-Kernel Overview 1.1 Pos
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1.2. SCALABILITY 3 • Middleware m
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Chapter 2 Concepts Underlying the T
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2.2. TASK STATES AND SCHEDULING RUL
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2.2. TASK STATES AND SCHEDULING RUL
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2.3. INTERRUPT HANDLING 11 preceden
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2.5. SYSTEM STATES 13 Transient sta
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2.7. MEMORY 15 which the upper and
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Chapter 3 Common T-Kernel Specifica
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3.2. SYSTEM CALLS 19 /* * Common co
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3.2. SYSTEM CALLS 21 Ordinarily a f
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3.3. HIGH-LEVEL LANGUAGE SUPPORT RO
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Chapter 4 T-Kernel/OS Functions Thi
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4.1. TASK MANAGEMENT FUNCTIONS 27 t
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4.1. TASK MANAGEMENT FUNCTIONS 29 #
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4.6. MEMORY POOL MANAGEMENT FUNCTIO
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4.7. TIME MANAGEMENT FUNCTIONS 163
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4.8. INTERRUPT MANAGEMENT FUNCTIONS
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4.9. SYSTEM MANAGEMENT FUNCTIONS 19
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4.10. SUBSYSTEM MANAGEMENT FUNCTION
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Chapter 5 T-Kernel/SM Details of th
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5.1. SYSTEM MEMORY MANAGEMENT FUNCT
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5.2. ADDRESS SPACE MANAGEMENT FUNCT
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5.2. ADDRESS SPACE MANAGEMENT FUNCT
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5.3. DEVICE MANAGEMENT FUNCTIONS 22
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5.5. IO PORT ACCESS SUPPORT FUNCTIO
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5.7. SYSTEM CONFIGURATION INFORMATI
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5.7. SYSTEM CONFIGURATION INFORMATI
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5.8. SUBSYSTEM AND DEVICE DRIVER ST
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260 CHAPTER 6. T-KERNEL/DS FUNCTION
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284 CHAPTER 7. REFERENCE INT tevptn
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286 CHAPTER 7. REFERENCE ER ercd =
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288 CHAPTER 7. REFERENCE INT ct = t
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290 CHAPTER 7. REFERENCE E OACV ERC
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