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Advanced UNIX IPC Facilities

After Haviland, et al.’s book. Advanced UNIX IPC Facilities. Introduction and Basic Concepts. UNIX offers a variety of advanced IPC mechanisms This makes UNIX an extremely rich system in the IPC area

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Advanced UNIX IPC Facilities

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  1. After Haviland, et al.’s book Advanced UNIX IPC Facilities

  2. Introduction and Basic Concepts • UNIX offers a variety of advanced IPC mechanisms • This makes UNIX an extremely rich system in the IPC area • Allows developer to use a variety of approaches when programming a system made up of cooperating tasks

  3. Categories of advanced IPC • Message passing • Semaphores • Shared memory

  4. IPC facility keys • UNIX has made the 3 categories of IPC as similar as possible for programming convenience • The most (?) important similarity is the IPC key • Keys are numbers used to identify an IPC object on a UNIX system • Much like a file name being used to identify files • Key allows IPC object to be shared by several processes

  5. Difference between keys and file names • Keys are not file names and carry less meaning • Actual key type is determined by implementation, and defined by key_t in <sys/types.h> • UNIX uses a simple library funciton that maps a file’s pathname to a key • Routine is called ftok( )‏

  6. IPC get operation • With the get operation, a program uses a key to • Create an IPC, or, • Gain access to an existing one Mqid = msgget((key_t)0100, 0644|IPC_CREAT|IPC_EXCL); For semaphores: semget For shared mem: shmget Msg queue key Non-neg id returned if successful

  7. Other IPC Operations • Control operations • Get status information • Set control values • Actual calls are msgctl, semctl, shmctl • More specific operations to perform various functions • Do interesting work • Called “IPC operations” • For example, for message queues we have: • Msgsend • msgrcv

  8. Status data structures • When an IPC object is created, the system also creates an IPC facility status structure • This contains any administrative info assoc with object • There is one type of status structure for messages, semaphores and shared mem • However all 3 contain a common permission structure, ipc_perm

  9. Message passing • A message is basically a sequence of characters of bytes, not necessarily null-terminated • One process creates a message queue using msgget • One a queue is established, a process with the right permissions can put messages into it with msgsnd • Another process can then read this message with msgrcv

  10. Form of msgget #include <sys/msg.h> int msgget(key_t key, int permflags);

  11. Permflags • Determines the exact action perform by msgget • Two constants are relevant here, defined in <sys/ipc.h> • Can be used alone or ORed together • IPC_CREAT • Tells msgget to create a message queue for the value key if one doesn't exist • Message queue will not be overwritten if it already exists • If not set, then msgget returns id of existing msgqueue, if exists • IPC_EXCL • If this and IPC_CREAT are both set, then only intend to create new msgqueue • Return -1 if already exists

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