The Linux Programming Interface: a linux and unix system Programming Handbook
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IPC Limits Since System V IPC objects consume system resources, the kernel places various limits on each class of IPC object in order to prevent resources from being exhausted. The methods for placing limits on System V IPC objects are not speci- fied by SUSv3, but most UNIX implementations (including Linux) follow a similar framework for the types of limits that may be placed. As we cover each System V IPC mechanism in the following chapters, we discuss the associated limits and note differences from other UNIX implementations. Although the types of limits that can be placed on each class of IPC object are generally similar across various UNIX implementations, the methods of viewing and changing these limits are not. The methods described in the following chapters are Linux-specific (they generally involve the use of files in the /proc/sys/kernel directory); things are done differently on other implementations. 936 Chapter 45 On Linux, the ipcs –l command can be used to list the limits on each of the IPC mechanisms. Programs can employ the Linux-specific IPC_INFO ctl operation to retrieve the same information. 45.9 Summary System V IPC is the name given to three IPC mechanisms that first appeared widely in System V, and have subsequently been ported to most UNIX implementations and incorporated into various standards. The three IPC mechanisms are message queues, which allow processes to exchange messages; semaphores, which allow pro- cesses to synchronize access to shared resources; and shared memory, which allows two or more processes to share the same pages of memory. The three IPC mechanisms have many similarities in their APIs and semantics. For each IPC mechanism, a get system call creates or opens an object. Given an integer key, the get calls return an integer identifier used to refer to the object in subsequent system calls. Each IPC mechanism also has a corresponding a ctl call that is used to delete an object and to retrieve and modify various attributes (e.g., ownership and permissions) in an object’s associated data structure. The algorithm used to generate identifiers for new IPC objects is designed to minimize the possibility of the same identifier being (immediately) reused if an object is deleted, even if the same key is used to create a new object. This enables client-server applications to function correctly—a restarted server process is able to detect and remove IPC objects created by its predecessor, and this action invali- dates the identifiers held by any clients of the previous server process. The ipcs command lists the System V IPC objects that currently exist on the system. The ipcrm command is used to remove System IPC objects. On Linux, files in the /proc/sysvipc directory can be used to obtain information about all of the System V IPC objects on the system. Each IPC mechanism has an associated set of limits that can be used to avoid exhaustion of system resources by preventing the creation of an arbitrary number of IPC objects. Various files under the /proc/sys/kernel directory can be used to view and modify these limits. tải về 6.93 Mb. Chia sẻ với bạn bè của bạn:
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