Abraham Silberschatz And Peter Baer Galvin Operating System Concepts Pdf


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Dynamic loading is a mechanism by which a computer program can, at run time , load a library or other binary into memory, retrieve the addresses of functions and variables contained in the library, execute those functions or access those variables, and unload the library from memory. It is one of the 3 mechanisms by which a computer program can use some other software; the other two are static linking and dynamic linking. Unlike static linking and dynamic linking, dynamic loading allows a computer program to start up in the absence of these libraries, to discover available libraries, and to potentially gain additional functionality. The main advantages are:. IBM 's strategic transaction processing system, CICS s onwards uses dynamic loading extensively both for its kernel and for normal application program loading.

abraham silberschatz peter baer galvin greg gagne

Dynamic loading is a mechanism by which a computer program can, at run time , load a library or other binary into memory, retrieve the addresses of functions and variables contained in the library, execute those functions or access those variables, and unload the library from memory. It is one of the 3 mechanisms by which a computer program can use some other software; the other two are static linking and dynamic linking. Unlike static linking and dynamic linking, dynamic loading allows a computer program to start up in the absence of these libraries, to discover available libraries, and to potentially gain additional functionality.

The main advantages are:. IBM 's strategic transaction processing system, CICS s onwards uses dynamic loading extensively both for its kernel and for normal application program loading. Shared libraries were added to Unix in the s, but initially without the ability to let a program load additional libraries after startup. Dynamic loading is most frequently used in implementing software plugins. Not all systems support dynamic loading.

Examples follow:. The result of dlsym or GetProcAddress has to be converted to a pointer of the appropriate type before it can be used. This can be problematic when the address of an object is to be retrieved rather than a function. However, usually one wants to extract functions anyway, so this is normally not a problem. On most systems in use today, function and object pointers are de facto convertible.

The following code snippet demonstrates one workaround which allows to perform the conversion anyway on many systems:. The above snippet will give a warning on some compilers: warning: dereferencing type-punned pointer will break strict-aliasing rules. Another workaround is:.

This makes use of the fact that reading from a different union member than the one most recently written to called " type punning " is common, and explicitly allowed even if strict aliasing is in force, provided the memory is accessed through the union type directly. Note that this trick may not work on platforms where the size of data pointers and the size of function pointers is not the same. The fact remains that any conversion between function and data object pointers has to be regarded as an inherently non-portable implementation extension, and that no "correct" way for a direct conversion exists, since in this regard the POSIX and ISO standards contradict each other.

Because of this problem, the POSIX documentation on dlsym for the outdated issue 6 stated that "a future version may either add a new function to return function pointers, or the current interface may be deprecated in favor of two new functions: one that returns data pointers and the other that returns function pointers".

If the contents of the library can be changed i. Since a pointer to a function pointer is itself an object pointer, this pointer can always be legally retrieved by call to dlsym and subsequent conversion. However, this approach requires maintaining separate pointers to all functions that are to be used externally, and the benefits are usually small. Loading a library causes memory to be allocated; the library must be deallocated in order to avoid a memory leak. Additionally, failure to unload a library can prevent filesystem operations on the file which contains the library.

However, unloading a DLL can lead to program crashes if objects in the main application refer to memory allocated within the DLL. For example, if a DLL introduces a new class and the DLL is closed, further operations on instances of that class from the main application will likely cause a memory access violation. Likewise, if the DLL introduces a factory function for instantiating dynamically loaded classes, calling or dereferencing that function after the DLL is closed leads to undefined behaviour.

The implementations of dynamic loading on UNIX-like operating systems and Windows allow programmers to extract symbols from the currently executing process. UNIX-like operating systems allow programmers to access the global symbol table, which includes both the main executable and subsequently loaded dynamic libraries.

Windows allows programmers to access symbols exported by the main executable. Windows does not use a global symbol table and has no API to search across multiple modules to find a symbol by name. In the Java programming language , classes can be dynamically loaded using the ClassLoader object.

For example:. The Reflection mechanism also provides a means to load a class if it isn't already loaded. It uses the classloader of the current class:. However, there is no simple way to unload a class in a controlled way. Loaded classes can only be unloaded in a controlled way, i. When doing so, various details need to be observed to ensure the class is really unloaded. This makes unloading of classes tedious.

Implicit unloading of classes, i. Until Java 1. Starting with Java 1. Starting with Java 6 classes can contain an internal marker indicating to the garbage collector they can be unloaded if the garbage collector desires to do so, independent of the classloader used to load the class.

The garbage collector is free to ignore this hint. Similarly, libraries implementing native methods are dynamically loaded using the System. There is no System.

Despite its promulgation in the s through UNIX and Windows, some systems still chose not to add—or even to remove—dynamic loading. For example, Plan 9 from Bell Labs and its successor 9front intentionally avoid dynamic linking, as they consider it to be "harmful". The Go runtime and any library functions are statically linked into the compiled binary. From Wikipedia, the free encyclopedia.

Mechanism by which a computer program can load a library or other binary into memory. Not to be confused with dynamic load. It is not to be confused with dynamically linked library.

Further information: Java Classloader. Computer programming portal. Archived from the original on Retrieved Wilson; Olsson, Ronald A.

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Operating System Concepts

In this section we find a set of exercises or problems of the subject , work or projects to be undertaken on the subject , case studies , directed studies , These exercises should be performed by the student obligatorily after each lesson theory. The aim is to establish the knowledge acquired and verify their capacity and study level. Operating System Concepts. ISBN: This book is intended as a general text of operating systems, can cover both the introductory part some aspects of designing them. Carretero, F.

List of ebooks and manuels about Operating system concepts 8th edition abraham silberschatz peter b galvin greg gagne. Chapter 5 Process Scheduling. Da-Wei Chang Chapter 1: Introduction. Chapter File System Implementation Silberschatz, Galvin and Gagne. Chapter 8: Main Memory 8. Download our operating system concepts 8th edition abraham silberschatz peter b galvin greg gagne eBooks for free and learn more about operating system concepts 8th edition abraham silberschatz peter b galvin greg gagne.

Operating system concepts 8th edition abraham silberschatz peter b galvin greg gagne

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Unlimited access to the largest selection of audiobooks and textbooks aligned to school curriculum on the only app specifically designed for struggling readers, like students dealing with dyslexia, blindness or other learning differences. Operating System Concepts with Java, now in its eighth edition, continues to provide a solid theoretical foundation for understanding operating systems in the language with which many students are most comfortable. The eighth edition now includes more coverage of the most current topics and applications, improved conceptual coverage and additional content to bridge the gap between concepts and actual implementations. New end-of-chapter problems, exercises, review questions, and programming exercises help to further reinforce important concepts, while WileyPLUS continues to motivate students and offer comprehensive support for the material in an interactive format. Add to Bookshelf.

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