Linkers And Loaders John Levine PdfBy Laban A. In and pdf 11.05.2021 at 00:32 10 min read
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In computing , a linker or link editor is a computer system program that takes one or more object files generated by a compiler or an assembler and combines them into a single executable file, library file, or another "object" file. A simpler version that writes its output directly to memory is called the loader , though loading is typically considered a separate process.
Typically, an object file can contain three kinds of symbols:. For most compilers, each object file is the result of compiling one input source code file. When a program comprises multiple object files, the linker combines these files into a unified executable program, resolving the symbols as it goes along. Linkers can take objects from a collection called a library or runtime library.
Most linkers do not include the whole library in the output; they include only the files [ clarification needed ] that are referenced by other object files or libraries. Library linking may thus be an iterative process, with some referenced modules requiring additional modules to be linked, and so on. Libraries exist for diverse purposes, and one or more system libraries are usually linked in by default. The linker also takes care of arranging the objects in a program's address space.
This may involve relocating code that assumes a specific base address into another base. Since a compiler seldom knows where an object will reside, it often assumes a fixed base location for example, zero. Relocating machine code may involve re-targeting of absolute jumps, loads and stores. The executable output by the linker may need another relocation pass when it is finally loaded into memory just before execution. This pass is usually omitted on hardware offering virtual memory : every program is put into its own address space, so there is no conflict even if all programs load at the same base address.
This pass may also be omitted if the executable is a position independent executable. Many operating system environments allow dynamic linking, deferring the resolution of some undefined symbols until a program is run.
That means that the executable code still contains undefined symbols, plus a list of objects or libraries that will provide definitions for these. Static linking is the result of the linker copying all library routines used in the program into the executable image. This may require more disk space and memory than dynamic linking, but is more portable, since it does not require the presence of the library on the system where it runs.
Static linking also prevents "DLL hell", since each program includes exactly the versions of library routines that it requires, with no conflict with other programs. A program using just a few routines from a library does not require the entire library to be installed.
As the compiler has no information on the layout of objects in the final output, it cannot take advantage of shorter or more efficient instructions that place a requirement on the address of another object.
For example, a jump instruction can reference an absolute address or an offset from the current location, and the offset could be expressed with different lengths depending on the distance to the target.
By first generating the most conservative instruction usually the largest relative or absolute variant, depending on platform and adding relaxation hints , it is possible to substitute shorter or more efficient instructions during the final link. In regard to jump optimizations this is also called automatic jump-sizing.
In general, the substituted sequences are shorter, which allows this process to always converge on the best solution given a fixed order of objects; if this is not the case, relaxations can conflict, and the linker needs to weigh the advantages of either option. While instruction relaxation typically occurs at link-time, inner-module relaxation can already take place as part of the optimizing process at compile-time.
In some cases, relaxation can also occur at load-time as part of the relocation process or combined with dynamic dead-code elimination techniques. One advantage of this is that it allows a program to be maintained without having to keep all of the intermediate object files, or without having to re-compile program sections that haven't changed.
It also permits program updates to be distributed in the form of small files originally card decks , containing only the object module to be replaced. In such systems, object code is in the form and format of byte punched-card images, so that updates can be introduced into a system using that medium.
It also allows one to add, change, or remove an overlay structure from an already linked load module. The term "linkage editor" should not be construed as implying that the program operates in a user-interactive mode like a text editor. It is intended for batch-mode execution, with the editing commands being supplied by the user in sequentially organized files, such as punched cards , DASD , or magnetic tape , and tapes were often used during the initial installation of the OS.
Linkage editing IBM nomenclature or consolidation or collection ICL nomenclature refers to the linkage editor's or consolidator's act of combining the various pieces into a relocatable binary, whereas the loading and relocation into an absolute binary at the target address is normally considered a separate step.
GNU ld runs the linker, which creates an executable file or a library from object files created during compilation of a software project.
A linker script may be passed to GNU ld to exercise greater control over the linking process. Two versions of ld are provided in binutils: the traditional GNU ld based on bfd , and an ELF-only version called gold. Possible origins of the name "ld" are " LoaD " and "Link eDitor".
From Wikipedia, the free encyclopedia. See also: Dynamic linker. Computer programming portal Free and open-source software portal. IBM Corporation. Archived PDF from the original on Retrieved Linkage editors and consolidators". In Floretin, J. John ed. Assemblers and Loaders.
August In Chivers, Ian D. Archived from the original on Application binary interface ABI. Binary code compatibility Foreign function interface Language binding Linker dynamic Loader Year problem.
Executable and object file formats. Comparison of formats. Unix command-line interface programs and shell builtins. Categories : Compilers Computer libraries Programming language implementation Utility software types. Namespaces Article Talk. Views Read Edit View history. Help Learn to edit Community portal Recent changes Upload file.
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CSE 131 - Compiler Construction
Anatomy of Programming Languages by William R. Cook, , pp, multiple formats. Applicative Computing: Its quarks, atoms and molecules by V. Wolfengagen, Touretzky, , pages, 1.
In computing , a linker or link editor is a computer system program that takes one or more object files generated by a compiler or an assembler and combines them into a single executable file, library file, or another "object" file. A simpler version that writes its output directly to memory is called the loader , though loading is typically considered a separate process. Typically, an object file can contain three kinds of symbols:. For most compilers, each object file is the result of compiling one input source code file. When a program comprises multiple object files, the linker combines these files into a unified executable program, resolving the symbols as it goes along. Linkers can take objects from a collection called a library or runtime library.
Linkers and Loaders
It's the cross-platform perspective that distinguishes this book. There's also a digestible guide to the computer architecture including registers, instruction formats, and memory addressing for each platform. Unix programmers will be pleased that the book has more information on non-Windows platforms than on Windows itself.
Software Engineering Stack Exchange is a question and answer site for professionals, academics, and students working within the systems development life cycle. It only takes a minute to sign up. Tried to search the web and couldn't find an answer. It might have something to do with "load", but that doesn't make much sense to me. Linkers in Linux were originally called loaders.
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Loading 1. Whatever your programming language, whatever your platform, you probably tap into linker and loader functions all the time. But do you know how to use them to their greatest possible advantage?
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