The remainder of the header word will contain various information depending upon the code. Specifically, Code Meaning 0 Start of segment. A segment name of up to 10 characters follows in the next two words. Bytes of the header contain a relative address. The following word is an absolute value which should be loaded into memory at the relative address. The following word should be loaded into that location and the address field of the loaded value should be modified by the addition of the base address of the current segment.
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Typically, an object file can contain three kinds of symbols: defined "external" symbols, sometimes called "public" or "entry" symbols, which allow it to be called by other modules, undefined "external" symbols, which reference other modules where these symbols are defined, and local symbols, used internally within the object file to facilitate relocation.
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. This may involve relocating code that assumes a specific base address into another base.
Since a compiler seldomly 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. See also: Dynamic linker 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. This approach offers two advantages: Often-used libraries for example the standard system libraries need to be stored in only one location, not duplicated in every single executable file, thus saving limited memory and disk space.
If a bug in a library function is corrected by replacing the library, all programs using it dynamically will benefit from the correction after restarting them. Programs that included this function by static linking would have to be re-linked first. There are also disadvantages: Known on the Windows platform as " DLL hell ", an incompatible updated library will break executables that depended on the behavior of the previous version of the library if the newer version is incorrectly not backward compatible.
A program, together with the libraries it uses, might be certified e. This also argues against automatic OS updates in critical systems; in both cases, the OS and libraries form part of a qualified environment. Static linking[ edit ] 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.
Relocation[ edit ] 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.
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. 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".
Chapter 3: Loader and Linkers - PPT, System Programming, Engg , Sem Notes | EduRev
It generates the executable module of a source program. It loads the executable module to the main memory. Input It takes as input, the object code generated by an assembler. It takes executable module generated by a linker. Function It combines all the object modules of a source code to generate an executable module.
Difference Between Linker and Loader