Linker (computing)

In

A simpler version that writes its output directly to memory is called the loader, though loading is typically considered a separate process.^[1][2]

Overview

Computer programs typically are composed of several parts or modules; these parts/modules do not need to be contained within a single object file, and in such cases refer to each other using symbols as addresses into other modules, which are mapped into memory addresses when linked for execution.

While the process of linking is meant to ultimately combine these independent parts, there are many good reasons to develop those separately at the

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. Loading the program will load these objects/libraries as well, and perform a final linking.

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 or performance is improved, 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
  backward compatible
  .
• A program, together with the libraries it uses, might be certified (e.g. as to correctness, documentation requirements, or performance) as a package, but not if components can be replaced (this also argues against automatic OS updates in critical systems; in both cases, the OS and libraries form part of a qualified environment).

Contained or virtual environments may further allow system administrators to mitigate or trade-off these individual pros and cons.

Static linking

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

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.^[4] This step can be performed only after all input objects have been read and assigned temporary addresses; the linker relaxation pass subsequently reassigns addresses, which may in turn allow more potential relaxations to occur. 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

Linkage editor

In IBM

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.

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.^[2]

Linker Control Scripts

In the beginning linkers gave users very limited control over the arrangement of generated output object files. As the target systems became complex with different memory requirements such as embedded systems, it became necessary to give users control to generate output object files with their specific requirements such as defining base addresses' of segments. Linkers control scripts were used for this.

Common implementations

On Unix and Unix-like systems, the linker is known as "ld". Origins of the name "ld" are "LoaDer" and "Link eDitor". The term "loader" was used to describe the process of loading external symbols from other programs during the process of linking.^[5]

GNU linker

The GNU linker (or GNU ld) is the

The command-line and linker script syntaxes of GNU ld is the de facto standard in much of the Unix-like world. The LLVM project's linker, lld, is designed to be drop-in compatible,^[7] and may be used directly with the GNU compiler. Another drop-in replacement, mold, is a highly parallelized and faster alternative which is also supported by GNU tools.^[8]

See also

References

Further reading

• Fraser, Christopher W.;
  S2CID 206508204
  .
• Operating System 360 - Linkage Editor (E) - Program Logic Manual (PDF) (3 ed.).
  International Business Machines Corporation. 1969-07-23 [June 1967]. Program number 360S-ED-510. File No. S360-31. Form Y28-6610-2. Archived from the original
  (PDF) on 2007-10-01. Retrieved 2020-03-07.
• S2CID 42995338
  .
• OCLC 42413382. Archived from the original on 2012-12-05. Retrieved 2020-01-12. Code: [1][2] Errata: [3]
• S2CID 5694671. Archived
  (PDF) from the original on 2020-03-07. Retrieved 2020-03-07. (19 pages)
• Ramsey, Norman (May 1996). "Relocating Machine Instructions by Currying" (PDF). ACM SIGPLAN Notices. 31 (5): 226–236.
  doi:10.1145/249069.231429. Archived
  (PDF) from the original on 2020-05-18.

External links

Look up linker in Wiktionary, the free dictionary.

• Ian Lance Justin's Linkers blog entries
• Linkers and Loaders, a Linux Journal article by Sandeep Grover
• Another Listing of Where to Get a Complete Collection of Free Tools for Assembly Language Development
• GNU linker manual
• LLD - The LLVM Linker
• ld(1): The GNU linker – Linux User Manual – User Commands