340 lines
10 KiB
Markdown
340 lines
10 KiB
Markdown
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---
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title: Linkage
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TARGET DECK: Obsidian::STEM
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FILE TAGS: c17::linkage
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tags:
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- c17
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- linkage
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---
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## Overview
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An identifier declared in different scopes or in the same scope multiple times can be made to refer to the same object or function by a process called **linkage**. There are three kinds of linkage: **external**, **internal**, and **none**.
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%%ANKI
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Basic
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Linkage is a property of what C construct?
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Back: Identifiers.
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Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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<!--ID: 1733010193273-->
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END%%
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%%ANKI
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Basic
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What capability does linkage allow identifiers to perform?
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Back: Reference to the same object or function.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1733010193275-->
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END%%
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%%ANKI
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Basic
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What are the three kinds of linkage?
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Back: External, internal, and none.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1733010193276-->
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END%%
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%%ANKI
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Basic
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External linkage denotes the same object across what?
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Back: The entire program.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1733010193277-->
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END%%
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%%ANKI
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Basic
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Internal linkage denotes the same object across what?
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Back: A single translation unit.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1733010193278-->
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END%%
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%%ANKI
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Basic
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What is the linkage between two different identifiers?
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Back: None.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1733010193279-->
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END%%
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%%ANKI
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Basic
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Which keyword is usually associated with external linkage?
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Back: `extern`
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1733010193280-->
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END%%
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%%ANKI
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Basic
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What linkage does a keyword annotated with `extern` have?
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Back: Internal or external.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1733010193281-->
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END%%
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%%ANKI
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Basic
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What linkage does a keyword associated with `static` have?
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Back: Internal.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1733010193282-->
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END%%
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%%ANKI
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Basic
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Which keyword is usually associated with internal linkage?
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Back: `static`
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1733010193283-->
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END%%
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%%ANKI
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Basic
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When can an identifier have both external and internal linkage?
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Back: When specified in two different translation units.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1733010193284-->
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END%%
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%%ANKI
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Basic
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Within a translation unit, what happens if an identifer has both external and internal linkage?
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Back: Undefined behavior.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1733010193285-->
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END%%
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%%ANKI
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Basic
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Consider the following translation unit. What is the linkage of `x`?
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```c
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int x;
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```
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Back: External.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1733010193286-->
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END%%
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%%ANKI
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Basic
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Consider the following translation unit. What is the linkage of `foo`?
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```c
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void foo(void) { }
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```
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Back: External.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1733010193287-->
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END%%
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%%ANKI
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Basic
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What is the linkage of an object identifier with file scope and no storage-class specifier?
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Back: External.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1733010193288-->
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END%%
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%%ANKI
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Basic
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*How* do you identify the linkage of an object identifier with file scope and no storage-class specifier?
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Back: Its always external.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1733010193289-->
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END%%
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%%ANKI
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Basic
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What is the linkage of a function identifier with file scope and no storage-class specifier?
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Back: Internal or external.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1733010193290-->
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END%%
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%%ANKI
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Basic
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Consider the following translation unit. What is the linkage of `foo`?
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```c
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static void foo(void);
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extern void foo(void);
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```
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Back: Internal.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1733010193291-->
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END%%
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%%ANKI
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Basic
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Consider the following translation unit. What is the linkage of `foo`?
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```c
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extern void foo(void);
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static void foo(void);
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```
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Back: N/A. Undefined behavior.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1733010193292-->
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END%%
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%%ANKI
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Basic
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Consider the following translation unit. What is the linkage of `foo`?
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```c
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void foo(void);
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static void foo(void);
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```
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Back: N/A. Undefined behavior.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1733010193293-->
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END%%
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%%ANKI
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Basic
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Consider the following translation unit. What is the linkage of `foo`?
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```c
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void foo(void);
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void foo(void);
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```
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Back: External.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1733010193294-->
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END%%
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%%ANKI
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Basic
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*How* do you identify the linkage of a function identifier with file scope and no storage-class specifier?
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Back: Its the same as if the function identifier were declared `extern`.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1733010193295-->
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END%%
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%%ANKI
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Basic
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What is the linkage of a function parameter?
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Back: None.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1733010193296-->
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END%%
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%%ANKI
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Basic
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What is the linkage of a local variable without a storage specified?
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Back: None.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1733010193297-->
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END%%
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%%ANKI
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Basic
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Consider the following translation unit. What is the linkage of the last `x`?
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```c
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int x;
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int main(void) { extern int x; }
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```
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Back: External.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1733010193298-->
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END%%
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%%ANKI
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Basic
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Consider the following translation unit. What is the linkage of the last `x`?
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```c
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int x;
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int main(void) { static int x; }
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```
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Back: Internal.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1733010193299-->
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END%%
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%%ANKI
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Basic
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Consider the following translation unit. What is the linkage of the last `x`?
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```c
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extern int x;
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int main(void) { static int x; }
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```
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Back: Internal.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1733010193300-->
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END%%
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%%ANKI
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Basic
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Consider the following translation unit. What is the linkage of the last `x`?
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```c
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extern int x;
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int main(void) { extern int x; }
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```
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Back: External.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1733010193301-->
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END%%
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%%ANKI
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Basic
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Consider the following translation unit. What is the linkage of the last `x`?
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```c
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static int x;
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extern int x;
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int main(void) { extern int x; }
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```
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Back: Internal.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1733010193302-->
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END%%
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%%ANKI
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Basic
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Consider the following translation unit. What is the linkage of the last `x`?
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```c
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extern int x;
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static int x;
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int main(void) { extern int x; }
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```
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Back: N/A. Undefined behavior.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1733010193303-->
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END%%
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%%ANKI
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Basic
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Consider the following translation unit. What is the linkage of the last `x`?
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```c
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extern int x;
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int main(void) { int x; }
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```
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Back: None.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1733010193304-->
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END%%
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%%ANKI
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Basic
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Consider the following translation unit. What is the linkage of the last `x`?
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```c
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static int x;
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int main(void) { int x; }
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```
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Back: None.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1733010193305-->
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END%%
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%%ANKI
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Basic
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Consider the following translation unit. What is the linkage of the last `x`?
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```c
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void foo(int x) { }
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```
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Back: None.
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Reference: “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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<!--ID: 1733010193306-->
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END%%
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## Bibliography
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* “ISO: Programming Languages - C,” April 12, 2011, [https://port70.net/~nsz/c/c11/n1570.pdf](https://port70.net/~nsz/c/c11/n1570.pdf).
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* Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
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