**Relocatable object files** are those, typically ending with a `.o` suffix, produced by the assembler. They contain binary code and data in a form that can be combined with other relocatable object files at compile time. The following diagram shows how one looks like when formatted using [[elf|ELF]]:
![[elf.png]]
%%ANKI
Basic
Relocatable object files are outputs of which compiler driver component?
Back: The assembler.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
Tags: c17
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END%%
%%ANKI
Basic
Relocatable object files are inputs into which compiler driver component?
Back: The linker.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
Tags: c17
<!--ID: 1734356868403-->
END%%
%%ANKI
Basic
A relocatable object file is typically broken up into what three regions?
Back: The header, sections, and the section header table.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
Tags: linker::elf
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END%%
%%ANKI
Basic
In a relocatable object file, what exists between the header and section header table?
Back: The sections.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
Tags: linker::elf
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END%%
%%ANKI
Cloze
A relocatable object file consists of a {header}, {sections}, and a {section header table}, in that order.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
Tags: linker::elf
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END%%
%%ANKI
Basic
Where in a relocatable object file does the section header table exist?
Back: At the end.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
Tags: linker::elf
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END%%
## Symbols
Every relocatable object module $m$ has a [[elf#`.symtab`|symbol table]] that contains information about the symbols defined and referenced by $m$. In the context of a linker, there are three different kinds of symbols:
1.**Global symbols** defined by $m$ and that can be referenced by other modules.
2. Global symbols referenced by $m$ but defined by another module.
3.**Local symbols** defined and referenced exclusively by $m$.
%%ANKI
Basic
With respect to linkers, how many kinds of symbols are there?
Back: Three.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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END%%
%%ANKI
Basic
With respect to linkers, what are the three kinds of symbols?
Back: Global (defined), global (referenced), and local.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1735159138628-->
END%%
%%ANKI
Basic
What distinguishes the two types of global symbols a linker understands?
Back: Whether or not the symbol is defined within the module in question.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1735159138632-->
END%%
%%ANKI
Basic
With respect to linkers, a global (defined) symbol corresponds to what kind of C construct?
Back: A nonstatic function or global variable defined in the given module.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
Tags: c17
<!--ID: 1735159138659-->
END%%
%%ANKI
Basic
With respect to linkers, a global (referenced) symbol corresponds to what kind of C construct?
Back: A nonstatic function or global variable defined in a different module.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
Tags: c17
<!--ID: 1735159138666-->
END%%
%%ANKI
Basic
With respect to linkers, a local symbol corresponds to what kind of C construct?
Back: A static function or global variable defined in the given module.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
There are three special pseudosections specified in the section header table that do not have entries in the section header table. Note pseudosections only exist in relocatable object files.
%%ANKI
Basic
How many types of pseudosections can be found in relocatable object files?
Back: Three.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1735343812817-->
END%%
%%ANKI
Basic
How many types of pseudosections can be found in executable object files?
Back: Zero.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1735343812819-->
END%%
%%ANKI
Basic
What are the three pseudosections possibly found in relocatable object files?
Back: ABS, UNDEF, and COMMON.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1735343812820-->
END%%
%%ANKI
Basic
In what region of an ELF file can references to pseudosections be found?
Back: The section header table.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1735343812821-->
END%%
%%ANKI
Basic
Why are ELF pseudosections named the way they are?
Back: They don't actually correspond to any ELF section.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1735343872318-->
END%%
### `ABS`
Marks symbols that should not be relocated.
%%ANKI
Basic
What does the ABS pseudosection indicate?
Back: The corresponding symbol should not be relocated.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1735343812822-->
END%%
%%ANKI
Basic
Why is the ABS pseudosection named the way it is?
Back: It's short for **abs**olute.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1735343812823-->
END%%
### `UNDEF`
Marks undefined symbols. These are referenced in the object module but (presumably) defined elsewhere.
%%ANKI
Basic
What does the UNDEF pseudosection indicate?
Back: The corresponding symbol is (presumably) defined elsewhere.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1735343812824-->
END%%
%%ANKI
Basic
Why is the UNDEF pseudosection named the way it is?
Back: It's short for **undef**ined.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1735343812825-->
END%%
### `COMMON`
Assuming `-fcommon`, marks unitialized data objects that are not yet allocated.
%%ANKI
Basic
What does the `COMMON` pseudosection indicate?
Back: The corresponding symbol is uninitialized and not yet allocated.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1735343812826-->
END%%
%%ANKI
Basic
What C variables are marked `COMMON` instead of put in `.bss`?
Back: Global uninitialized variables.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1735343812827-->
END%%
%%ANKI
Basic
What C variables are put in `.bss` instead of marked `COMMON`?
Back: Static variables or global variables initialized to zero.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1735343812828-->
END%%
%%ANKI
Basic
Assuming `-fcommon`, what kind of C variables does the `.bss` section contain?
Back: Static variables or global and static variables initialized to zero.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
Tags: c17
<!--ID: 1735343812829-->
END%%
%%ANKI
Basic
Assuming `-fcommon`, what kind of C variables does the `COMMON` section contain?
Back: Global uninitialized variables.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
Tags: c17
<!--ID: 1735343812830-->
END%%
%%ANKI
Basic
Assuming `-fcommon`, which ELF section contains uninitialized global C variables?
Back: N/A. These are "placed" into the `COMMON` pseudosection.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
Tags: c17
<!--ID: 1735343812831-->
END%%
%%ANKI
Basic
Assuming `-fcommon`, which ELF section contains global C variables initialized to a zero value?
Back: `.bss`
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
Tags: c17
<!--ID: 1735343812832-->
END%%
%%ANKI
Basic
Consider the following translation unit. Assuming `-fcommon`, which ELF section will `foo` end up in?
```c
int foo = 0;
```
Back: `.bss`
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
Tags: c17
<!--ID: 1735343812833-->
END%%
%%ANKI
Basic
Consider the following translation unit. Assuming `-fcommon`, which ELF section will `foo` end up in?
```c
int foo;
```
Back: N/A. It is "placed" into the `COMMON` pseudosection.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
Tags: c17
<!--ID: 1735343812834-->
END%%
At compile time, the compiler exports each global symbol as either **strong** or **weak**, and the assembler encodes this information in the symbol table. Functions and initialized global variables get strong symbols whereas uninitialized global variables get weak symbols. The linker then resolves global symbols as follows:
1. Multiple strong symbols with the same name are not allowed.
2. Given a strong symbol and multiple weak symbols with the same name, choose the strong symbol.
3. Given multiple weak symbols with the same name, choose *any* of the weak symbols.
%%ANKI
Basic
Assuming `-fcommon`, global symbols are further categorized into what two buckets?
Back: Strong and weak.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1735343812835-->
END%%
%%ANKI
Basic
Which component of the compiler driver indicates whether a global variable is strong or weak?
Back: The compiler.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
Tags: c17
<!--ID: 1735343812836-->
END%%
%%ANKI
Basic
Does a function correspond to a strong or weak symbol?
Back: Strong.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1735343812837-->
END%%
%%ANKI
Basic
Does a globally initialized variable correspond to a strong or weak symbol?
Back: Strong.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1735343812838-->
END%%
%%ANKI
Basic
Does a globally uninitialized variable correspond to a strong or weak symbol?
Back: Weak.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1735343812839-->
END%%
%%ANKI
Basic
Does a static variable correspond to a strong or weak symbol?
Back: N/A. Strong and weak describe global variables.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1735343812840-->
END%%
%%ANKI
Basic
Is `foo` considered strong or weak in the following translation unit?
```c
int foo;
```
Back: Weak.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1735343812841-->
END%%
%%ANKI
Basic
Is `foo` considered strong or weak in the following translation unit?
```c
int foo = 0;
```
Back: Strong.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1735343812842-->
END%%
%%ANKI
Basic
Is `foo` considered strong or weak in the following translation unit?
```c
int foo = 1;
```
Back: Strong.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1735343812843-->
END%%
%%ANKI
Basic
Is `foo` considered strong or weak in the following translation unit?
```c
int foo();
```
Back: Strong.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1735343812844-->
END%%
%%ANKI
Basic
How does a linker resolve multiple strong symbols with the same name?
Back: N/A. It throws an error.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1735343812845-->
END%%
%%ANKI
Basic
How does a linker resolve one strong symbol and multiple weak symbols with the same name?
Back: It prefers the strong symbol.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1735343812846-->
END%%
%%ANKI
Basic
How does a linker resolve one weak symbol and multiple strong symbols with the same name?
Back: N/A. It throws an error.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1735343812847-->
END%%
%%ANKI
How does a linker resolve multiple weak symbols with the same name?
Back: By arbitrarily picking one.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
END%%
%%ANKI
Cloze
Assuming `-fcommon`, {1:strong} is to {2:`.bss`} whereas {2:weak} is to {1:`COMMON`}.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1735343812848-->
END%%
%%ANKI
Basic
Why is `COMMON` considered in conflict with the C standard?
Back: C only allows a single definition for any object.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
A **static library** is a format for packaging multiple relocatable object files together. When the linker builds the output executable, it only copies the object modules in the library referenced by the application program.
On Linux systems, static libraries are typically stored on disk as an **archive**. An archive is a collection of concatenated relocatable object files with a header that describes the size and location of each member object file.
%%ANKI
Cloze
A {static library} packages multiple {relocatable object files} together.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1735352006086-->
END%%
%%ANKI
Basic
A static library is a collection of what kind of files?
Back: Relocatable object files.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1735352006090-->
END%%
%%ANKI
Basic
What memory-saving strategy does static libraries allow linkers to employ?
Back: Only copying relocatable object files actually used by the application program.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1735352006092-->
END%%
%%ANKI
Basic
Linux typically uses what file format for its static libraries?
Back: Archives.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1735352006095-->
END%%
%%ANKI
Cloze
On Linux machines, an {archive} typically has a {`.a`} suffix.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1735352006098-->
END%%
%%ANKI
Basic
On Linux machines, what kind of files usually have a `.a` suffix?
Back: Archives.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1735352006100-->
END%%
%%ANKI
Basic
A Linux archive file is a specific example of what more general kind of file?
Back: A static library.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
* Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
