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Notes on linkers.

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Joshua Potter 2024-12-16 10:48:51 -07:00
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- [ ] Sheet Music (10 min.) - [ ] Sheet Music (10 min.)
- [ ] Korean (Read 1 Story) - [ ] Korean (Read 1 Story)
* Notes on [[strings#Copying Functions|memcpy and memmove]]. * Notes on [[c17/strings/index#Copying Functions|memcpy and memmove]].

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- [ ] OGS - [ ] OGS
- [ ] Sheet Music (10 min.) - [ ] Sheet Music (10 min.)
- [ ] Korean (Read 1 Story) - [ ] Korean (Read 1 Story)
* Notes on sections found within an [[elf|ELF]] file.

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---
title: Strings
TARGET DECK: Obsidian::STEM
FILE TAGS: c17
tags:
- c17
---
## Overview
A contiguous sequence of characters terminated by the `NUL` character (refer to [[ascii|ASCII]]). Text data is said to be more platform-independent than [[endianness|binary]] data since it is unaffected by word size or byte ordering.
%%ANKI
Basic
What is a C-style string?
Back: A character array terminated with a `NUL` character.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1707758281264-->
END%%
%%ANKI
Basic
What character terminates all C-style strings?
Back: `NUL`
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1707758281266-->
END%%
%%ANKI
Basic
What is the decimal value of `NUL` in ASCII encoding?
Back: `0`
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1707758281268-->
END%%
%%ANKI
Basic
Text is more platform-independent than e.g. integer encodings because it is unaffected by what two properties?
Back: Word size and byte ordering.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1707758281270-->
END%%
%%ANKI
Basic
What kind of array can be expressed as a literal?
Back: Strings.
Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
<!--ID: 1728244147668-->
END%%
%%ANKI
Cloze
A {string} is a {`NUL`}-terminated array of {`char`}.
Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
<!--ID: 1728244147671-->
END%%
%%ANKI
Cloze
`<string.h>` uses prefix {`mem`} to refer to {array} arguments.
Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
<!--ID: 1728244147673-->
END%%
%%ANKI
Cloze
`<string.h>` uses prefix {`str`} to refer to {string} arguments.
Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
<!--ID: 1728244147676-->
END%%
## Escape Sequences
C has a standard for processing different escape sequences. Many languages built with C in mind parse these escape sequences in a similar way.
* `\ooo`: Consists of one to three octal digits.
* [[bash/index|Bash]] supports this sequence as `$'\ooo'`.
%%ANKI
Basic
How are C escape sequences for octal digits denoted?
Back: As `\ooo`.
Reference: Brian W. Kernighan and Dennis M. Ritchie, *The C Programming Language*, 2nd ed (Englewood Cliffs, N.J: Prentice Hall, 1988).
<!--ID: 1706975891805-->
END%%
%%ANKI
Basic
In C, `\ooo` allows specifying how many octal digits?
Back: One to three.
Reference: Brian W. Kernighan and Dennis M. Ritchie, *The C Programming Language*, 2nd ed (Englewood Cliffs, N.J: Prentice Hall, 1988).
<!--ID: 1706975891810-->
END%%
%%ANKI
Basic
How are C escape sequences exposed in bash?
Back: Using ANSI-C quoting, i.e. `$$'string'`.
Reference: Mendel Cooper, “Advanced Bash-Scripting Guide,” n.d., 916.
Tags: bash
<!--ID: 1721387296231-->
END%%
* `\xhh`: Consists of one or more [[radices#Hexadecimal|hexadecimal]] digits. The `x` prefix is required to distinguish from octal escape sequences.
* [[bash/index|Bash]] supports this sequence as `$'\xhh'`. One or two digits is supported.
%%ANKI
Basic
How are C escape sequences for hexadecimal digits denoted?
Back: As `\xhh`.
Reference: Brian W. Kernighan and Dennis M. Ritchie, *The C Programming Language*, 2nd ed (Englewood Cliffs, N.J: Prentice Hall, 1988).
<!--ID: 1706975891820-->
END%%
%%ANKI
Basic
In C, `\x` allows specifying how many hexadecimal digits?
Back: One or more.
Reference: Brian W. Kernighan and Dennis M. Ritchie, *The C Programming Language*, 2nd ed (Englewood Cliffs, N.J: Prentice Hall, 1988).
<!--ID: 1706975891824-->
END%%
%%ANKI
Basic
What footgun does C's `\x` sequence expose?
Back: Using more than two hexadecimal digits can produce undefined results.
Reference: Arnold D. Robbins, “GAWK: Effective AWK Programming,” October 2023, [https://www.gnu.org/software/gawk/manual/gawk.pdf](https://www.gnu.org/software/gawk/manual/gawk.pdf).
<!--ID: 1706975891828-->
END%%
* `\uhhhh`: Introduced in C11 to represent Unicode code points. *Must* have exactly four hexadecimal characters specified with `0` leading padding if necessary.
* [[bash/index|Bash]] supports this sequence as `$'uhhhh'`. One to four hex digits is supported.
%%ANKI
Basic
What two ways are C escape sequences for unicode denoted?
Back: As `\uhhhh` or `\Uhhhhhhhh`.
Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
Tags: encoding::unicode
<!--ID: 1706975891832-->
END%%
%%ANKI
Basic
In C, `\u` allows specifying how many hexadecimal digits?
Back: Exactly four.
Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
Tags: encoding::unicode
<!--ID: 1706975891835-->
END%%
%%ANKI
Basic
In what standard were C's `\u` and `\U` escape sequences introduced?
Back: C11.
Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
Tags: encoding::unicode
<!--ID: 1706975891839-->
END%%
* `\Uhhhhhhhh`: Introduced in C11 to represent larger unicode code points. *Must* have exactly eight hexadecimal characters specified with `0` leading padding if necessary.
%%ANKI
Basic
In C, `\U` allows specifying how many hexadecimal digits?
Back: Exactly eight.
Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
Tags: encoding::unicode
<!--ID: 1706975891843-->
END%%
%%ANKI
Basic
Why does C have both `\u` and `\U`?
Back: `\U` accommodates for larger code point values.
Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
Tags: encoding::unicode
<!--ID: 1706976705750-->
END%%
## Copying Functions
The two primary functions used for copying memory are `memcpy` and `memmove`:
```c
void* memcpy(void* restrict s1, const void* restrict s2, size_t n);
void* memmove(void* s1, const void* s2, size_t n);
```
%%ANKI
Basic
What are the two primary functions provided by the standard library for copying memory?
Back: `memcpy` and `memmove`.
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
<!--ID: 1732843837986-->
END%%
%%ANKI
Basic
What is the function prototype of `memcpy`?
Back:
```c
void* memcpy(void* restrict s1, const void* restrict s2, size_t n);
```
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
<!--ID: 1732843837987-->
END%%
%%ANKI
Basic
What does the `memcpy` function do?
Back: Copies a specified number of bytes from one object to another.
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
<!--ID: 1732843837989-->
END%%
%%ANKI
Basic
Disregarding out of bounds errors, when does `memcpy` invoke undefined behavior?
Back: When copying occurs between objects that overlap in memory.
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
<!--ID: 1732843837990-->
END%%
%%ANKI
Basic
What does the first argument of `memcpy` refer to?
Back: The destination address bytes are copied to.
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
<!--ID: 1732843837991-->
END%%
%%ANKI
Basic
What does the second argument of `memcpy` refer to?
Back: The source address bytes are copied from.
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
<!--ID: 1732843837993-->
END%%
%%ANKI
Basic
What does the third argument of `memcpy` refer to?
Back: The number of bytes to copy from the source address.
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
<!--ID: 1732843837994-->
END%%
%%ANKI
Basic
Which of `memcpy` or `memmove` is faster?
Back: `memcpy`
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
<!--ID: 1732843837995-->
END%%
%%ANKI
Basic
*Why* is `memmove` slower than `memcpy`?
Back: It's implemented as if copying first into a temporary array that does not overlap the arguments in memory.
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
<!--ID: 1732843837996-->
END%%
%%ANKI
Basic
When should you prefer using `memcpy` instead of `memmove`?
Back: When the copying occurs between objects that do not overlap in memory.
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
<!--ID: 1732843837997-->
END%%
%%ANKI
Basic
Which standard header library defines `memcpy`?
Back: `<string.h>`
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
<!--ID: 1732843837998-->
END%%
%%ANKI
Basic
What is the function prototype of `memmove`?
Back:
```c
void* memmove(void* s1, const void* s2, size_t n);
```
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
<!--ID: 1732843837999-->
END%%
%%ANKI
Basic
What does the first argument of `memmove` refer to?
Back: The destination address bytes are copied to.
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
<!--ID: 1732843838000-->
END%%
%%ANKI
Basic
What does the second argument of `memmove` refer to?
Back: The source address bytes are copied from.
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
<!--ID: 1732843838001-->
END%%
%%ANKI
Basic
What does the third argument of `memmove` refer to?
Back: The number of bytes to copy from the source address.
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
<!--ID: 1732843838002-->
END%%
%%ANKI
Basic
Disregarding out of bounds errors, when does `memmove` invoke undefined behavior?
Back: N/A.
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
<!--ID: 1732843838003-->
END%%
%%ANKI
Basic
When should you prefer using `memmove` instead of `memcopy`?
Back: When the copying occurs between objects that overlap in memory.
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
<!--ID: 1732843838004-->
END%%
%%ANKI
Basic
Which standard header library defines `memmove`?
Back: `<string.h>`
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
<!--ID: 1732843838005-->
END%%
%%ANKI
Basic
What does the `memmove` function do?
Back: Copies a specified number of bytes from one object to another.
Reference: “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
<!--ID: 1732843838006-->
END%%
## Bibliography
* Arnold D. Robbins, “GAWK: Effective AWK Programming,” October 2023, [https://www.gnu.org/software/gawk/manual/gawk.pdf](https://www.gnu.org/software/gawk/manual/gawk.pdf).
* Brian W. Kernighan and Dennis M. Ritchie, *The C Programming Language*, 2nd ed (Englewood Cliffs, N.J: Prentice Hall, 1988).
* Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
* “ISO: Programming Languages - C17,” April 2017, [https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf](https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf).
* Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
* Mendel Cooper, “Advanced Bash-Scripting Guide,” n.d., 916.

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---
title: ELF
TARGET DECK: Obsidian::STEM
FILE TAGS: linker::elf x86-64
tags:
- elf
- linker
- x86-64
---
## Overview
Modern [[x86-64/index|x86-64]] Linux systems use the ELF (Executable and Linkable Format) object file format.
![[elf.png]]
%%ANKI
Basic
What object file format do modern x86-64 Linux machines typically use?
Back: ELF.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1734356868437-->
END%%
%%ANKI
Basic
What is the ELF object file format an acronym for?
Back: **E**xecutable and **L**inkable **F**ormat.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1734356868442-->
END%%
%%ANKI
Basic
What is ELF an example of?
Back: An object file format.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1734356868446-->
END%%
### Relocatable Object Files
A typical ELF relocatable object file contains the following sections:
#### `.text`
The machine code of the compiled program.
%%ANKI
Basic
What does the `.text` section of an ELF file contain?
Back: The machine code of the compiled program.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1734367304868-->
END%%
%%ANKI
Basic
Which ELF section typically contains the machine code of the compiled program?
Back: `.text`
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1734367304871-->
END%%
%%ANKI
Cloze
The {`.text`} section contains the {machine code} of the compiled program.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1734367304872-->
END%%
#### `.rodata`
Read-only data such as the format strings in [[c17/strings/printf|printf]] statements and [[conditions#JMP|jump tables]] for switch statements.
%%ANKI
Basic
What does the `.rodata` section of an ELF file contain?
Back: Read-only data.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1734367304874-->
END%%
%%ANKI
Basic
Which ELF section typically contains read-only data?
Back: `.rodata`
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1734367304875-->
END%%
%%ANKI
Basic
Why is the `.rodata` section named the way it is?
Back: It stands for **r**ead-**o**nly **data**.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1734367304876-->
END%%
%%ANKI
Basic
In what section of an ELF file would a `printf` format string be found in?
Back: `.rodata`
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
Tags: c17::printf
<!--ID: 1734367304877-->
END%%
%%ANKI
Basic
In what section of an ELF file would a jump table be found in?
Back: `.rodata`
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1734367304878-->
END%%
#### `.data`
Global and static C variables initialized to a non-zero value.
%%ANKI
Basic
What kind of global/static C variables does the `.data` section of an ELF file contain?
Back: Those initialized to a non-zero value.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1734368832070-->
END%%
%%ANKI
Basic
Which ELF section contains global C variables initialized to a non-zero value?
Back: `.data`
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1734368832073-->
END%%
%%ANKI
Basic
Which ELF section contains static C variables initialized to a non-zero value?
Back: `.data`
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1734368832075-->
END%%
%%ANKI
Basic
Which ELF section contains local C variables initialized to a non-zero value?
Back: N/A.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1734368832076-->
END%%
%%ANKI
Basic
Consider the following translation unit. Which ELF section will `foo` end up in?
```c
int foo = 1;
```
Back: `.data`
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1734369188095-->
END%%
%%ANKI
Basic
Consider the following translation unit. Which ELF section will `bar` end up in?
```c
int foo() {
int bar = 0;
}
```
Back: N/A.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1734369188101-->
END%%
%%ANKI
Basic
Consider the following translation unit. Which ELF section will `bar` end up in?
```c
int foo() {
static int bar = 1;
}
```
Back: `.data`.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1734369188110-->
END%%
#### `.bss`
Uninitialized global and static C variables, along with any global or static variables initialized to zero.
%%ANKI
Basic
What kind of global/static C variables does the `.bss` section of an ELF file contain?
Back: Unitialized variables or those 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: 1734368832077-->
END%%
%%ANKI
Basic
Which ELF section contains uninitialized global C variables?
Back: `.bss`
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1734368832078-->
END%%
%%ANKI
Basic
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.
<!--ID: 1734368832079-->
END%%
%%ANKI
Basic
Which two ELF sections contain global/static C variables?
Back: `.data` and `.bss`
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1734368832080-->
END%%
%%ANKI
Basic
Consider the following translation unit. 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.
<!--ID: 1734369188113-->
END%%
%%ANKI
Basic
Consider the following translation unit. Which ELF section will `foo` end up in?
```c
int foo;
```
Back: `.bss`
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1734369188117-->
END%%
%%ANKI
Basic
Both `.bss` and `.data` hold what kind of C variables?
Back: Global or static C variables.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1734369188121-->
END%%
%%ANKI
Basic
Which of `.data` and/or `.bss` is considered a placeholder?
Back: `.bss`
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1734369188126-->
END%%
%%ANKI
Basic
What does Bryant et al. mean by saying `.bss` is "merely a placeholder"?
Back: The `.bss` section occupies no actual space in the object file.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1734369188131-->
END%%
%%ANKI
Basic
How does the size of the `.bss` section compare to that of the `.data` section?
Back: Less than or equal since the `.bss` section is always empty.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1734369188136-->
END%%
%%ANKI
Basic
What mneumonic does Bryant et al. suggest to remember the `.bss` and `.data` distinction?
Back: `.bss` is a backronym for "**B**etter **S**ave **S**pace!"
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1734369188140-->
END%%
%%ANKI
Basic
Consider the following translation unit. Which ELF section will `bar` end up in?
```c
int foo() {
static int bar = 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.
<!--ID: 1734369188106-->
END%%
#### `.symtab`
A symbol table with information about functions and global variables defined and referenced in the program.
%%ANKI
Basic
What does the `.symtab` section of an ELF file contain?
Back: A symbol table for functions and 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: 1734370180072-->
END%%
%%ANKI
Basic
Which ELF section contains a symbol table for functions and global variables?
Back: `.symtab`
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1734370180078-->
END%%
%%ANKI
Basic
Why is the `.symtab` ELF section named the way it is?
Back: It's short for **sym**bol **tab**le.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1734370180083-->
END%%
## Bibliography
* 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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<!--ID: 1733671136117--> <!--ID: 1733671136117-->
END%% END%%
## Object Files
Object files come in three forms:
1. **Relocatable object files**. Contains binary code and data in a form that can be combined with other relocatable object files at compile time.
2. **Executable object files**. Contains binary code and data in a form that can be copied directly into memory and executed.
3. **Shared object files**. A special type of relocatable object file that can be loaded into memory and linked dynamically, at either load time or run time.
An **object module** is a sequence of bytes whereas an **object file** is an object module stored on disk.
%%ANKI
Basic
What are the three types of object files?
Back: Relocatable, executable, and shared.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1734356868367-->
END%%
%%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
<!--ID: 1734356868394-->
END%%
%%ANKI
Basic
Executable object files are outputs of 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: 1734356868399-->
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
Executable object files are inputs into which compiler driver component?
Back: N/A.
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: 1734356868406-->
END%%
%%ANKI
Cloze
A {shared} object file is a special case of a {relocatable} object file.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1734356868428-->
END%%
%%ANKI
Basic
What is an object module?
Back: A sequence of bytes.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1734356868413-->
END%%
%%ANKI
Basic
In terms of object modules, what is an object file?
Back: An object module stored on disk.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1734356868418-->
END%%
%%ANKI
Basic
What distinguishes an object module from an object file?
Back: An object file is an object module stored on disk.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1734356868423-->
END%%
%%ANKI
Basic
In what way is the term "object file" misused by Bryant et al.?
Back: Technically this term only refers to object modules on disk.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1734356868433-->
END%%
## Bibliography ## Bibliography
* 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.

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---
title: Object Files
TARGET DECK: Obsidian::STEM
FILE TAGS: linker::object-file
tags:
- linker
- object-file
---
## Overview
Object files come in three forms:
1. **Relocatable object files**. Contains binary code and data in a form that can be combined with other relocatable object files at compile time.
2. **Executable object files**. Contains binary code and data in a form that can be copied directly into memory and executed.
3. **Shared object files**. A special type of relocatable object file that can be loaded into memory and linked dynamically, at either load time or run time.
%%ANKI
Basic
What are the two types of object files?
Back: Relocatable and executable.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1734356868367-->
END%%
%%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
<!--ID: 1734356868394-->
END%%
%%ANKI
Basic
Executable object files are outputs of 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: 1734356868399-->
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
Executable object files are inputs into which compiler driver component?
Back: N/A.
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: 1734356868406-->
END%%
%%ANKI
Cloze
A {shared} object file is a special case of a {relocatable} object file.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1734356868428-->
END%%
An **object module** is a sequence of bytes whereas an **object file** is an object module stored on disk.
%%ANKI
Basic
What is an object module?
Back: A sequence of bytes.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1734356868413-->
END%%
%%ANKI
Basic
In terms of object modules, what is an object file?
Back: An object module stored on disk.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1734356868418-->
END%%
%%ANKI
Basic
What distinguishes an object module from an object file?
Back: An object file is an object module stored on disk.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1734356868423-->
END%%
%%ANKI
Basic
In what way is the term "object file" misused by Bryant et al.?
Back: Technically this term only refers to object modules on disk.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
<!--ID: 1734356868433-->
END%%
Object files are organized to specific **object file formats**:
* Windows uses the PE (Portable Executable) format.
* Mac OS-X uses the Mach-O format.
* Modern x86-64 Linux systems use the ELF (Executable and Linkable Format) format.
%%ANKI
Basic
What object file format do x86-64 Linux machines typically use?
Back: ELF.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
Tags: x86-64
<!--ID: 1734356868437-->
END%%
%%ANKI
Basic
What is the ELF object file format an acronym for?
Back: **E**xecutable and **L**inkable **F**ormat.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
Tags: x86-64
<!--ID: 1734356868442-->
END%%
%%ANKI
Basic
What is ELF an example of?
Back: An object file format.
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
Tags: x86-64
<!--ID: 1734356868446-->
END%%
## Bibliography
* Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.