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