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title TARGET DECK FILE TAGS tags
Alignment Obsidian::STEM c17::alignment x86-64
c17
x86-64

Overview

For a large class of modern ISAs, storage for basic C datatypes respect self-alignment. This means chars can start on any byte address, shorts on any even address, 4-byte ints and floats must start on an address divisible by 4, and doubles must start on an address divisible by 8. Likewise pointers are also self-aligned.

%%ANKI Basic What does self-alignment refer to? Back: The placement of C datatypes on an address divisible by the size of the datatype. Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. http://www.catb.org/esr/structure-packing/.

END%%

Wasted space introduced solely for alignment purposes is referred to as slop.

%%ANKI Cloze With respect to memory alignment, {slop} is {waste space for padding datatypes to their alignment}. Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. http://www.catb.org/esr/structure-packing/.

END%%

Structures

A struct's stride address refers to the first address following the struct data that has the same alignment as the struct. In general the compiler adds various constraints to how a struct is layed out:

  1. The struct's alignment follows that of its widest scalar member.
    • This guarantees each member satisfies its own self-alignment requirement.
  2. The struct introduces trailing padding up to its stride address.
    1. This ensures each element in an array satsifies its self-alignment requirement.

The sizeof operator on a struct returns the total space used between its leading address and its stride address.

%%ANKI Basic What addresses can a char be stored at? Back: Any address. Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. http://www.catb.org/esr/structure-packing/.

END%%

%%ANKI Basic What addresses can an int be stored at? Back: Any address divisible by 4. Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. http://www.catb.org/esr/structure-packing/.

END%%

%%ANKI Basic What addresses can a short be stored at? Back: Any address divisible by 2. Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. http://www.catb.org/esr/structure-packing/.

END%%

%%ANKI Basic What addresses can a double be stored at (outside of a struct)? Back: Any address divisible by 8. Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. http://www.catb.org/esr/structure-packing/.

END%%

%%ANKI Basic What addresses can a double be stored at (inside of a struct)? Back: Depending on compiler, addresses divisible by 4 or 8. Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. http://www.catb.org/esr/structure-packing/.

END%%

%%ANKI Basic What addresses can a pointer be stored at? Back: Any address divisible by 8. Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. http://www.catb.org/esr/structure-packing/.

END%%

%%ANKI Basic How does self-alignment make access faster? Back: It enables single instruction fetches and puts. Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. http://www.catb.org/esr/structure-packing/.

END%%

%%ANKI Basic What value is slop initialized to? Back: Undefined. Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. http://www.catb.org/esr/structure-packing/.

END%%

%%ANKI Basic Why isn't equality for structs well-defined? Back: The value of slop is undefined. Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. http://www.catb.org/esr/structure-packing/.

END%%

%%ANKI Basic Why isn't inequality for structs well-defined? Back: The value of slop is undefined. Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. http://www.catb.org/esr/structure-packing/.

END%%

%%ANKI Basic Assume allocated order matches source order. How are the following variables aligned?

char *p;
char c;
short x;

Back:

char *p;     // 8 bytes
char c;      // 1 byte
char pad[1]; // 1 byte
short x;     // 2 bytes

Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. http://www.catb.org/esr/structure-packing/.

END%%

%%ANKI Basic Assume allocated order matches source order. How are the following variables aligned?

long x;
short c;
float f;

Back:

long x;      // 8 bytes
short c;     // 2 bytes
char pad[2]; // 2 bytes
float f;     // 4 bytes

Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. http://www.catb.org/esr/structure-packing/.

END%%

%%ANKI Basic Assume allocated order matches source order. What is the value of N?

char c;
char pad1[M];
char *p;
char pad2[N];
int x;

Back: 0 Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. http://www.catb.org/esr/structure-packing/.

END%%

%%ANKI Basic Assume allocated order matches source order. What is the value of M?

char c;
char pad1[M];
char *p;
char pad2[N];
int x;

Back: Between 0 and 7 inclusive. Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. http://www.catb.org/esr/structure-packing/.

END%%

%%ANKI Basic Consider short A[N]. What is the internal padding of this array? Back: 0, i.e. A has no internal padding. Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. http://www.catb.org/esr/structure-packing/.

END%%

%%ANKI Basic Consider char* A[N]. What is the internal padding of this array? Back: 0, i.e. A has no internal padding. Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. http://www.catb.org/esr/structure-packing/.

END%%

%%ANKI Basic What is the alignment of a struct instance? Back: That of its widest scalar member. Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. http://www.catb.org/esr/structure-packing/.

END%%

%%ANKI Basic Why is the alignment of a struct instance that of its widest scalar member? Back: It is an easy way to ensure all members are self-aligned. Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. http://www.catb.org/esr/structure-packing/.

END%%

%%ANKI Basic Why does a struct sometimes introduce trailing padding? Back: To ensure each element of an array of these structs is self-aligned. 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 The address of a struct is the same as its {first member}. Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. http://www.catb.org/esr/structure-packing/.

END%%

%%ANKI Basic How much leading padding does a struct instance have? Back: 0 Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. http://www.catb.org/esr/structure-packing/.

END%%

%%ANKI Basic How are members of the following struct aligned?

struct foo {
  char *p;
  char c;
  long x;
};

Back:

struct foo {
  char *p;     // 8 bytes
  char c;      // 1 byte
  char pad[7]; // 7 bytes
  long x;      // 8 bytes
};

Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. http://www.catb.org/esr/structure-packing/.

END%%

%%ANKI Basic How are members of the following struct aligned?

struct foo {
  char c;
  char *p;
  long x;
};

Back:

struct foo {
  char c;      // 1 byte
  char pad[7]; // 7 bytes
  char *p;     // 8 bytes
  long x;      // 8 bytes
};

Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. http://www.catb.org/esr/structure-packing/.

END%%

%%ANKI Cloze A struct's {stride address} is {the first address following the struct's data with the same alignment as the struct}. Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. http://www.catb.org/esr/structure-packing/.

END%%

%%ANKI Basic What is the result of sizeof(struct foo)?

struct foo {
  char *p;
  char c;
};

Back: 16 Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. http://www.catb.org/esr/structure-packing/.

END%%

%%ANKI Basic What is the result of sizeof(struct foo)?

struct foo {
  short s;
  char c;
};

Back: 4 Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. http://www.catb.org/esr/structure-packing/.

END%%

%%ANKI Basic What "hidden" space is returned by sizeof on struct types? Back: Slop. Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. http://www.catb.org/esr/structure-packing/.

END%%

%%ANKI Basic How are members of the following struct aligned?

struct foo {
  char c;
  struct bar {
    char *p;
    short x;
  } bar;
};

Back:

struct foo {
  char c;         // 1 byte
  char pad[7];    // 7 bytes
  struct bar {
    char *p;      // 8 bytes
    short x;      // 2 bytes
    char pad2[6]; // 6 bytes
  } bar;
};

Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. http://www.catb.org/esr/structure-packing/.

END%%

%%ANKI Basic How are members of the following struct aligned?

struct foo {
  char c;
  struct bar {
    short x;
  } bar;
};

Back:

struct foo {
  char c;         // 1 byte
  char pad[1];    // 1 byte
  struct bar {
    short x;      // 2 bytes
  } bar;
};

Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. http://www.catb.org/esr/structure-packing/.

END%%

%%ANKI Basic What is the alignment of a nested struct? Back: That of its widest scalar member. Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. http://www.catb.org/esr/structure-packing/.

END%%

%%ANKI Basic The sizeof operator on structs returns the space used between what two addresses? Back: The struct's leading address and stride address. Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. http://www.catb.org/esr/structure-packing/.

END%%

%%ANKI Basic What is the offset of field u in the following?

struct foo {
  char u;
  int v;
};

Back: 0 Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What is the offset of field v in the following?

struct foo {
  char u;
  int v;
};

Back: 4 Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

Unions

The sizeof operator on a union returns that of its widest member.

%%ANKI Basic The sizeof operator on unions returns what? Back: The sizeof that of its widest member. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What is the offset of field u in the following?

union foo {
  char u;
  int v;
};

Back: 0 Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What is the offset of field v in the following?

union foo {
  char u;
  int v;
};

Back: 0 Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

Bit-Fields

A member of a structure or union may be declared to consist of a specified number of bits (including a sign bit, if any). Such a member is called a bit-field; its width is preceded by a colon. Its width must be an ICE with a nonnegative value that does not exceed the width of an object of the type that would be specified were the colon and expression omitted.

A bit-field shall have a type is a qualified or unqualified version of _Bool, signed int, unsigned int, or some other implementation-defined type.

%%ANKI Basic A bit-field can be declared as members of what? Back: structs or unions. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic How is a bit-field declared? Back: As a member of a struct or union with width preceded by a colon. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic What kind of expression must the width of a bit-field be? Back: An ICE. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic What non-implementation-defined types can a bit-field be declared with? Back: _Bool, signed int, or unsigned int. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic What types can a bit-field be declared with? Back: _Bool, signed int, unsigned int, or some other implementation-defined type. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic What is the sign of the following bit-field?

struct foo { int bar : 1; };

Back: N/A. This is implementation-defined. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic What is the sign of the following bit-field?

struct foo { signed bar : 1; };

Back: Signed. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic What is the sign of the following bit-field?

struct foo { unsigned bar : 1; };

Back: Unsigned. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic Which unary operator cannot be applied to a bit-field object? Back: & Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic What compilation error exists in the following translation unit?

#include <stdio.h>

struct foo { int a; int b : 4; };

int main(void) {
	struct foo bar = { .a = 1, .b = 1 };
	printf("%p\n", (void *)&bar.b);
}

Back: Cannot take the address-of a bit-field. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic What compilation error exists in the following translation unit?

#include <stdio.h>

struct foo { int a; int b : 4; };

int main(void) {
	struct foo bar = { .a = 1, .b = 1 };
	printf("%p\n", (void *)&bar.a);
}

Back: N/A. This compiles correctly. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic What is an unnamed bit-field? Back: A bit-field with no declarator. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

Packing

An implementation may allocate any addressable storage unit large enough to hold a bit-field. If enough space remains, a bit-field that immediately follows another in a structure shall be packed into adjacent bits of the same unit.

If insufficient space remains, whether a bit-field that does not fit is put into the next unit or overlaps adjacent units is implementation-defined.

A bit-field structure member with a width of 0 indicates that no further bit-field is to be packed into the unit in which the previous bit-field, if any, was placed.

%%ANKI Basic What are the addressable storage units supported in x86-64? Back: A byte, word, double word, or quad word. 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

END%%

%%ANKI Basic What does the C standard mean by an "addressable storage unit"? Back: A memory unit that can be directly addressed/manipulated by the processor. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic Does x86-64 allow bit-fields to overlap adjacent addressable storage units? Back: No. Reference: Raymond, Eric. “The Lost Art of Structure Packing.” Accessed November 4, 2024. http://www.catb.org/esr/structure-packing/. Tags: x86-64

END%%

%%ANKI Basic What does a bit-field of width 0 indicate? Back: No subsequent bit-field can be packed into the unit in which the previous bit-field, if any, was placed. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic What is wrong with the following struct definition?

struct foo { unsigned bar : 0; };

Back: A bit-field of width 0 cannot have a declarator. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic How is the following struct correctly written?

struct foo { unsigned bar : 0; };

Back:

struct foo { unsigned : 0; };

Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic Assume a 4-byte unsigned int. What is wrong with the following struct definition?

struct foo { unsigned bar : 31; };

Back: N/A. This is correct. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf. Tags: x86-64

END%%

%%ANKI Basic Assume a 4-byte unsigned int. What is wrong with the following struct definition?

struct foo { unsigned bar : 32; };

Back: N/A. This is correct. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf. Tags: x86-64

END%%

%%ANKI Basic Assume a 4-byte unsigned int. What is wrong with the following struct definition?

struct foo { unsigned bar : 33; };

Back: The width of a bit-field cannot exceed its types (in this case unsigned int). Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf. Tags: x86-64

END%%

%%ANKI Basic Assume a byte-sized storage unit, no overlapping units, and low-to-high order. How is the following packed in memory?

struct foo {
  signed a : 4;
  signed b : 2;
  signed c : 4;
};

Back:

struct foo {
  _padding : 2;  // 2 bits
  signed b : 2;  // 2 bits
  signed a : 4;  // 4 bits
  _padding : 4;  // 4 bits
  signed c : 4;  // 4 bits
};

Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic Assume a byte-sized storage unit, overlapping units, and low-to-high order. How is the following packed in memory?

struct foo {
  signed a : 4;
  signed   : 0;
  signed c : 4;
};

Back:

struct foo {
  _padding : 4;  // 4 bits
  signed a : 4;  // 4 bits
  _padding : 4;  // 4 bits
  signed c : 4;  // 4 bits
};

Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic Assume a byte-sized storage unit, overlapping units, and high-to-low order. How is the following packed in memory?

struct foo {
  signed a : 4;
  signed   : 0;
  signed c : 4;
};

Back:

struct foo {
  signed a : 4;  // 4 bits
  _padding : 4;  // 4 bits
  signed c : 4;  // 4 bits
  _padding : 4;  // 4 bits
};

Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic Assume a 32-bit storage unit, no overlapping units, and low-to-high order. How is the following packed in memory?

struct foo {
  signed a : 4;
  signed b : 2;
  signed c : 4;
};

Back:

struct foo {
  _padding : 22; // 22 bits
  signed c : 4;  // 4 bits
  signed b : 2;  // 2 bits
  signed a : 4;  // 4 bits
};

Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic Assume a byte-sized storage unit, overlapping units, and low-to-high order. How is the following packed in memory?

struct foo {
  signed a : 4;
  signed b : 2;
  signed c : 4;
};

Back:

struct foo {
  _padding : 6;  // 6 bits
  signed c : 4;  // 4 bits
  signed b : 2;  // 2 bits
  signed a : 4;  // 4 bits
};

Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic Assume a 32-bit storage unit, no overlapping units, and low-to-high order. How is the following packed in memory?

struct foo {
  signed a : 4;
  signed   : 0;
  signed c : 4;
};

Back:

struct foo {
  _padding : 28; // 28 bits
  signed c : 4;  // 4 bits
  _padding : 28; // 28 bits
  signed a : 4;  // 4 bits
};

Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic Assume a byte-sized storage unit, overlapping units, and high-to-low order. How is the following packed in memory?

struct foo {
  signed a : 4;
  signed b : 2;
  signed c : 4;
};

Back:

struct foo {
  signed a : 4;  // 4 bits
  signed b : 2;  // 2 bits
  signed c : 4;  // 4 bits
  _padding : 6;  // 6 bits
};

Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic Assume a byte-sized storage unit, no overlapping units, and high-to-low order. How is the following packed in memory?

struct foo {
  signed a : 4;
  signed b : 2;
  signed c : 4;
};

Back:

struct foo {
  signed a : 4;  // 4 bits
  signed b : 2;  // 2 bits
  _padding : 2;  // 2 bits
  signed c : 4;  // 4 bits
  _padding : 4;  // 4 bits
};

Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic Assume a 32-bit storage unit, overlapping units, and high-to-low order. How is the following packed in memory?

struct foo {
  signed a : 4;
  signed b : 2;
  signed c : 4;
};

Back:

struct foo {
  signed a : 4;  // 4 bits
  signed b : 2;  // 2 bits
  signed c : 4;  // 4 bits
  _padding : 22; // 22 bits
};

Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic What implementation-defined property guarantees tightly-packed bit-fields? Back: Whether bit-fields can overlap adjacent addressable storage units. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Cloze An implementation may allocate any {addressable storage} unit large enough to hold a bit-field. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic What does it mean for bit-fields to be allocated low-to-high? Back: They are packed starting from the least significant bit of the addressable storage unit. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic What does it mean for bit-fields to be allocated high-to-low? Back: They are packed starting from the most significant bit of the addressable storage unit. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

Bibliography