C Dialect and Translation Assumptions for Xen

This document specifies the C language dialect used by Xen and the assumptions Xen makes on the translation toolchain. It covers, in particular:

1. the used language extensions;

2. the translation limits that the translation toolchains must be able to accommodate;

3. the implementation-defined behaviors upon which Xen may depend.

All points are of course relevant for portability. In addition, programming in C is impossible without a detailed knowledge of the implementation-defined behaviors. For this reason, it is recommended that Xen developers have familiarity with this document and the documentation referenced therein.

This document needs maintenance and adaptation in the following circumstances:

• whenever the compiler is changed or updated;

• whenever the use of a certain language extension is added or removed;

• whenever code modifications cause exceeding the stated translation limits.

Applicable C Language Standard

Xen is written in C99 with extensions. The relevant ISO standard is

ISO/IEC 9899:1999/Cor 3:2007: Programming Languages - C, Technical Corrigendum 3. ISO/IEC, Geneva, Switzerland, 2007.

Reference Documentation

The following documents are referred to in the sequel:

GCC_MANUAL:

https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gcc.pdf

CPP_MANUAL:

https://gcc.gnu.org/onlinedocs/gcc-12.1.0/cpp.pdf

ARM64_ABI_MANUAL:

https://github.com/ARM-software/abi-aa/blob/60a8eb8c55e999d74dac5e368fc9d7e36e38dda4/aapcs64/aapcs64.rst

X86_64_ABI_MANUAL:

https://gitlab.com/x86-psABIs/x86-64-ABI/-/jobs/artifacts/master/raw/x86-64-ABI/abi.pdf?job=build

C Language Extensions

The following table lists the extensions currently used in Xen. The table columns are as follows:

Extension

a terse description of the extension;

Architectures

a set of Xen architectures making use of the extension;

References

when available, references to the documentation explaining the syntax and semantics of (each instance of) the extension.

Extension	Architectures	References
Non-standard tokens	ARM64, X86_64	_Static_assert: see Section “2.1 C Language” of GCC_MANUAL. asm, __asm__: see Sections “6.48 Alternate Keywords” and “6.47 How to Use Inline Assembly Language in C Code” of GCC_MANUAL. __volatile__: see Sections “6.48 Alternate Keywords” and “6.47.2.1 Volatile” of GCC_MANUAL. __const__: see Section “6.48 Alternate Keywords” of GCC_MANUAL. typeof, __typeof__: see Section “6.7 Referring to a Type with typeof” of GCC_MANUAL. __alignof__, __alignof: see Sections “6.48 Alternate Keywords” and “6.44 Determining the Alignment of Functions, Types or Variables” of GCC_MANUAL. __attribute__: see Section “6.39 Attribute Syntax” of GCC_MANUAL. __builtin_types_compatible_p: see Section “6.59 Other Built-in Functions Provided by GCC” of GCC_MANUAL. __builtin_va_arg: non-documented GCC extension. __builtin_offsetof: see Section “6.53 Support for offsetof” of GCC_MANUAL.
Empty initialization list	ARM64, X86_64	Non-documented GCC extension.
Arithmetic operator on pointer to void	ARM64, X86_64	See Section “6.24 Arithmetic on void- and Function-Pointers” of GCC_MANUAL.”
Statements and declarations in expressions	ARM64, X86_64	See Section “6.1 Statements and Declarations in Expressions” of GCC_MANUAL.
Structure or union definition with no members	ARM64, X86_64	See Section “6.19 Structures with No Members” of GCC_MANUAL.
Zero size array type	ARM64, X86_64	See Section “6.18 Arrays of Length Zero” of GCC_MANUAL.
Binary conditional expression	ARM64, X86_64	See Section “6.8 Conditionals with Omitted Operands” of GCC_MANUAL.
‘Case’ label with upper/lower values	ARM64, X86_64	See Section “6.30 Case Ranges” of GCC_MANUAL.
Unnamed field that is not a bit-field	ARM64, X86_64	See Section “6.63 Unnamed Structure and Union Fields” of GCC_MANUAL.
Empty declaration	ARM64, X86_64	Non-documented GCC extension. Note: an empty declaration is caused by a semicolon at file scope with nothing before it (not to be confused with an empty statement).
Incomplete enum declaration	ARM64	See Section “6.49 Incomplete enum Types” of GCC_MANUAL.
Implicit conversion from a pointer to an incompatible pointer	ARM64, X86_64	Non-documented GCC extension. The documentation for option -Wincompatible-pointer-types in Section “3.8 Options to Request or Suppress Warnings” of GCC_MANUAL is possibly relevant.
Pointer to a function is converted to a pointer to an object or a pointer to an object is converted to a pointer to a function	X86_64	Non-documented GCC extension. The information provided in https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83584 is possibly relevant.
Token pasting of ‘,’ and __VA_ARGS__	ARM64, X86_64	See Section “6.21 Macros with a Variable Number of Arguments” of GCC_MANUAL.
Named variadic macro arguments	ARM64, X86_64	See Section “6.21 Macros with a Variable Number of Arguments” of GCC_MANUAL.
No arguments for ‘…’ parameter of variadic macro	ARM64, X86_64	See Section “6.21 Macros with a Variable Number of Arguments” of GCC_MANUAL.
void function returning void expression	ARM64, X86_64	See the documentation for -Wreturn-type in Section “3.8 Options to Request or Suppress Warnings” of GCC_MANUAL.
GNU statement expressions from macro expansion	ARM64, X86_64	See Section “6.1 Statements and Declarations in Expressions” of GCC_MANUAL.
Invalid application of sizeof to a void type	ARM64, X86_64	See Section “6.24 Arithmetic on void- and Function-Pointers” of GCC_MANUAL.
Redeclaration of already-defined enum	ARM64, X86_64	See Section “6.49 Incomplete enum Types” of GCC_MANUAL.
struct with flexible array member nested in a struct	ARM64, X86_64	See Section “6.18 Arrays of Length Zero” of GCC_MANUAL.
struct with flexible array member used as an array element	ARM64, X86_64	See Section “6.18 Arrays of Length Zero” of GCC_MANUAL.
enumerator value outside the range of int	ARM64, X86_64	Non-documented GCC extension.
Extended integer types	X86_64	See Section “6.9 128-bit Integers” of GCC_MANUAL.
Designated initializer for a range of elements	ARM64, X86_64	See Section “6.29 Designated Initializers” of GCC_MANUAL
Signed << compiler-defined behavior	All architectures	See Section “4.5 Integers” of GCC_MANUAL. As an extension to the C language, GCC does not use the latitude given in C99 and C11 only to treat certain aspects of signed << as undefined.
Signed >> acts on negative numbers by sign extension	All architectures	See Section “4.5 Integers” of GCC_MANUAL.

Translation Limits

The following table lists the translation limits that a toolchain has to satisfy in order to translate Xen. The numbers given are a compromise: on the one hand, many modern compilers have very generous limits (in several cases, the only limitation is the amount of available memory); on the other hand we prefer setting limits that are not too high, because compilers do not have any obligation of diagnosing when a limit has been exceeded, and not too low, so as to avoid frequently updating this document. In the table, only the limits that go beyond the minima specified by the relevant C Standard are listed.

The table columns are as follows:

Limit

a terse description of the translation limit;

Architectures

a set relevant of Xen architectures;

Threshold

a value that the Xen project does not wish to exceed for that limit (this is typically below, often much below what the translation toolchain supports);

References

when available, references to the documentation providing evidence that the translation toolchain honors the threshold (and more).

Limit	Architectures	Threshold	References
Size of an object	ARM64, X86_64	8388608	The maximum size of an object is defined in the MAX_SIZE macro, and for a 32 bit architecture is 8MB. The maximum size for an array is defined in the PTRDIFF_MAX and in a 32 bit architecture is 2^30-1. See occurrences of these macros in GCC_MANUAL.
Characters in one logical source line	ARM64	5000	See Section “11.2 Implementation limits” of CPP_MANUAL.
Characters in one logical source line	X86_64	12000	See Section “11.2 Implementation limits” of CPP_MANUAL.
Nesting levels for #include files	ARM64	24	See Section “11.2 Implementation limits” of CPP_MANUAL.
Nesting levels for #include files	X86_64	32	See Section “11.2 Implementation limits” of CPP_MANUAL.
case labels for a switch statement (excluding those for any nested switch statements)	X86_64	1500	See Section “4.12 Statements” of GCC_MANUAL.
Number of significant initial characters in an external identifier	ARM64, X86_64	63	See Section “4.3 Identifiers” of GCC_MANUAL.

Implementation-Defined Behaviors

The following table lists the C language implementation-defined behaviors relevant for MISRA C:2012 Dir 1.1 upon which Xen may possibly depend.

The table columns are as follows:

I.-D.B.

a terse description of the implementation-defined behavior;

Architectures

a set relevant of Xen architectures;

Value(s)

for i.-d.b.’s with values, the values allowed;

References

when available, references to the documentation providing details about how the i.-d.b. is resolved by the translation toolchain.

I.-D.B.	Architectures	Value(s)	References
Allowable bit-field types other than _Bool, signed int, and unsigned int	ARM64, X86_64	All explicitly signed integer types, all unsigned integer types, and enumerations.	See Section “4.9 Structures, Unions, Enumerations, and Bit-Fields”.
#pragma preprocessing directive that is documented as causing translation failure or some other form of undefined behavior is encountered	ARM64, X86_64	pack, GCC visibility	#pragma pack: see Section “6.62.11 Structure-Layout Pragmas” of GCC_MANUAL. #pragma GCC visibility: see Section “6.62.14 Visibility Pragmas” of GCC_MANUAL.
The number of bits in a byte	ARM64	8	See Section “4.4 Characters” of GCC_MANUAL and Section “8.1 Data types” of ARM64_ABI_MANUAL.
The number of bits in a byte	X86_64	8	See Section “4.4 Characters” of GCC_MANUAL and Section “3.1.2 Data Representation” of X86_64_ABI_MANUAL.
Whether signed integer types are represented using sign and magnitude, two’s complement, or one’s complement, and whether the extraordinary value is a trap representation or an ordinary value	ARM64, X86_64	Two’s complement	See Section “4.5 Integers” of GCC_MANUAL.
Any extended integer types that exist in the implementation	X86_64	__uint128_t	See Section “6.9 128-bit Integers” of GCC_MANUAL.
The number, order, and encoding of bytes in any object	ARM64		See Section “4.15 Architecture” of GCC_MANUAL and Chapter 5 “Data types and alignment” of ARM64_ABI_MANUAL.
The number, order, and encoding of bytes in any object	X86_64		See Section “4.15 Architecture” of GCC_MANUAL and Section “3.1.2 Data Representation” of X86_64_ABI_MANUAL.
Whether a bit-field can straddle a storage-unit boundary	ARM64		See Section “4.9 Structures, Unions, Enumerations, and Bit-Fields of GCC_MANUAL and Section “8.1.8 Bit-fields” of ARM64_ABI_MANUAL.
Whether a bit-field can straddle a storage-unit boundary	X86_64		See Section “4.9 Structures, Unions, Enumerations, and Bit-Fields” of GCC_MANUAL and Section “3.1.2 Data Representation” of X86_64_ABI_MANUAL.
The order of allocation of bit-fields within a unit	ARM64		See Section “4.9 Structures, Unions, Enumerations, and Bit-Fields of GCC_MANUAL and Section “8.1.8 Bit-fields” of ARM64_ABI_MANUAL.
The order of allocation of bit-fields within a unit	X86_64		See Section “4.9 Structures, Unions, Enumerations, and Bit-Fields” of GCC_MANUAL and Section “3.1.2 Data Representation” of X86_64_ABI_MANUAL.
What constitutes an access to an object that has volatile-qualified type	ARM64, X86_64		See Section “4.10 Qualifiers” of GCC_MANUAL.
The values or expressions assigned to the macros specified in the headers <float.h>, <limits.h>, and <stdint.h>	ARM64		See Section “4.15 Architecture” of GCC_MANUAL and Chapter 5 “Data types and alignment” of ARM64_ABI_MANUAL.
The values or expressions assigned to the macros specified in the headers <float.h>, <limits.h>, and <stdint.h>	X86_64		See Section “4.15 Architecture” of GCC_MANUAL and Section “3.1.2 Data Representation” of X86_64_ABI_MANUAL.
Character not in the basic source character set is encountered in a source file, except in an identifier, a character constant, a string literal, a header name, a comment, or a preprocessing token that is never converted to a token	ARM64	UTF-8	See Section “1.1 Character sets” of CPP_MANUAL. We assume the locale is not restricting any UTF-8 characters being part of the source character set.
The value of a char object into which has been stored any character other than a member of the basic execution character set	ARM64		See Section “4.4 Characters” of GCC_MANUAL and Section “8.1 Data types” of ARM64_ABI_MANUAL.
The value of a char object into which has been stored any character other than a member of the basic execution character set	X86_64		See Section “4.4 Characters” of GCC_MANUAL and Section “3.1.2 Data Representation” of X86_64_ABI_MANUAL.
The value of an integer character constant containing more than one character or containing a character or escape sequence that does not map to a single-byte execution character	ARM64		See Section “4.4 Characters” of GCC_MANUAL and Section “8.1 Data types” of ARM64_ABI_MANUAL.
The value of an integer character constant containing more than one character or containing a character or escape sequence that does not map to a single-byte execution character	X86_64		See Section “4.4 Characters” of GCC_MANUAL and Section “3.1.2 Data Representation” of X86_64_ABI_MANUAL.
The mapping of members of the source character set	ARM64, X86_64		See Section “4.4 Characters” of GCC_MANUAL and the documentation for -finput-charset=charset in the same manual.
The members of the source and execution character sets, except as explicitly specified in the Standard	ARM64, X86_64	UTF-8	See Section “4.4 Characters” of GCC_MANUAL
The values of the members of the execution character set	ARM64, X86_64		See Section “4.4 Characters” of GCC_MANUAL and the documentation for -fexec-charset=charset in the same manual.
How a diagnostic is identified	ARM64, X86_64		See Section “4.1 Translation” of GCC_MANUAL.
The places that are searched for an included < > delimited header, and how the places are specified or the header is identified	ARM64, X86_64		See Chapter “2 Header Files” of CPP_MANUAL.
How the named source file is searched for in an included “ “ delimited header	ARM64, X86_64		See Chapter “2 Header Files” of CPP_MANUAL.
How sequences in both forms of header names are mapped to headers or external source file names	ARM64, X86_64		See Chapter “2 Header Files” of CPP_MANUAL.
Whether the # operator inserts a character before the character that begins a universal character name in a character constant or string literal	ARM64, X86_64		See Section “3.4 Stringizing” of CPP_MANUAL.
The current locale used to convert a wide string literal into corresponding wide character codes	ARM64, X86_64		See Section “4.4 Characters” of GCC_MANUAL and Section “11.1 Implementation-defined behavior” of CPP_MANUAL.
The value of a string literal containing a multibyte character or escape sequence not represented in the execution character set	X86_64		See Section “4.4 Characters” of GCC_MANUAL and Section “11.1 Implementation-defined behavior” of CPP_MANUAL.
The behavior on each recognized #pragma directive	ARM64, X86_64	pack, GCC visibility	See Section “4.13 Preprocessing Directives” of GCC_MANUAL and Section “7 Pragmas” of CPP_MANUAL.
The method by which preprocessing tokens (possibly resulting from macro expansion) in a #include directive are combined into a header name	X86_64		See Section “4.13 Preprocessing Directives” of GCC_MANUAL and Section “11.1 Implementation-defined behavior” of CPP_MANUAL.

Sizes of Integer types

Xen expects System V ABI on x86_64:

https://gitlab.com/x86-psABIs/x86-64-ABI

Xen expects AAPCS32 on ARMv8-A AArch32 and ARMv7-A:

https://github.com/ARM-software/abi-aa/blob/main/aapcs32/aapcs32.rst

Xen expects AAPCS64 LP64 on ARMv8-A AArch64:

https://github.com/ARM-software/abi-aa/blob/main/aapcs64/aapcs64.rst

A summary table of data types, sizes and alignment is below:

Type	Size	Alignment	Architectures
char	8 bits	8 bits	x86_32, ARMv8-A AArch32, ARMv8-R AArch32, ARMv7-A, x86_64, ARMv8-A AArch64, RV64, PPC64
short	16 bits	16 bits	x86_32, ARMv8-A AArch32, ARMv8-R AArch32, ARMv7-A, x86_64, ARMv8-A AArch64, RV64, PPC64
int	32 bits	32 bits	x86_32, ARMv8-A AArch32, ARMv8-R AArch32, ARMv7-A, x86_64, ARMv8-A AArch64, RV64, PPC64
long	32 bits	32 bits	x86_32, ARMv8-A AArch32, ARMv8-R AArch32, ARMv7-A
long	64 bits	64 bits	x86_64, ARMv8-A AArch64, RV64, PPC64
long long	64-bit	32-bit	x86_32
long long	64-bit	64-bit	x86_64, ARMv8-A AArch64, RV64, PPC64, ARMv8-A AArch32, ARMv8-R AArch32, ARMv7-A
pointer	32-bit	32-bit	x86_32, ARMv8-A AArch32, ARMv8-R AArch32, ARMv7-A
pointer	64-bit	64-bit	x86_64, ARMv8-A AArch64, RV64, PPC64

END OF DOCUMENT.