Note

IMPORTANT All MISRA C rules, text, and examples are copyrighted by the MISRA Consortium Limited and used with permission.

Please refer to https://www.misra.org.uk/ to obtain a copy of MISRA C, or for licensing options for other use of the rules.

The following is the list of MISRA C rules that apply to the Xen hypervisor.

It is possible that in specific circumstances it is best not to follow a rule because it is not possible or because the alternative leads to better code quality. Those cases are called "deviations". They are permissible as long as they are documented. For details, please refer to docs/misra/documenting-violations.rst and docs/misra/deviations.rst

Other documentation mechanisms are work-in-progress.

The existing codebase is not 100% compliant with the rules. Some of the violations are meant to be documented as deviations, while some others should be fixed. Both compliance and documenting deviations on the existing codebase are work-in-progress.

The list below might need to be updated over time. Reach out to THE REST maintainers if you want to suggest a change.

Dir number | Severity | Summary | Notes
--- | --- | --- | ---
Dir 1.1 | Required | Any implementation-defined behaviour on which the output of the program depends shall be documented and understood |
Dir 2.1 | Required | All source files shall compile without any compilation errors |
Dir 4.1 | Required | Run-time failures shall be minimized | The strategies adopted by Xen to prevent certain classes of runtime failures is documented by C-runtime-failures.rst
Dir 4.7 | Required | If a function returns error information then that error information shall be tested |
Dir 4.10 | Required | Precautions shall be taken in order to prevent the contents of a header file being included more than once | Files that are intended to be included more than once do not need to conform to the directive
Dir 4.11 | Required | The validity of values passed to library functions shall be checked | We do not have libraries in Xen (libfdt and others are not considered libraries from MISRA C point of view as they are imported in source form)
Dir 4.14 | Required | The validity of values received from external sources shall be checked |

Rule number | Severity | Summary | Notes
--- | --- | --- | ---
Rule 1.1 | Required | The program shall contain no violations of the standard C syntax and constraints, and shall not exceed the implementation's translation limits | We make use of several compiler extensions as documented by C-language-toolchain.rst
Rule 1.3 | Required | There shall be no occurrence of undefined or critical unspecified behaviour |
Rule 1.4 | Required | Emergent language features shall not be used | Emergent language features, such as C11 features, should not be confused with similar compiler extensions, which we use. When the time comes to adopt C11, this rule will be revisited.

Rule 2.1

Required

A project shall not contain unreachable code

The following are allowed:
  - Invariantly constant conditions, e.g. if(IS_ENABLED(CONFIG_HVM)) { S; }
  - Switch with a controlling value statically determined not to match one or more case statements
  - Functions that are intended to be referenced only from assembly code (e.g. 'do_trap_fiq')
  - asm-offsets.c, as they are not linked deliberately, because they are used to generate definitions for asm modules
  - Declarations without initializer are safe, as they are not executed

Rule 2.6 | Advisory | A function should not contain unused label declarations |
Rule 3.1 | Required | The character sequences /* and // shall not be used within a comment | Comments containing hyperlinks inside C-style block comments are safe
Rule 3.2 | Required | Line-splicing shall not be used in // comments |
Rule 4.1 | Required | Octal and hexadecimal escape sequences shall be terminated |
Rule 4.2 | Advisory | Trigraphs should not be used |
Rule 5.1 | Required | External identifiers shall be distinct | The Xen characters limit for identifiers is 63. Public headers (xen/include/public/) are allowed to retain longer identifiers for backward compatibility.
Rule 5.2 | Required | Identifiers declared in the same scope and name space shall be distinct | The Xen characters limit for identifiers is 63. Public headers (xen/include/public/) are allowed to retain longer identifiers for backward compatibility.
Rule 5.3 | Required | An identifier declared in an inner scope shall not hide an identifier declared in an outer scope | Using macros as macro parameters at invocation time is allowed even if both macros use identically named local variables, e.g. max(var0, min(var1, var2))
Rule 5.4 | Required | Macro identifiers shall be distinct | The Xen characters limit for macro identifiers is 63. Public headers (xen/include/public/) are allowed to retain longer identifiers for backward compatibility.

Rule 5.5

Required

Identifiers shall be distinct from macro names

Macros expanding to their own name are allowed, e.g.:

    #define x x

Clashes between names of function-like macros and identifiers of non-callable entities are allowed. Callable entities having an identifier that is the same of the name of a function-like macro are not allowed. Example (not allowed):

    #define f(x, y) f(x, y)
    void f(int x, int y);

Rule 5.6 | Required | A typedef name shall be a unique identifier |
Rule 6.1 | Required | Bit-fields shall only be declared with an appropriate type | In addition to the C99 types, we also consider appropriate types enum and all explicitly signed / unsigned integer types.
Rule 6.2 | Required | Single-bit named bit fields shall not be of a signed type |
Rule 7.1 | Required | Octal constants shall not be used |

Rule 7.2

Required

A "u" or "U" suffix shall be applied to all integer constants that are represented in an unsigned type

The rule asks that any integer literal that is implicitly unsigned is made explicitly unsigned by using one of the indicated suffixes. As an example, on a machine where the int type is 32-bit wide, 0x77777777 is signed whereas 0x80000000 is (implicitly) unsigned. In order to comply with the rule, the latter should be rewritten as either 0x80000000u or 0x80000000U. Consistency considerations may suggest using the same suffix even when not required by the rule. For instance, if one has:

Original: f(0x77777777); f(0x80000000);

one should do

Solution 1: f(0x77777777U); f(0x80000000U);

over

Solution 2: f(0x77777777); f(0x80000000U);

after having ascertained that "Solution 1" is compatible with the intended semantics.

Rule 7.3 | Required | The lowercase character l shall not be used in a literal suffix |
Rule 7.4 | Required | A string literal shall not be assigned to an object unless the object type is pointer to const-qualified char | All "character types" are permitted, as long as the string element type and the character type match. (There should be no casts.) Assigning a string literal to any object with type "pointer to const-qualified void" is allowed.
Rule 8.1 | Required | Types shall be explicitly specified |
Rule 8.2 | Required | Function types shall be in prototype form with named parameters | Clarification: both function and function pointers types shall have named parameters.
Rule 8.3 | Required | All declarations of an object or function shall use the same names and type qualifiers | The type ret_t maybe be deliberately used and defined as int or long depending on the type of guest to service
Rule 8.4 | Required | A compatible declaration shall be visible when an object or function with external linkage is defined | Allowed exceptions: asm-offsets.c, definitions for asm modules not called from C code, gcov_base.c
Rule 8.5 | Required | An external object or function shall be declared once in one and only one file |
Rule 8.6 | Required | An identifier with external linkage shall have exactly one external definition | Declarations without definitions are allowed (specifically when the definition is compiled-out or optimized-out by the compiler)
Rule 8.8 | Required | The static storage class specifier shall be used in all declarations of objects and functions that have internal linkage |
Rule 8.10 | Required | An inline function shall be declared with the static storage class | gnu_inline (without static) is allowed.
Rule 8.12 | Required | Within an enumerator list the value of an implicitly-specified enumeration constant shall be unique |
Rule 8.14 | Required | The restrict type qualifier shall not be used |
Rule 9.1 | Mandatory | The value of an object with automatic storage duration shall not be read before it has been set | Rule clarification: do not use variables before they are initialized. An explicit initializer is not necessarily required. Try reducing the scope of the variable. If an explicit initializer is added, consider initializing the variable to a poison value.
Rule 9.2 | Required | The initializer for an aggregate or union shall be enclosed in braces |
Rule 9.3 | Required | Arrays shall not be partially initialized | {} is also allowed to specify explicit zero-initialization
Rule 9.4 | Required | An element of an object shall not be initialized more than once |

Rule 10.1

Required

Operands shall not be of an inappropriate essential type

The following are allowed:
  - Value-preserving conversions of integer constants
  - Bitwise and, or, xor, one's complement, bitwise and assignment, bitwise or assignment, bitwise xor assignment (bitwise and, or, xor are safe on non-negative integers; also Xen assumes two's complement representation)
  - Left shift, right shift, left shift assignment, right shift assignment (see C-language-toolchain.rst for uses of compilers' extensions)
  - Implicit conversions to boolean for conditionals (?: if while for) and logical operators (! || &&)
  - The essential type model allows the constants defined by anonymous enums (e.g., enum { A, B, C }) to be used as operands to arithmetic operators, as they have a signed essential type.

Rule 10.2 | Required | Expressions of essentially character type shall not be used inappropriately in addition and subtraction operations |

Rule 10.3

Required

The value of an expression shall not be assigned to an object with a narrower essential type or of a different essential type category

Please beware that this rule has many violations in the Xen codebase today, and its adoption is aspirational. However, when submitting new patches please try to decrease the number of violations when possible.

gcc has a helpful warning that can help you spot and remove violations of this kind: conversion. For instance, you can use it as follows:

CFLAGS="-Wconversion -Wno-error=sign-conversion -Wno-error=conversion" make -C xen

Rule 10.4

Required

Both operands of an operator in which the usual arithmetic conversions are performed shall have the same essential type category

Please beware that this rule has many violations in the Xen codebase today, and its adoption is aspirational. However, when submitting new patches please try to decrease the number of violations when possible.

gcc has a helpful warning that can help you spot and remove violations of this kind: arith-conversion. For instance, you can use it as follows:

CFLAGS="-Warith-conversion -Wno-error=arith-conversion" make -C xen

Rule 11.1 | Required | Conversions shall not be performed between a pointer to a function and any other type | All conversions to integer types are permitted if the destination type has enough bits to hold the entire value. Conversions to bool and void* are permitted.
Rule 11.2 | Required | Conversions shall not be performed between a pointer to an incomplete type and any other type | All conversions to integer types are permitted if the destination type has enough bits to hold the entire value. Conversions to bool and void* are permitted.
Rule 11.3 | Required | A cast shall not be performed between a pointer to object type and a pointer to a different object type |
Rule 11.6 | Required | A cast shall not be performed between pointer to void and an arithmetic type | All conversions to integer types are permitted if the destination type has enough bits to hold the entire value. Conversions to bool are permitted.
Rule 11.7 | Required | A cast shall not be performed between pointer to object and a noninteger arithmetic type |
Rule 11.8 | Required | A cast shall not remove any const or volatile qualification from the type pointed to by a pointer |
Rule 11.9 | Required | The macro NULL shall be the only permitted form of null pointer constant |
Rule 12.2 | Required | The right hand operand of a shift operator shall lie in the range zero to one less than the width in bits of the essential type of the left hand operand | We rely on gcc -fsanitize=undefined to check for dangerious violations to this rule and to ensure compliance
Rule 12.5 | Mandatory | The sizeof operator shall not have an operand which is a function parameter declared as "array of type" |
Rule 13.1 | Required | Initializer lists shall not contain persistent side effects |

Rule 13.2

Required

The value of an expression and its persistent side-effects shall be the same under all permitted evaluation orders

Be aware that the static analysis tool Eclair might report several findings for Rule 13.2 of type "caution". These are instances where Eclair is unable to verify that the code is valid in regard to Rule 13.2. Caution reports might not be violations. The rule should be followed in any case in new code submitted.

Rule 13.6 | Required | The operand of the sizeof operator shall not contain any expression which has potential side effects | In addition to sizeof, we also want to apply the rule to typeof and alignof
Rule 14.1 | Required | A loop counter shall not have essentially floating type |

Rule 14.3

Required

Controlling expressions shall not be invariant

Due to the extensive usage of IS_ENABLED, sizeof compile-time checks, and other constructs that are detected as errors by MISRA C scanners, managing the configuration of a MISRA C scanner for this rule would be unmanageable. Thus, this rule is adopted with a project-wide deviation on if, ?:, switch(sizeof(...)), and switch(offsetof(...)) statements.

while(0) and while(1) and alike are allowed.

Rule 14.4 | Required | The controlling expression of an if-statement and the controlling expression of an iteration-statement shall have essentially Boolean type | Automatic conversions of integer types to bool are permitted. Automatic conversions of pointer types to bool are permitted. This rule still applies to enum types.
Rule 16.3 | Required | An unconditional break statement shall terminate every switch-clause | In addition to break, also other unconditional flow control statements such as continue, return, goto are allowed.

Rule 16.4

Required

Every switch statement shall have a default label

Switch statements with enums as controlling expression don't need a default label as gcc -Wall enables -Wswitch which warns (and breaks the build as we use -Werror) if one of the enum labels is missing from the switch.

Switch statements with integer types as controlling expression should have a default label:

- if the switch is expected to handle all possible cases explicitly, then a default label shall be added to handle unexpected error conditions, using BUG(), ASSERT(), WARN(), domain_crash(), or other appropriate methods;

- if the switch is expected to handle a subset of all possible cases, then an empty default label shall be added with an in-code comment on top of the default label with a reason and when possible a more detailed explanation. Example:

      default:
          /* Notifier pattern */
          break;

Rule 16.2 | Required | A switch label shall only be used when the most closely-enclosing compound statement is the body of a switch statement | The x86 emulator (xen/arch/x86/x86_emulate*) is exempt from compliance with this rule. Efforts to make the x86 emulator adhere to Rule 16.2 would result in increased complexity and maintenance difficulty, and could potentially introduce bugs.
Rule 16.6 | Required | Every switch statement shall have at least two switch-clauses | Single-clause switches are allowed when they do not involve a default label.
Rule 16.7 | Required | A switch-expression shall not have essentially Boolean type |
Rule 17.1 | Required | The features of <stdarg.h> shall not be used |
Rule 17.2 | Required | Functions shall not call themselves, either directly or indirectly | Limited forms of recursion are allowed if the recursion is bound (there is an upper limit and the upper limit is enforced.) The bounding should be explained in a comment or in a deviation.
Rule 17.3 | Mandatory | A function shall not be declared implicitly |
Rule 17.4 | Mandatory | All exit paths from a function with non-void return type shall have an explicit return statement with an expression |
Rule 17.5 | Advisory | The function argument corresponding to a parameter declared to have an array type shall have an appropriate number of elements |
Rule 17.6 | Mandatory | The declaration of an array parameter shall not contain the static keyword between the [ ] |
Rule 17.7 | Required | The value returned by a function having non-void return type shall be used | Please beware that this rule has many violations in the Xen codebase today, and its adoption is aspirational. However, when submitting new patches please try to decrease the number of violations when possible.
Rule 18.1 | Required | A pointer resulting from arithmetic on a pointer operand shall address an element of the same array as that pointer operand |

Rule 18.2

Required

Subtraction between pointers shall only be applied to pointers that address elements of the same array

Be aware that the static analysis tool Eclair might report several findings for Rule 18.2 of type "caution". These are instances where Eclair is unable to verify that the code is valid in regard to Rule 18.2. Caution reports might not be violations. The rule should be followed in any case in new code submitted.

Rule 18.3 | Required | The relational operators > >= < and <= shall not be applied to objects of pointer type except where they point into the same object |
Rule 18.8 | Required | Variable-length array types shall not be used |
Rule 18.6 | Required | The address of an object with automatic storage shall not be copied to another object that persists after the first object has ceased to exist |
Rule 19.1 | Mandatory | An object shall not be assigned or copied to an overlapping object | Be aware that the static analysis tool Eclair might report several findings for Rule 19.1 of type "caution". These are instances where Eclair is unable to verify that the code is valid in regard to Rule 19.1. Caution reports are not violations.
Rule 20.2 | Required | The ', " or characters and the /* or // character sequences shall not occur in a header file name |
Rule 20.3 | Required | The #include directive shall be followed by either a <filename> or "filename" sequence |
Rule 20.4 | Required | A macro shall not be defined with the same name as a keyword |
Rule 20.6 | Required | Tokens that look like a preprocessing directive shall not occur within a macro argument |

Rule 20.7

Required

Expressions resulting from the expansion of macro parameters shall be enclosed in parentheses

Extra parentheses are not required when macro parameters are used as function arguments, as macro arguments, array indices, lhs in assignments or as initializers in initalizer lists. In addition, the use of a named variable argument in a macro that would constitute a violation of the rule is allowed by ECLAIR as an extension of the MISRA guideline, since it may not always be possible to parenthesize such argument and the feature is non-standard:

    #define M(args...) args
    #if M(1) + 0

Rule 20.9 | Required | All identifiers used in the controlling expression of #if or #elif preprocessing directives shall be #define'd before evaluation |
Rule 20.11 | Required | A macro parameter immediately following a # operator shall not immediately be followed by a ## operator |
Rule 20.12 | Required | A macro parameter used as an operand to the # or ## operators, which is itself subject to further macro replacement, shall only be used as an operand to these operators | Variadic macros are allowed to violate the rule.
Rule 20.13 | Required | A line whose first token is # shall be a valid preprocessing directive |
Rule 20.14 | Required | All #else #elif and #endif preprocessor directives shall reside in the same file as the #if #ifdef or #ifndef directive to which they are related |

Rule 21.1

Required

#define and #undef shall not be used on a reserved identifier or reserved macro name

Identifiers starting with an underscore followed by another underscore or an upper-case letter are reserved. Today Xen uses many, such as header guards and bitwise manipulation functions. Upon analysis it turns out Xen identifiers do not clash with the identifiers used by modern GCC, but that is not a guarantee that there won't be a naming clash in the future or with another compiler. For these reasons we discourage the introduction of new reserved identifiers in Xen, and we see it as positive the reduction of reserved identifiers. At the same time, certain identifiers starting with two underscores are also commonly used in Linux (e.g. __set_bit) and we don't think it would be an improvement to rename them.

Rule 21.2 | Required | A reserved identifier or reserved macro name shall not be declared | See comment for Rule 21.1
Rule 21.3 | Required | The memory allocation and deallocation functions of <stdlib.h> shall not be used | Xen doesn't provide, use, or link against a Standard Library1
Rule 21.4 | Required | The standard header file <setjmp.h> shall not be used | Xen doesn't provide, use, or link against a Standard Library2
Rule 21.5 | Required | The standard header file <signal.h> shall not be used | Xen doesn't provide, use, or link against a Standard Library3
Rule 21.6 | Required | The Standard Library input/output routines shall not be used | Xen doesn't provide, use, or link against a Standard Library4
Rule 21.7 | Required | The Standard Library functions atof, atoi, atol and atoll of <stdlib.h> shall not be used | Xen doesn't provide, use, or link against a Standard Library5
Rule 21.8 | Required | The Standard Library functions abort, exit and system of <stdlib.h> shall not be used | Xen doesn't provide, use, or link against a Standard Library6
Rule 21.9 | Required | The library functions bsearch and qsort of <stdlib.h> shall not be used | Xen doesn't provide, use, or link against a Standard Library7
Rule 21.10 | Required | The Standard Library time and date routines shall not be used | Xen doesn't provide, use, or link against a Standard Library8
Rule 21.11 | Required | The standard header file <tgmath.h> shall not be used | Xen doesn't provide, use, or link against a Standard Library9
Rule 21.12 | Advisory | The exception handling features of <fenv.h> should not be used | Xen doesn't provide, use, or link against a Standard Library10
Rule 21.13 | Mandatory | Any value passed to a function in <ctype.h> shall be representable as an unsigned char or be the value EOF |
Rule 21.14 | Required | The Standard Library function memcmp shall not be used to compare null terminated strings |
Rule 21.15 | Required | The pointer arguments to the Standard Library functions memcpy, memmove and memcmp shall be pointers to qualified or unqualified versions of compatible types |
Rule 21.16 | Required | The pointer arguments to the Standard Library function memcmp shall point to either a pointer type, an essentially signed type, an essentially unsigned type, an essentially Boolean type or an essentially enum type | void* arguments are allowed
Rule 21.17 | Mandatory | Use of the string handling functions from <string.h> shall not result in accesses beyond the bounds of the objects referenced by their pointer parameters |
Rule 21.18 | Mandatory | The size_t argument passed to any function in <string.h> shall have an appropriate value |
Rule 21.19 | Mandatory | The pointers returned by the Standard Library functions localeconv, getenv, setlocale or, strerror shall only be used as if they have pointer to const-qualified type |
Rule 21.20 | Mandatory | The pointer returned by the Standard Library functions asctime ctime gmtime localtime localeconv getenv setlocale or strerror shall not be used following a subsequent call to the same function |
Rule 21.21 | Required | The Standard Library function system of <stdlib.h> shall not be used |
Rule 22.1 | Required | All resources obtained dynamically by means of Standard Library functions shall be explicitly released | Xen doesn't provide, use, or link against a Standard Library11
Rule 22.2 | Mandatory | A block of memory shall only be freed if it was allocated by means of a Standard Library function |
Rule 22.3 | Required | The same file shall not be open for read and write access at the same time on different streams | Xen doesn't provide, use, or link against a Standard Library12
Rule 22.4 | Mandatory | There shall be no attempt to write to a stream which has been opened as read-only |
Rule 22.5 | Mandatory | A pointer to a FILE object shall not be dereferenced |
Rule 22.6 | Mandatory | The value of a pointer to a FILE shall not be used after the associated stream has been closed |
Rule 22.7 | Required | The macro EOF shall only be compared with the unmodified return value from any Standard Library function capable of returning EOF | Xen doesn't provide, use, or link against a Standard Library13
Rule 22.8 | Required | The value of errno shall be set to zero prior to a call to an errno-setting-function | Xen doesn't provide, use, or link against a Standard Library14
Rule 22.9 | Required | The value of errno shall be tested against zero after calling an errno-setting-function | Xen doesn't provide, use, or link against a Standard Library15
Rule 22.10 | Required | The value of errno shall only be tested when the last function to be called was an errno-setting-function | Xen doesn't provide, use, or link against a Standard Library16

# 1 Terms & Definitions

A switch clause can be defined as: "the non-empty list of statements which follows a non-empty list of case/default labels". A formal definition is available within the amplification of MISRA C:2012 Rule 16.1.

Footnotes

1. Xen implements itself a few functions with names that match the corresponding function names of the Standard Library for developers' convenience. These functions are part of the Xen code and subject to analysis.↩︎

2. Xen implements itself a few functions with names that match the corresponding function names of the Standard Library for developers' convenience. These functions are part of the Xen code and subject to analysis.↩︎

3. Xen implements itself a few functions with names that match the corresponding function names of the Standard Library for developers' convenience. These functions are part of the Xen code and subject to analysis.↩︎

4. Xen implements itself a few functions with names that match the corresponding function names of the Standard Library for developers' convenience. These functions are part of the Xen code and subject to analysis.↩︎

5. Xen implements itself a few functions with names that match the corresponding function names of the Standard Library for developers' convenience. These functions are part of the Xen code and subject to analysis.↩︎

6. Xen implements itself a few functions with names that match the corresponding function names of the Standard Library for developers' convenience. These functions are part of the Xen code and subject to analysis.↩︎

7. Xen implements itself a few functions with names that match the corresponding function names of the Standard Library for developers' convenience. These functions are part of the Xen code and subject to analysis.↩︎

8. Xen implements itself a few functions with names that match the corresponding function names of the Standard Library for developers' convenience. These functions are part of the Xen code and subject to analysis.↩︎

9. Xen implements itself a few functions with names that match the corresponding function names of the Standard Library for developers' convenience. These functions are part of the Xen code and subject to analysis.↩︎

10. Xen implements itself a few functions with names that match the corresponding function names of the Standard Library for developers' convenience. These functions are part of the Xen code and subject to analysis.↩︎

11. Xen implements itself a few functions with names that match the corresponding function names of the Standard Library for developers' convenience. These functions are part of the Xen code and subject to analysis.↩︎

12. Xen implements itself a few functions with names that match the corresponding function names of the Standard Library for developers' convenience. These functions are part of the Xen code and subject to analysis.↩︎

13. Xen implements itself a few functions with names that match the corresponding function names of the Standard Library for developers' convenience. These functions are part of the Xen code and subject to analysis.↩︎

14. Xen implements itself a few functions with names that match the corresponding function names of the Standard Library for developers' convenience. These functions are part of the Xen code and subject to analysis.↩︎

15. Xen implements itself a few functions with names that match the corresponding function names of the Standard Library for developers' convenience. These functions are part of the Xen code and subject to analysis.↩︎

16. Xen implements itself a few functions with names that match the corresponding function names of the Standard Library for developers' convenience. These functions are part of the Xen code and subject to analysis.↩︎