C Coding Conventions and Style Guide

These are conventions which we attempt to follow in all of our code. They do not apply to external libraries, where we stick with the existing conventions.

Much of our code pre-dates the conventions, so it may not all comply. However, please try to ensure that any new code does follow the rules. In general, we strive to abide by Robert Baden-Powell’s rule for scouting: “Try and leave [the code] a little better than you found it.”

This guide is in three parts: first, we list general coding conventions; then we divide the rest of the conventions between kernel and user-level. Please read the appropriate guide, as kernel and user-level conventions sometimes contradict each other.

General C code

This guide applies to general C code at user-level. For developing the seL4 kernel and other verified code, please refer to the verification requirements below, which override the general conventions.

Compiler options

Code should compile without warnings, with -Wall.

Automatic formatting

For automatic formatting of C code, we use astyle, version 2.05.1, with the settings declared in our astylerc.

Spacing and braces

  • Indent using four (4) spaces. Do not use tabs for indentation at all.
  • Use no space between * and the variable name in pointer expressions (int *foo).
  • Use space between keywords and parentheses; e.g., if (condition).
  • Put the opening brace of a function implementation on the line after the function’s return type, name, and argument list.
    int atoi(const char *nptr)
        /* ... */
  • Use the “one true brace style” (1TBS); use braces everywhere the syntax allows (including single-statement scopes).
    if (x == FOO) {
    } else if (y == BAR) {
    } else {
  • When a function argument list gets too long for one line, indent the remaining arguments on the next line just inboard of the parenthesis on the line above. For example:
    void myfunc(my_ridiculously_long_type_t foo,
                my_ridiculously_long_type_t bar)
        /* ... */

Choosing data types

When using integral types (char, int, long, etc.), qualify them explicitly as unsigned except where negative values are meaningful and must be handled. (Overflow of signed integral types is undefined in the C standard.)

Naming of symbols (variables), types, and type aliases (typedef)

  • Use typedef to create aliases for all struct types.
  • The names of type aliases should always end in _t.
  • Function pointer type aliases should always end in _fn_t.
  • Do not alias pointer types with typedef; we keep them explicit.
    typedef tick_count unsigned int;
    typedef tick_count_ptr_t *tick_count; /* NO */
    tick_count *tick_count_ptr; /* yes */
  • Use snake_case to name a multi-word variable or type.
  • Non-static functions should be prefixed with appropriate names to avoid polluting the namespace.
    • The convention in library code is to use the name of the library.
    • A good guide for application code is to use the filename.
  • Name Boolean variables with a verb phrase including a verb like is_, has_, or want_ so that the intent of the variable is clear in expressions and conditionals.
  • Avoid magic numbers; define meaningful constants. Prefer C language symbols over preprocessor symbols, because the former are visible in a symbolic debugger like gdb.
  • Use SCREAMING_SNAKE_CASE for, and only for, preprocessor symbols and values of enumeration type.
  • Prefix architecture-specific code with arch_, platform-specific code with plat_, and mode-specific code with mode_.


  • Avoid pointer arithmetic unless necessary.
  • Always use preprocessor macros for bit manipulation (e.g., use BIT(7) instead of 1 << 7).


  • Put architecture- and platform-specific code in dedicated directories.
  • Only static inline functions are permitted in header files, and then only for performance.
  • Variables at file scope must be marked as static.
  • Use public/private header files; avoid extern unless necessary.
  • General-purpose utility functions belong in shared libraries, not library-specific files, and vice versa. (Consider checking libutils in the util_libs Git repository for existing functionality, and consider making useful additions there.)
  • Prefer static inline functions over preprocessor macros unless preprocessor features are required or the macro is simple.
  • Put a comment after the #endif of an #if, #ifdef, or #ifndef blocks that refers to the preprocessor symbol(s) upon which the code is guarded.
    #ifdef CONFIG_BLAH
    /* ... */
    #endif /* CONFIG_BLAH */

Memory allocation

For memory that should be zeroed, use calloc rather than malloc followed by memset or bzero.


  • Prefer /* this style */ comments over // this style.
  • Use Javadoc-style comments on function prototypes in headers.
  • Document function parameters and return types, at minimum.

User-level code

Type conventions

Be aware that our code needs to be portable across 32- and 64-bit platforms.

  • Use seL4_Word for word-sized things.
  • Use void * for untyped addresses.
  • Use uintptr_t for pointer arithmetic…
  • …but avoid pointer arithmetic unless necessary.
  • Use size_t for sizes of objects (including pointers themselves).
  • Use unsigned int for values known to be small.
  • Use fixed-width types only when necessary, e.g., in device drivers.
  • Use long variants of built-in functions (CLZL) for 64-bit compatibility, unless operating on a fixed-width type.
  • Use typedef to create aliases for all enum types.

Format strings

  • Use 64-bit-friendly printing macros for printf (e.g., PRIi32 for uint32_t).
  • Use %p for pointers.
  • Use %zd and %zu for ssize_t and size_t respectively.

Header guards

Use #pragma once in header files to avoid duplicated includes.

Error handling

  • Always check error codes.
  • Use assert only to test invariants in code.
  • Use ZF_LOGF for fatal errors: it will call abort().
  • For all other error reporting, use the appropriate ZF_LOG level.
  • Use only ZF_LOG macros for diagnostic messages–not printf.
  • Try to follow a transactional approach to error handling: check input and report errors before modifying state.

Kernel code

  • Use word_t for word-sized things.
  • Do not explicitly typecast between pointers and references (i.e., integers used as, e.g., offsets into structures): use object-specific macros for this purpose, such as TCB_REF and TCB_PTR.

Format strings

The in-kernel printf implementation is limited.

  • Use %p for pointers.
  • Use %lu for words, and cast to avoid warnings.

Header guards

Do not use #pragma once; use header guards.

Error handling

Within seL4, the decode stage must only check conditions; the invoke stage can alter state.

Verification requirements

Verified code (such as that in the seL4 kernel) must follow these requirements. Note that the verified code requirements override anything in the general guide.

  • Follow the C99 standard. (The link is to the final draft before ratification; the official standard document cannot be distributed freely.)
  • Avoid taking the address of a variable of automatic storage class. (In most C implementations, this means stack-allocated locals.)
  • Do not use floating-point types, e.g., double or float.
  • Do not use restrict.
  • union types cannot be used; use the bitfield generator instead.
  • Do not preincrement or predecrement variables (++x, --y).
  • Do not use variable names that duplicate typedef type aliases:
    typedef int A;
    A A;
  • Do not use fall-through cases in switch statements.
  • Do not use variadic argument lists.
  • Declare functions that take no arguments as taking a void argument.
  • References to linker addresses must be via extern char[] declarations rather than declarations of other types (such as extern char or extern void *).
  • Do not pass arrays as arguments to functions expecting pointers.
    void foo(int *some_pointer);
    int my_array[10];
  • Do not declare local variables as static.
  • Prefix struct fields with the name of the struct to avoid namespace conflicts in the proof. (Much existing kernel code does not follow this directive; new code should adopt the recommended practice to avoid namespace collisions in proofs.)
  • Do not use typedef to create a type alias for an enum; always specify enum types as word_t (otherwise, the enum size is determined by the compiler).

Further resources