Coding Style

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We strive to maintain a consistent coding style in the M5 source code to make the source more readable and maintainable. This necessarily involves compromise among the multiple developers who work on this code. We feel that we have been successful in finding such a compromise, as each of the primary M5 developers is annoyed by at least one of the rules below. We ask that you abide by these guidelines as well if you develop code that you would like to contribute back to M5. An Emacs c++-mode style embodying the indentation rules is available in the source tree at util/emacs/m5-c-style.el.

Indentation and Line Breaks

Indentation will be 4 spaces per level, though namespaces should not increase the indentation.

  • Exception: labels followed by colons (case and goto labels and public/private/protected modifiers) are indented two spaces from the enclosing context.

Indentation should use spaces only (no tabs), as tab widths are not always set consistently, and tabs make output harder to read when used with tools such as diff.

Lines must be a maximum of 79 characters long.


For control blocks (if, while, etc.), opening braces must be on the same line as the control keyword with a space between the closing parenthesis and the opening brace.

  • Exception: for multi-line expressions, the opening brace may be placed on a separate line to distinguish the control block from the statements inside the block.
if (...) {

// exception case
for (...;
     ...) // brace could be up here
{ // but this is optionally OK *only* when the 'for' spans multiple lines

'Else' keywords should follow the closing 'if' brace on the same line, as follows:

if (...) {
} else if (...) {
} else {

Blocks that consist of a single statement that fits on a single line may optionally omit the braces. Braces are still required if the single statement spans multiple lines, or if the block is part of an else/if chain where other blocks have braces.

// This is OK with or without braces
if (a > 0)

// In the following cases, braces are still required
if (a > 0) {

if (a > 0) {
} else {
    underflow = true;
    warn("underflow on a");

For function definitions or class declarations, the opening brace must be in the first column of the following line.

In function definitions, the return type should be on one line, followed by the function name, left-justified, on the next line. As mentioned above, the opening brace should also be on a separate line following the function name.

See examples below:


class ExampleClass

Functions should be preceded by a block comment describing the function.

Inline function declarations longer than one line should not be placed inside class declarations. Most functions longer than one line should not be inline anyway.


There should be:

  • one space between keywords (if, for, while, etc.) and opening parentheses
  • one space around binary operators (+, -, <, >, etc.) including assignment operators (=, +=, etc.)
  • no space around '=' when used in parameter/argument lists, either to bind default parameter values (in Python or C++) or to bind keyword arguments (in Python)
  • no space between function names and opening parentheses for arguments
  • no space immediately inside parentheses, except for very complex expressions. Complex expressions are preferentially broken into multiple simpler expressions using temporary variables.

For pointer and reference argument declarations, either of the following are acceptable:

FooType *fooPtr;
FooType &fooRef;


FooType* fooPtr;
FooType& fooRef;

However, style should be kept consistent within a file. If you are editing an existing file, please keep consistent with the existing code. If you are writing new code in a new file, feel free to choose the style of your preference.


Class and type names are mixed case, start with an uppercase letter, and do not contain underscores (e.g., ClassName). Exception: names that are acronyms should be all upper case (e.g., CPU). Class member names (method and variables, including const variables) are mixed case, start with a lowercase letter, and do not contain underscores (e.g., aMemberVariable). Class members that have accessor methods should have a leading underscore to indicate that the user should be using an accessor. The accessor functions themselves should have the same name as the variable without the leading underscore.

Local variables are lower case, with underscores separating words (e.g., local_variable).

C preprocessor symbols (constants and macros) should be all caps with underscores. However, these are deprecated, and should be replaced with const variables and inline functions, respectively, wherever possible.

class FooBarCPU
    static const int minLegalFoo = 100;  // consts are formatted just like other vars
    int _fooVariable;   // starts with '_' because it has public accessor functions
    int barVariable;    // no '_' since it's internal use only

    // short inline methods can go all on one line
    int fooVariable() const { return _fooVariable; }

    // longer inline methods should be formatted like regular functions,
    // but indented
    fooVariable(int new_value)
        assert(new_value >= minLegalFoo);
        _fooVariable = new_value;


Whenever possible favor C++ includes over C include. E.g. choose cstdio, not stdio.h.

The block of #includes at the top of the file should be organized. We keep several sorted groups. This makes it easy to find #include and to avoid duplicate #includes.

Always include Python.h first if you need that header. This is mandated by the integration guide. The next header file should be your main header file (e.g., for you'd include foo.hh first). Having this header first ensures that it is independent and can be included in other places without missing dependencies.

// Include Python.h first if you need it.
#include <Python.h>

// Include your main header file before any other non-Python headers (i.e., the one with the same name as your cc source file)
#include "main_header.hh"

// C includes in sorted order
#include <fcntl.h>
#include <sys/time.h>

// C++ includes
#include <cerrno>
#include <cstdio>
#include <string>
#include <vector>

// Shared headers living in include/. These are used both in the simulator and utilities such as the m5 tool.
#include <gem5/asm/generic/m5ops.h>

// M5 includes
#include "base/misc.hh"
#include "cpu/base.hh"
#include "params/BaseCPU.hh"
#include "sim/system.hh"

File structure and modularity

Source files (.cc files) should never contain extern declarations; instead, include the header file associated with the .cc file in which the object is defined. This header file should contain extern declarations for all objects exported from that .cc file. This header should also be included in the defining .cc file. The key here is that we have a single external declaration in the .hh file that the compiler will automatically check for consistency with the .cc file. (This isn't as important in C++ as it was in C, since linker name mangling will now catch these errors, but it's still a good idea.)

When sufficient (i.e., when declaring only pointers or references to a class), header files should use forward class declarations instead of including full header files.

Header files should never contain using namespace declarations at the top level. This forces all the names in that namespace into the global namespace of any source file including that header file, which basically completely defeats the point of using namespaces. It is OK to use using namespace declarations at the top level of a source (.cc) file since the effect is entirely local to that .cc file. It's also OK to use them in _impl.hh files, since for practical purposes these are source (not header) files despite their extension.

Documenting the code

Each file/class/member should be documented using doxygen style comments. The documentation style to use is presented here: Documentation Guidelines

M5 Status Messages

Fatal v. Panic

There are two error functions defined in src/base/misc.hh: panic() and fatal(). While these two functions have roughly similar effects (printing an error message and terminating the simulation process), they have distinct purposes and use cases. The distinction is documented in the comments in the header file, but is repeated here for convenience because people often get confused and use the wrong one.

  • panic() should be called when something happens that should never ever happen regardless of what the user does (i.e., an actual m5 bug). panic() calls abort() which can dump core or enter the debugger.
  • fatal() should be called when the simulation cannot continue due to some condition that is the user's fault (bad configuration, invalid arguments, etc.) and not a simulator bug. fatal() calls exit(1), i.e., a "normal" exit with an error code.

The reasoning behind these definitions is that there's no need to panic if it's just a silly user error; we only panic if m5 itself is broken. On the other hand, it's not hard for users to make errors that are fatal, that is, errors that are serious enough that the m5 process cannot continue.

Inform, Warn and Hack

The file src/base/misc.hh also houses 3 functions that alert the user to various conditions happening within the simulation: inform(), warn() and hack(). The purpose of these functions is strictly to provide simulation status to the user so none of these functions will stop the simulator from running.

  • inform() and inform_once() should be called for informative messages that users should know, but not worry about. inform_once() will only display the status message generated by the inform_once function the first time it is called.
  • warn() and warn_once() should be called when some functionality isn't necessarily implemented correctly, but it might work well enough. The idea behind a warn() is to inform the user that if they see some strange behavior shortly after a warn() the description might be a good place to go looking for an error.
  • hack() should be called when some functionality isn't implemented nearly as well as it could or should be but for expediency or history sake hasn't been fixed.
  • inform() Provides status messages and normal operating messages to the console for the user to see, without any connotations of incorrect behavior. For example it's used when secondary CPUs being executing code on ALPHA.