4. C Coding Guidelines

Pacemaker is a large project accepting contributions from developers with a wide range of skill levels and organizational affiliations, and maintained by multiple people over long periods of time. Following consistent guidelines makes reading, writing, and reviewing code easier, and helps avoid common mistakes.

Some existing Pacemaker code does not follow these guidelines, for historical reasons and API backward compatibility, but new code should.

4.1. Code Organization

Pacemaker’s C code is organized as follows:

Directory Contents
daemons the Pacemaker daemons (pacemakerd, pacemaker-based, etc.)
include header files for library APIs
lib libraries
tools command-line tools

Source file names should be unique across the entire project, to allow for individual tracing via PCMK_trace_files.

4.2. Pacemaker Libraries

Library Symbol prefix Source location API Headers Description
libcib cib lib/cib
include/crm/cib.h
include/crm/cib/*

API for pacemaker-based IPC and the CIB

libcrmcluster pcmk lib/cluster
include/crm/cluster.h
include/crm/cluster/*

Abstract interface to underlying cluster layer

libcrmcommon pcmk lib/common
include/crm/common/*
some of include/crm/*

Everything else

libcrmservice svc lib/services
include/crm/services.h

Abstract interface to supported resource types (OCF, LSB, etc.)

liblrmd lrmd lib/lrmd
include/crm/lrmd*.h

API for pacemaker-execd IPC

libpacemaker pcmk lib/pacemaker
include/pacemaker*.h
include/pcmki/*

High-level APIs equivalent to command-line tool capabilities (and high-level internal APIs)

libpe_rules pe lib/pengine
include/crm/pengine/*

Scheduler functionality related to evaluating rules

libpe_status pe lib/pengine
include/crm/pengine/*

Low-level scheduler functionality

libstonithd stonith lib/fencing
include/crm/stonith-ng.h
include/crm/fencing/*

API for pacemaker-fenced IPC

4.2.1. Public versus Internal APIs

Pacemaker libraries have both internal and public APIs. Internal APIs are those used only within Pacemaker; public APIs are those offered (via header files and documentation) for external code to use.

Generic functionality needed by Pacemaker itself, such as string processing or XML processing, should remain internal, while functions providing useful high-level access to Pacemaker capabilities should be public. When in doubt, keep APIs internal, because it’s easier to expose a previously internal API than hide a previously public API.

Internal APIs can be changed as needed.

The public API/ABI should maintain a degree of stability so that external applications using it do not need to be rewritten or rebuilt frequently. Many OSes/distributions avoid breaking API/ABI compatibility within a major release, so if Pacemaker breaks compatibility, that significantly delays when OSes can package the new version. Therefore, changes to public APIs should be backward-compatible (as detailed throughout this chapter), unless we are doing a (rare) release where we specifically intend to break compatibility.

External applications known to use Pacemaker’s public C API include sbd and dlm_controld.

4.2.2. API Documentation

Pacemaker uses Doxygen to automatically generate its online API documentation, so all public API (header files, functions, structs, enums, etc.) should be documented with Doxygen comment blocks. Other code may be documented in the same way if desired, with an \internal tag in the Doxygen comment.

Simple example of an internal function with a Doxygen comment block:

/*!
 * \internal
 * \brief Return string length plus 1
 *
 * Return the number of characters in a given string, plus one.
 *
 * \param[in] s  A string (must not be NULL)
 *
 * \return The length of \p s plus 1.
 */
static int
f(const char *s)
{
   return strlen(s) + 1;
}

4.2.3. API Header File Naming

  • Internal API headers should be named ending in _internal.h, in the same location as public headers, with the exception of libpacemaker, which for historical reasons keeps internal headers in include/pcmki/pcmki_*.h).
  • If a library needs to share symbols just within the library, header files for these should be named ending in _private.h and located in the library source directory (not include). Such functions should be declared as G_GNUC_INTERNAL, to aid compiler efficiency (glib defines this symbol appropriately for the compiler).

Header files that are not library API are located in the same locations as other source code.

4.2.4. API Symbol Naming

Exposed API symbols (non-static function names, struct and typedef names in header files, etc.) must begin with the prefix appropriate to the library (shown in the table at the beginning of this section). This reduces the chance of naming collisions when external software links against the library.

The prefix is usually lowercase but may be all-caps for some defined constants and macros.

Public API symbols should follow the library prefix with a single underbar (for example, pcmk_something), and internal API symbols with a double underbar (for example, pcmk__other_thing).

File-local symbols (such as static functions) and non-library code do not require a prefix, though a unique prefix indicating an executable (controld, crm_mon, etc.) can be helpful to indicate symbols shared between multiple source files for the executable.

4.2.5. Public API Deprecation

Public APIs may not be removed in most Pacemaker releases, but they may be deprecated.

When a public API is deprecated, it is moved to a header whose name ends in compat.h. The original header includes the compatibility header only if the PCMK_ALLOW_DEPRECATED symbol is undefined or defined to 1. This allows external code to continue using the deprecated APIs, but internal code is prevented from using them because the crm_internal.h header defines the symbol to 0.

4.3. C Boilerplate

Every C file should start with a short copyright and license notice:

/*
 * Copyright <YYYY[-YYYY]> the Pacemaker project contributors
 *
 * The version control history for this file may have further details.
 *
 * This source code is licensed under <LICENSE> WITHOUT ANY WARRANTY.
 */

<LICENSE> should follow the policy set forth in the COPYING file, generally one of “GNU General Public License version 2 or later (GPLv2+)” or “GNU Lesser General Public License version 2.1 or later (LGPLv2.1+)”.

Header files should additionally protect against multiple inclusion by defining a unique symbol of the form PCMK__<capitalized_header_name>__H. For example:

#ifndef PCMK__MY_HEADER__H
#  define PCMK__MY_HEADER__H

// header code here

#endif // PCMK__MY_HEADER__H

Public API header files should additionally declare “C” compatibility for inclusion by C++, and give a Doxygen file description. For example:

#ifdef __cplusplus
extern "C" {
#endif

/*!
 * \file
 * \brief My brief description here
 * \ingroup core
 */

// header code here

#ifdef __cplusplus
}
#endif

4.4. Line Formatting

  • Indentation must be 4 spaces, no tabs.
  • Do not leave trailing whitespace.
  • Lines should be no longer than 80 characters unless limiting line length hurts readability.

4.5. Comments

/* Single-line comments may look like this */

// ... or this

/* Multi-line comments should start immediately after the comment opening.
 * Subsequent lines should start with an aligned asterisk. The comment
 * closing should be aligned and on a line by itself.
 */

4.6. Variables

  • Pointers:
/* (1) The asterisk goes by the variable name, not the type;
 * (2) Avoid leaving pointers uninitialized, to lessen the impact of
 *     use-before-assignment bugs
 */
char *my_string = NULL;

// Use space before asterisk and after closing parenthesis in a cast
char *foo = (char *) bar;
  • Global variables should be avoided in libraries when possible. State information should instead be passed as function arguments (often as a structure). This is not for thread safety – Pacemaker’s use of forking ensures it will never be threaded – but it does minimize overhead, improve readability, and avoid obscure side effects.
  • Time intervals are sometimes represented in Pacemaker code as user-defined text specifications (for example, “10s”), other times as an integer number of seconds or milliseconds, and still other times as a string representation of an integer number. Variables for these should be named with an indication of which is being used (for example, use interval_spec, interval_ms, or interval_ms_s instead of interval).

4.7. Operators

// Operators have spaces on both sides
x = a;

/* (1) Do not rely on operator precedence; use parentheses when mixing
 *     operators with different priority, for readability.
 * (2) No space is used after an opening parenthesis or before a closing
 *     parenthesis.
 */
x = a + b - (c * d);

4.8. Control Statements (if, else, while, for, switch)

/*
 * (1) The control keyword is followed by a space, a left parenthesis
 *     without a space, the condition, a right parenthesis, a space, and the
 *     opening bracket on the same line.
 * (2) Always use braces around control statement blocks, even if they only
 *     contain one line. This makes code review diffs smaller if a line gets
 *     added in the future, and avoids the chance of bad indenting making a
 *     line incorrectly appear to be part of the block.
 * (3) The closing bracket is on a line by itself.
 */
if (v < 0) {
    return 0;
}

/* "else" and "else if" are on the same line with the previous ending brace
 * and next opening brace, separated by a space. Blank lines may be used
 * between blocks to help readability.
 */
if (v > 0) {
    return 0;

} else if (a == 0) {
    return 1;

} else {
    return 2;
}

/* Do not use assignments in conditions. This ensures that the developer's
 * intent is always clear, makes code reviews easier, and reduces the chance
 * of using assignment where comparison is intended.
 */
// Do this ...
a = f();
if (a) {
    return 0;
}
// ... NOT this
if (a = f()) {
    return 0;
}

/* It helps readability to use the "!" operator only in boolean
 * comparisons, and explicitly compare numeric values against 0,
 * pointers against NULL, etc. This helps remind the reader of the
 * type being compared.
 */
int i = 0;
char *s = NULL;
bool cond = false;

if (!cond) {
    return 0;
}
if (i == 0) {
    return 0;
}
if (s == NULL) {
    return 0;
}

/* In a "switch" statement, indent "case" one level, and indent the body of
 * each "case" another level.
 */
switch (expression) {
    case 0:
        command1;
        break;
    case 1:
        command2;
        break;
    default:
        command3;
        break;
}

4.9. Structures

Changes to structures defined in public API headers (adding or removing members, or changing member types) are generally not possible without breaking API compatibility. However, there are exceptions:

  • Public API structures can be designed such that they can be allocated only via API functions, not declared directly or allocated with standard memory functions using sizeof.
    • This can be enforced simply by documentating the limitation, in which case new struct members can be added to the end of the structure without breaking compatibility.
    • Alternatively, the structure definition can be kept in an internal header, with only a pointer type definition kept in a public header, in which case the structure definition can be changed however needed.

4.10. Enumerations

  • Enumerations should not have a typedef, and do not require any naming convention beyond what applies to all exposed symbols.
  • New values should usually be added to the end of public API enumerations, because the compiler will define the values to 0, 1, etc., in the order given, and inserting a value in the middle would change the numerical values of all later values, breaking code compiled with the old values. However, if enum numerical values are explicitly specified rather than left to the compiler, new values can be added anywhere.
  • When defining constant integer values, enum should be preferred over #define or const when possible. This allows type checking without consuming memory.

4.10.1. Flag groups

Pacemaker often uses flag groups (also called bit fields or bitmasks) for a collection of boolean options (flags/bits).

This is more efficient for storage and manipulation than individual booleans, but its main advantage is when used in public APIs, because using another bit in a bitmask is backward compatible, whereas adding a new function argument (or sometimes even a structure member) is not.

#include <stdint.h>

/* (1) Define an enumeration to name the individual flags, for readability.
 *     An enumeration is preferred to a series of "#define" constants
 *     because it is typed, and logically groups the related names.
 * (2) Define the values using left-shifting, which is more readable and
 *     less error-prone than hexadecimal literals (0x0001, 0x0002, 0x0004,
 *     etc.).
 * (3) Using a comma after the last entry makes diffs smaller for reviewing
 *     if a new value needs to be added or removed later.
 */
enum pcmk__some_bitmask_type {
    pcmk__some_value    = (1 << 0),
    pcmk__other_value   = (1 << 1),
    pcmk__another_value = (1 << 2),
};

/* The flag group itself should be an unsigned type from stdint.h (not
 * the enum type, since it will be a mask of the enum values and not just
 * one of them). uint32_t is the most common, since we rarely need more than
 * 32 flags, but a smaller or larger type could be appropriate in some
 * cases.
 */
uint32_t flags = pcmk__some_value|pcmk__other_value;

/* If the values will be used only with uint64_t, define them accordingly,
 * to make compilers happier.
 */
enum pcmk__something_else {
    pcmk__whatever    = (UINT64_C(1) << 0),
};

We have convenience functions for checking flags (see pcmk_any_flags_set(), pcmk_all_flags_set(), and pcmk_is_set()) as well as setting and clearing them (see pcmk__set_flags_as() and pcmk__clear_flags_as(), usually used via wrapper macros defined for specific flag groups). These convenience functions should be preferred to direct bitwise arithmetic, for readability and logging consistency.

4.11. Booleans

4.11.1. Boolean Types

Booleans in C can be represented by an integer type, bool, or gboolean.

Integers are sometimes useful for storing booleans when they must be converted to and from a string, such as an XML attribute value (for which crm_element_value_int() can be used). Integer booleans use 0 for false and nonzero (usually 1) for true.

gboolean should be used with glib APIs that specify it. gboolean should always be used with glib’s TRUE and FALSE constants.

Otherwise, bool should be preferred. bool should be used with the true and false constants from the stdbool.h header.

4.11.2. Testing Booleans

Do not use equality operators when testing booleans. For example:

// Do this
if (bool1) {
    fn();
}
if (!bool2) {
    fn2();
}

// Not this
if (bool1 == true) {
    fn();
}
if (bool2 == false) {
    fn2();
}

// Otherwise there's no logical end ...
if ((bool1 == false) == true) {
    fn();
}

Conversely, equality operators should be used with non-boolean variables, even when just testing zero or nonzero:

int var1 = fn();

// Prefer this, because it gives a hint to the type when reading it
if (var1 == 0) {
    fn2();
}

// Not this, because a reader could mistakenly assume it is a boolean
if (!var1) {
    fn2();
}

4.12. Functions

Function names should be unique across the entire project, to allow for individual tracing via PCMK_trace_functions, and make it easier to search code and follow detail logs.

4.12.1. Function Definitions

/*
 * (1) The return type goes on its own line
 * (2) The opening brace goes by itself on a line
 * (3) Use "const" with pointer arguments whenever appropriate, to allow the
 *     function to be used by more callers.
 */
int
my_func1(const char *s)
{
    return 0;
}

/* Functions with no arguments must explicitly list them as void,
 * for compatibility with strict compilers
 */
int
my_func2(void)
{
    return 0;
}

/*
 * (1) For functions with enough arguments that they must break to the next
 *     line, align arguments with the first argument.
 * (2) When a function argument is a function itself, use the pointer form.
 * (3) Declare functions and file-global variables as ``static`` whenever
 *     appropriate. This gains a slight efficiency in shared libraries, and
 *     helps the reader know that it is not used outside the one file.
 */
static int
my_func3(int bar, const char *a, const char *b, const char *c,
         void (*callback)())
{
    return 0;
}

4.12.2. Return Values

Functions that need to indicate success or failure should follow one of the following guidelines. More details, including functions for using them in user messages and converting from one to another, can be found in include/crm/common/results.h.

  • A standard Pacemaker return code is one of the pcmk_rc_* enum values or a system errno code, as an int.
  • crm_exit_t (the CRM_EX_* enum values) is a system-independent code suitable for the exit status of a process, or for interchange between nodes.
  • Other special-purpose status codes exist, such as enum ocf_exitcode for the possible exit statuses of OCF resource agents (along with some Pacemaker-specific extensions). It is usually obvious when the context calls for such.
  • Some older Pacemaker APIs use the now-deprecated “legacy” return values of pcmk_ok or the positive or negative value of one of the pcmk_err_* constants or system errno codes.
  • Functions registered with external libraries (as callbacks for example) should use the appropriate signature defined by those libraries, rather than follow Pacemaker guidelines.

Of course, functions may have return values that aren’t success/failure indicators, such as a pointer, integer count, or bool.

4.12.3. Public API Functions

Unless we are doing a (rare) release where we break public API compatibility, new public API functions can be added, but existing function signatures (return type, name, and argument types) should not be changed. To work around this, an existing function can become a wrapper for a new function.

4.13. Memory Management

  • Always use calloc() rather than malloc(). It has no additional cost on modern operating systems, and reduces the severity and security risks of uninitialized memory usage bugs.
  • Ensure that all dynamically allocated memory is freed when no longer needed, and not used after it is freed. This can be challenging in the more event-driven, callback-oriented sections of code.
  • Free dynamically allocated memory using the free function corresponding to how it was allocated. For example, use free() with calloc(), and g_free() with most glib functions that allocate objects.

4.14. Logging and Output

4.14.1. Logging Vs. Output

Log messages and output messages are logically similar but distinct. Oversimplifying a bit, daemons log, and tools output.

Log messages are intended to help with troubleshooting and debugging. They may have a high level of technical detail, and are usually filtered by severity – for example, the system log by default gets messages of notice level and higher.

Output is intended to let the user know what a tool is doing, and is generally terser and less technical, and may even be parsed by scripts. Output might have “verbose” and “quiet” modes, but it is not filtered by severity.

4.14.2. Common Guidelines for All Messages

  • When format strings are used for derived data types whose implementation may vary across platforms (pid_t, time_t, etc.), the safest approach is to use %lld in the format string, and cast the value to long long.
  • Do not rely on %s handling NULL values properly. While the standard library functions might, not all Pacemaker API using them does, and it’s safest to get in the habit of always ensuring format values are non-NULL. For debug and trace messages, the crm_str() macro is sufficient and will map NULL to the string “<null>”, but for other messages an understandable string appropriate to the context should be used when the value is NULL.
  • The convenience macros pcmk__plural_s() and pcmk__plural_alt() are handy when logging a word that may be singular or plural.

4.14.3. Logging

Pacemaker uses libqb for logging, but wraps it with a higher level of functionality (see include/crm/common/logging*h).

A few macros crm_err(), crm_warn(), etc. do most of the heavy lifting.

By default, Pacemaker sends logs at notice level and higher to the system log, and logs at info level and higher to the detail log (typically /var/log/pacemaker/pacemaker.log). The intent is that most users will only ever need the system log, but for deeper troubleshooting and developer debugging, the detail log may be helpful, at the cost of being more technical and difficult to follow.

The same message can have more detail in the detail log than in the system log, using libqb’s “extended logging” feature:

/* The following will log a simple message in the system log, like:

       warning: Action failed: Node not found

   with extra detail in the detail log, like:

       warning: Action failed: Node not found | rc=-1005 id=hgjjg-51006
*/
crm_warn("Action failed: %s " CRM_XS " rc=%d id=%s",
         pcmk_rc_str(rc), rc, id);

4.14.4. Output

Pacemaker has a somewhat complicated system for tool output. The main benefit is that the user can select the output format with the --output-as option (usually “text” for human-friendly output or “xml” for reliably script-parsable output, though crm_mon additionally supports “console” and “html”).

A custom message can be defined with a unique string identifier, plus implementation functions for each supported format. The caller invokes the message using the identifier. The user selects the output format via --output-as, and the output code automatically calls the appropriate implementation function.

The interface (most importantly pcmk__output_t) is declared in include/crm/common/output*h. See the API comments and existing tools for examples.

4.15. String Handling

4.15.1. Define Constants for Magic Strings

A “magic” string is one used for control purposes rather than human reading, and which must be exactly the same every time it is used. Examples would be configuration option names, XML attribute names, or environment variable names.

These should always be defined constants, rather than using the string literal everywhere. If someone mistypes a defined constant, the code won’t compile, but if they mistype a literal, it could go unnoticed until a user runs into a problem.

4.15.2. Library Functions

Pacemaker’s libcrmcommon has a large number of functions to assist in string handling. The most commonly used ones are:

  • pcmk__str_eq() tests string equality (similar to strcmp()), but can handle NULL, and takes options for case-insensitive, whether NULL should be considered a match, etc.
  • crm_strdup_printf() takes printf()-style arguments and creates a string from them (dynamically allocated, so it must be freed with free()). It asserts on memory failure, so the return value is always non-NULL.

String handling functions should almost always be internal API, since Pacemaker isn’t intended to be used as a general-purpose library. Most are declared in include/crm/common/strings_internal.h. util.h has some older ones that are public API (for now, but will eventually be made internal).

4.15.3. char*, gchar*, and GString

When using dynamically allocated strings, be careful to always use the appropriate free function.

  • char* strings allocated with something like calloc() must be freed with free(). Most Pacemaker library functions that allocate strings use this implementation.
  • glib functions often use gchar* instead, which must be freed with g_free().
  • Occasionally, it’s convenient to use glib’s flexible GString* type, which must be freed with g_string_free().

4.16. Regular Expressions

  • Use REG_NOSUB with regcomp() whenever possible, for efficiency.
  • Be sure to use regfree() appropriately.

4.17. Makefiles

Pacemaker uses automake for building, so the Makefile.am in each directory should be edited rather than Makefile.in or Makefile, which are automatically generated.

  • Public API headers are installed (by adding them to a HEADERS variable in Makefile.am), but internal API headers are not (by adding them to noinst_HEADERS).

4.18. vim Settings

Developers who use vim to edit source code can add the following settings to their ~/.vimrc file to follow Pacemaker C coding guidelines:

" follow Pacemaker coding guidelines when editing C source code files
filetype plugin indent on
au FileType c   setlocal expandtab tabstop=4 softtabstop=4 shiftwidth=4 textwidth=80
autocmd BufNewFile,BufRead *.h set filetype=c
let c_space_errors = 1