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.
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.
| 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 |
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 [https://github.com/ClusterLabs/sbd] and dlm_controld.
Pacemaker uses Doxygen [https://www.doxygen.nl/manual/docblocks.html] to automatically generate its online API documentation [https://clusterlabs.org/pacemaker/doxygen/], 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;
}
Header files that are not library API are located in the same locations as other source code.
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.
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 [https://github.com/ClusterLabs/pacemaker/blob/master/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
/* 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.
*/
/* (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;
// 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);
/*
* (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;
}
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:
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.
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.
/*
* (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;
}
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.
Of course, functions may have return values that aren’t success/failure indicators, such as a pointer, integer count, or bool.
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.
Pacemaker uses automake [https://www.gnu.org/software/automake/manual/automake.html] for building, so the Makefile.am in each directory should be edited rather than Makefile.in or Makefile, which are automatically generated.
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