The OpenVPN 2.4+ coding style is based on the Allman style, e.g.:

myfunction(int a, int b)
    const char *dummy = "Hello world!";
    if (a == b)


  • Indentation is 4 spaces, no tabs ever.
  • Opening and closing brackets get their own line, and must match indentation.
  • Line length is 80 characters (soft limit) but may be extended up to 120 if wrapping makes the result less readable / more ugly
  • C99 is allowed. E.g. for (int i = 0; i < max; i++).
  • Only use /* */-style comments

Line wrapping

The maximum line length is 80 characters. When statements exceed this length, wrap them by aligning with the appropriate (, or otherwise using a single indent (i.e. 4 spaces) on the new line. For example:

    if (variable_with_artificially_long_name_1 != 0
        && variable_with_artificially_long_name_2 != 0)
        return variable_with_artificially_long_name_1
            + variable_with_artificially_long_name_2
            + variable_with_artificially_long_name_3;

If you find yourself continuously exceeding the line limit, you might want to consider breaking up your function into smaller functions to reduce nesting.

Single lines may use up to 120 characters if the resulting code is more readable than if wrapped. But this should be an exception, no whole code blocks should be written with "more than 80 as a general norm".

Some additional secure coding style rules

This are style rules, not general secure coding guide lines. See e.g. for useful guidelines on writing secure C.

All branches must use braces

For example:

if (a)
    return true;

We wouldn't be the first to not spot missing braces in an if statement, like what happened in Apple's goto fail; bug. Always using braces prevents this kind of mistake.

Cases in a switch statement should break or explicitly fall through

The only exception to this rule are empty case statements, to not overly clutter the code with comments.

For example:

switch (a)
    case ONE:
    case TWO:
        one_or_two = true;
        /* Fall through */
    case THREE:

This makes the intent immediately clear to the reader / reviewer.

Portability concerns

Printing time_t and suseconds_t values

POSIX specifies that time_t is an integer type but does not specify its size. The underlying type might be int, long or long long, some systems might make it unsigned too. This makes it hard to portably print time_t values using printf-like APIs, as a mismatch between the format string and the actual time_t size can lead to crashes or bogus representations.

The safest way is to print time_t values after casting to int64_t and using PRIi64 as format specifier. Casting to a 64 bits wide int avoids y2038 problems and PRIi64 is preferred over %lld as the latter is not supported in the windows runtime targeted by mingw.

Similarly, POSIX specifies suseconds_t as a signed integer, but does not specify its size. Casting it as a long is enough.

  struct timeval now;
  gettimeofday(&now, NULL);                                                                                                                                                                                    
  printf("%"PRIi64".%06ld\n", (int64_t)now.tv_sec, (long)now.tv_usec);

Windows specific

OpenVPN core uses UTF-8 encoding for strings. These may be converted to and from UTF-16 used in Windows as required. Declare all strings explicitly as char * or wchar_t *. Use of TCHAR is not allowed. Call Windows API functions using their explicit ANSI or wide character variants: e.g., CreateFileW or CreateFileA instead of CreateFile. The core executable for Windows is built without -DUNICODE and -D_UNICODE.

Sources for Windows-only executables and libraries such as openvpnserv.exe, tapctl.exe and openvpnmscia.dll partially depend on UNICODE (and in some cases _UNICODE) defined. Also there is a widespread use of TCHAR as well as function names that resolve to single-byte or wide character variants depending on whether UNICODE is defined or not.

However, in spite of appearances to the contrary, non-unicode builds are no longer supported. In new code declare wide strings as wchar_t * or WCHAR *. One may implicitly assume that all existing TCHAR would evaluate to WCHAR. For runtime and API calls use explicit function names instead of depending on UNICODE and/or _UNICODE are defined or not.

In particular, do not use _tprintf family of stdio functions. Instead, use wprintf or printf variants as required. Use %ls as the print format specifications for wide strings. For narrow strings, %s may be used in printf family of functions, but in wprintf family use %hs. Do not use %S.

For example, given the existing function,

MsgToEventLog(DWORD flags, LPCTSTR lpszMsg, ...);

a new code snippet that calls it would be written as

const wchar_t *username;
const char *config;
MsgtoEvenLog(M_SYSERR, L"Failed to authorize profile <%hs> for user <%ls>", config, username);

Uncrustify rules

The most fit tool for our purpose (because it can be told to just leave certain constructs alone, while changing others) seems to be

uncrustify -c dev-tools/uncrustify.conf tun.c

You can find our current config in the dev-tools directory in the git repository:

Last modified 8 months ago Last modified on 10/31/23 13:02:54