Pitfalls in Writing Portable C++98 Code


I’ve now made enough submissions to CRAN that have crashed and burned on Solaris, that I think it’s now time to put some of the pitfalls I’ve bumped into in writing. My goal is for this blog post to be a mini-checklist package authors can run through before submitting a C++-code containing package to CRAN. (If you happen to be a CRAN maintainer who has stumbled upon this blog post, I apologize for all the trouble I’ve caused in my various package submissions. Let this be my recompense.)

For the unaware, the CRAN build machines encompass four main platforms: Linux, OS X (macOS), Windows, and Solaris. The compilers used on the Solaris machines (Oracle Solaris Studio, or more recently, Oracle Developer Studio) are very picky when it comes to C++ code that should respect the C++98 standard. This implies that

  • many of the features that became part of the C99 standard, along with
  • the ‘minor’ features of the C++11 standard (that might be considered ‘bug fixes’, for things accidentally omitted from the C++98 standard)

may not be available when compiling against the C++98 standard. Notably, gcc and clang often make these features available when compiling with -std=c++98, or otherwise make them available if the -std flag is not explicitly specified, so it’s very easy to accidentally write code that unbeknownst to you does not adhere to the C++98 standard.

The CRAN Solaris build machines use the Oracle Studio compilers, alongside their stlport4 C++ standard library. Together, these adopt a very strict interpretation of the C++98 standard. While gcc and clang often take a “I know what you mean” approach to compiling your code, the Solaris compilers take a more “this is what the standard says” approach.

With that said, let’s get started.

C++11

If you can use C++11 (or greater) when compiling your package’s C++ code, just do it. The C++11 standard adds a slew of excellent features, while fixing a few omissions of the C++98 standard. Overall, C++11 makes it easier to write cross-platform, portable C++ programs. All of the platforms used on CRAN support C++11 now – most recently, Windows joined the club with the toolchain update spearheaded by Jeroen Ooms et. al, and with much help from Duncan Murdoch + others.

The only reason not to use C++11 nowadays is if your package needs to build on older machines (Red Hat Enterprise Linux 5 + gcc-4.4, I’m looking at you), but even then one can compile with -std=c++0x to get a subset of C++11 features. (R users in such environments will likely need to manually set CXX1XSTD = -std=c++0x in a local ~/.R/Makevars, or something similar.)

For R packages, using C++11 is as simple as placing the following line in your src/Makevars and src/Makevars.win files:

CXX_STD = CXX11

The development versions of R even come with support for the C++14 standard. So, if you can, please use modern C++ – for your own sanity, and also as an extra layer of protection versus the common portability pitfalls.

Rule: If you can, use the most recent version of the C++ standard available.

Standard Library Headers

The following code may fail to compile on Solaris:

#include <cstring>
size_t string_length(const char* string) {
  return ::strlen(string);
}
kevin@soularis:~/scratch
$ CC -library=stlport4 string_length.cpp 
"string_length.cpp", line 3: Error: size_t is not defined.
"string_length.cpp", line 4: Error: strlen is not defined.
2 Error(s) detected.

The C++ standard library headers that ‘wrap’ their C counterparts are typically prefixed with a c and contain no extension, e.g. <cstring>; while the C headers themselves are typically given a .h extension, e.g. <string.h>. When the <cstring> header is included in a translation unit, the C++98 standard dictates that the compiler:

  • Must define its members (e.g. strlen) in the std:: namespace, and
  • May define its members (e.g. strlen) in the global namespace.

In fact, gcc and clang both accept the above code, but the Solaris compilers do not. (The Solaris compilers do not populate the global namespace when including these headers.)

Rule: If you include a C++-style standard library header, reference symbols from the std namespace. Prefer using C++-style standard library headers over the original C counterpart. Prefer referencing these symbols explicitly, with the std:: prefix.

C99

The C++98 standard was ratified at an unfortunate time – it came just one year before the C99 standard, and the C99 standard introduced a number of tools that make it easier to write safe + correct C code. Some of the newer pieces that became part of the C99 standard made it into C++98, but some didn’t. Examples that I’ve bumped into thus far are:

  • long long
  • snprintf / vsnprintf
  • isblank
  • Fixed-width integer types (uint8_t etc., from <cstdint>)
  • Variadic macros

long long in particular has been a thorny issue, as many modern C++ libraries, such as Boost (which you might be using through the CRAN BH package), will just assume that your compiler defines the long long type, regardless of what version of the standard you attempt to compile the associated code with. Attempts to compile such code may cause your compiler to generate warnings, and CRAN may reject your package on the basis of these warnings. (Because your package would crash and burn on Solaris.) In Boost’s case, you often need to add something like this to your src/Makevars[.win]:

PKG_CXXFLAGS = -DBOOST_NO_INT64_T -DBOOST_NO_INTEGRAL_INT64_T -DBOOST_NO_LONG_LONG

See the dplyr Makevars file for an example of this.

Most compiler suites that compile C++ also compile C and support the C99 standard, and often make those symbols available to C++ code. However, strictly speaking, these are not available in the C++98 standard, and so expect compiler errors on Solaris if you use these features.

Rule: Avoid using symbols defined newly in the C99 standard, as the Solaris compilers may not make them available when compiling in C++98 mode.

Exceptions

The following code may fail to compile on Solaris:

#include <stdexcept>
void ouch() {
  throw std::logic_error("ouch!");
}

Can you guess why? In fact, the logic_error class has two constructors:

  1. explicit logic_error(const std::string& what_arg);
  2. explicit logic_error(const char* what_arg);

The second constructor was added only in C++11, so a strictly conforming C++98 compiler may not provide it. And, because the constructor is marked explicit, the compiler will not attempt to convert the user-provided const char* to std::string, to invoke the first constructor. As you can imagine, most friendly compilers will accept your code either way as the intention is obvious, but don’t expect Solaris to be friendly.

This omission is not unique to logic_error; it seems to be common to all of the exception classes defined in <stdexcept>.

Rule: Avoid constructing exception objects with C strings.

Be Careful with <cctype>

Have you ever wanted to know whether a particular char is a letter, a number, a whitespace character, or something else? The <cctype> header provides utilities for assessing this, with e.g. std::isspace. Unfortunately, these functions are dangerous for one main reason:

  • The behavior is undefined if the value of ch is not representable as unsigned char and is not equal to EOF.

Together, this implies a very counter-intuitive result for the following program on Solaris:

#include <cstdio>
#include <cctype>

int main() {
    char ch = '\xa1'; // '¡' in latin-1 locales + UTF-8
    std::printf("is whitespace: %i\n", std::isspace(ch));
    return 0;
}

Compiled and run on Solaris, I see:

kevin@soularis:~/scratch
$ CC -library=stlport4 whitespace.cpp && ./a.out 
is whitespace: 8

What happened here? Well:

  1. '\xa1' (normally, the ‘¡’ character in latin-1 or UTF-8 locales) is assigned to a char,
  2. Because the integer value of '\xa1' (161) lies outside the range of a char, it’s converted to -95 (161 - 256, wrapping around),
  3. Because -95 is not representable as an unsigned char, the program is undefined,
  4. Solaris takes the ‘undefined’ interpretation literally, and gives you an unexpected result over an expected result.

Now, you might argue that the behavior is clearly documented and it’s the authors fault for writing a program that exhibits this behavior, but it’s unfortunately easy to do. For example:

#include <cctype>

int countWhitespace(const char* bytes) {
  int count = 0;
  while (std::isspace(*bytes++))
    ++count;
  return count;
}

This is the kind of program that only looks obviously wrong if you’re an expert, and I think even experts could miss this. The solution is to explicitly cast any chars to unsigned char before passing them to <cctype> functions.

Or, just write your own wrapper functions that accept const char* and do the right thing. For example, it might suffice to just use:

boolean isWhitespace(char ch) {
  return
    ch == ' ' ||
    ch == '\f' ||
    ch == '\r' ||
    ch == '\n' ||
    ch == '\t' ||
    ch == '\v';
}

This of course does not capture all kinds of whitespace characters. For example, the Unicode standard defines a whole slew of multibyte white space characters; figuring out how to handle all of that is beyond the scope of this post.

Interestingly, the wide character analogues defined in <cwctype> don’t appear to come with the same caveat, and hence should be safer to use.

Rule: Be careful when using <cctype> – either write your own wrappers, or ensure you cast to unsigned char first.

The R Manuals

If you’re not already aware, the R Manuals, and Writing R Extensions in particular, are excellent references for common issues encountered when using R. They’re not perfect – I often find it difficult to remember which manual contains which bit of relevant information I’m looking for, and where in the manual that information lies – but they are incredibly comprehensive and actively updated by the R Core team.

The section in Writing R Extensions on Portable C and C++ code is a very nice reference for common portability pitfalls in the C++ code used by R packages that the CRAN maintainers have seen throughout the years. Treat this section as another mini-checklist before submitting an R package containing C++ code to CRAN.

Martyn Plummer also has an article in the R Journal, Portable C++ for R Packages, which (having discovered after writing an initial iteration of this blog post) discusses many of the same points in this blog post, in some more detail.

Rule: Review the ‘Portable C and C++ Code’ section before submitting your package to CRAN.

Leaky Compilers

Unfortunately, some compilers will leak macro definitions that can conflict with your code unexpectedly. Some such symbols, as discussed in Portable C and C++ code, are:

  • ERR
  • zero
  • I
  • CS, DS, ES, FS, GS and SS

Solaris isn’t the only leaky compiler – gcc also ‘leaks’ major and minor macro definitions when including <sys/sysmacros.h>, and this header might find its way into your program when including, for example, <iterator>. (See here for one such example.)

Rule: Be aware of macro pollution.

Don’t Panic

Let’s be honest. It’s really easy to make mistakes when attempting to write C++ code that strictly adheres to the C++98 standard, and portability issues create frustration for everyone (you and the CRAN maintainers included). The net result is still a better package that is more likely to be portable outside of the gcc / clang ecosystem.

Do your best, be friendly and up-front with the CRAN maintainers when you’re submitting (or re-submitting) a package containing C++ code that produces compiler errors, and try to learn a little bit more each time.

Rule: Remember to take a deep breath, and be patient.

Wrapping Up

Hopefully, this post will help you (or possibly just future me) to avoid headaches with your next CRAN package submission containing C++ code.

And, to re-iterate once more, if you can use C++11 (or greater), do it!


Comments