#pragma pack instructs the compiler to pack structure members with particular alignment. Most compilers, when you declare a struct, will insert padding between members to ensure that they are aligned to appropriate addresses in memory (usually a multiple of the type’s size). This avoids the performance penalty (or outright error) on some architectures associated with … Read more
The Predefined Macros for OS site has a very complete list of checks. Here are a few of them, with links to where they’re found: Windows _WIN32 Both 32 bit and 64 bit _WIN64 64 bit only __CYGWIN__ Unix (Linux, *BSD, but not Mac OS X) See this related question on some of the pitfalls of using … Read more
Yes, use -E -dM options instead of -c. Example (outputs them to stdout): echo | gcc -dM -E – echo | clang -dM -E – For C++ echo | g++ -dM -E -x c++ – echo | clang++ -dM -E -x c++ – From the gcc manual: Instead of the normal output, generate a list … Read more
There are predefined macros that are used by most compilers, you can find the list here. GCC compiler predefined macros can be found here. Here is an example for gcc: #if defined(WIN32) || defined(_WIN32) || defined(__WIN32__) || defined(__NT__) //define something for Windows (32-bit and 64-bit, this part is common) #ifdef _WIN64 //define something for Windows … Read more
extern “C” doesn’t really change the way that the compiler reads the code. If your code is in a .c file, it will be compiled as C, if it is in a .cpp file, it will be compiled as C++ (unless you do something strange to your configuration). What extern “C” does is affect linkage. … Read more
For a long time, CMake had the add_definitions command for this purpose. However, recently the command has been superseded by a more fine grained approach (separate commands for compile definitions, include directories, and compiler options). An example using the new add_compile_definitions: add_compile_definitions(OPENCV_VERSION=${OpenCV_VERSION}) add_compile_definitions(WITH_OPENCV2) Or: add_compile_definitions(OPENCV_VERSION=${OpenCV_VERSION} WITH_OPENCV2) The good part about this is that it circumvents … Read more
Where are MIN and MAX defined in C, if at all? They aren’t. What is the best way to implement these, as generically and type safe as possible (compiler extensions/builtins for mainstream compilers preferred). As functions. I wouldn’t use macros like #define MIN(X, Y) (((X) < (Y)) ? (X) : (Y)), especially if you plan … Read more
Wow, this requirement goes way back to the 1970s. In the very early days of pre-standardised C, if you wanted to invoke the preprocessor, then you had to write a # as the first thing in the first line of a source file. Writing only a # at the top of the file affords flexibility … Read more
Those are called #include guards. Once the header is included, it checks if a unique value (in this case HEADERFILE_H) is defined. Then if it’s not defined, it defines it and continues to the rest of the page. When the code is included again, the first ifndef fails, resulting in a blank file. That prevents … Read more
The do … while and if … else are there to make it so that a semicolon after your macro always means the same thing. Let’s say you had something like your second macro. #define BAR(X) f(x); g(x) Now if you were to use BAR(X); in an if … else statement, where the bodies of … Read more