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I have a C file that needs a specific header file. If that header file does not exist, I want the preprocessor to issue a specific warning. Something like:
#if !(#include <special.h>)
#warning "Don't worry, you can fix this."
#warning "You just need to update this other repo over here:"
#endif
Is this possible with the C preprocessor?
367
Without a header file, you cannot run or compile your program code. In C, all program codes must include the stdio. h header file.
In C language, header files contain the set of predefined standard library functions. You request to use a header file in your program by including it with the C preprocessing directive “#include”. All the header file have a '. h' an extension.
To check if an header file has been included or not in a C or C++ code, we need to check if the macro defined in the header file is being defined in the client code. Standard header files like math. h have their own unique macro (like _MATH_H ) which we need to check. Consider this example of checking if math.
I know this is a old question, but I also needed a way to do this and I was disappointed when I read these answers. It turns out that clang has a nice __has_include("header name") directive that works flawlessly to do what you need :)
Maybe you want to check if the directive is available first by calling #if defined(__has_include)
124
No, this is not possible. All you can do is try to #include the header, and if it doesn't exist, the compiler will give you an error.
An alternative would be to use a tool like GNU autoconf. autoconf generates a shell script called configure which analyzes your build system and determines things like whether or not you have certain header files installed, and it generates a header file consisting of macros indicating that information.
For example, it might define HAVE_SYS_TIME_H if your build system includes the header <sys/time.h>, so you can then write code such as:
#if HAVE_SYS_TIME_H
#include <sys/time.h>
#else
#warning "Don't worry, you can fix this."
#endif
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