可以使用 C 预处理器来判断文件是否存在吗? [英] Can the C preprocessor be used to tell if a file exists?

问题描述

我有一个非常大的代码库(阅读:数千个模块)，其中的代码在许多项目中共享，这些项目都运行在具有不同 C++ 编译器的不同操作系统上.不用说，维护构建过程可能是一件苦差事.

I have a very large codebase (read: thousands of modules) that has code shared across numerous projects that all run on different operating systems with different C++ compilers. Needless to say, maintaining the build process can be quite a chore.

如果文件在当前文件夹.有谁知道实现这一目标的方法吗?

There are several places in the codebase where it would clean up the code substantially if only there were a way to make the pre-processor ignore certain #includes if the file didn't exist in the current folder. Does anyone know a way to achieve that?

目前，我们在共享文件中的 #include 周围使用 #ifdef，并使用第二个项目特定文件 #defines#include 存在于项目中.这有效，但它很丑.人们在从项目中添加或删除文件时经常忘记正确更新定义.我已经考虑编写一个预构建工具来保持这个文件是最新的，但是如果有一种独立于平台的方式来使用预处理器来做到这一点，我宁愿这样做.有什么想法吗?

Presently, we use an #ifdef around the #include in the shared file, with a second project-specific file that #defines whether or not the #include exists in the project. This works, but it's ugly. People often forget to properly update the definitions when they add or remove files from the project. I've contemplated writing a pre-build tool to keep this file up to date, but if there's a platform-independent way to do this with the preprocessor I'd much rather do it that way instead. Any ideas?

推荐答案

通常这是通过使用脚本来完成的，该脚本在尝试包含文件时尝试运行预处理器.根据预处理器是否返回错误，脚本会使用适当的#define(或#undef)更新生成的.h 文件.在 bash 中，脚本可能看起来像这样:

Generally this is done by using a script that tries running the preprocessor on an attempt at including the file. Depending on if the preprocessor returns an error, the script updates a generated .h file with an appropriate #define (or #undef). In bash, the script might look vaguely like this:

cat > .test.h <<'EOM'
#include <asdf.h>
EOM
if gcc -E .test.h
 then
  echo '#define HAVE_ASDF_H 1' >> config.h
 else 
  echo '#ifdef HAVE_ASDF_H' >> config.h
  echo '# undef HAVE_ASDF_H' >> config.h
  echo '#endif' >> config.h
 fi

autoconf 是一个非常全面的框架，用于可移植地处理此类(以及数千个其他)可移植性检查.

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