CMake中#cmakedefine替换结果不一致的原因咨询

The core issue here is how CMake's #cmakedefine directive works in your config.h.in file. Let's break down exactly what's happening:

When you use #cmakedefine VAR_NAME ... in your template, CMake checks the CMake variable named VAR_NAME (not the value you're inserting with @...@). Here's the play-by-play for your lines:

1. First line: #cmakedefine HAVE_GETOPT @HAVE_GETOPT@

   • CMake looks for the CMake variable HAVE_GETOPT, which you set via check_function_exists(getopt HAVE_GETOPT). Since it evaluates to true (1), this line becomes #define HAVE_GETOPT 1 as expected.

2. Second line: #cmakedefine STDLIB_HAS_GETOPT @HAVE_GETOPT@

   • CMake now checks for the CMake variable STDLIB_HAS_GETOPT—but you never set this variable in your CMakeLists.txt! Even though you're inserting @HAVE_GETOPT@ as the value, the #cmakedefine directive depends entirely on the variable name immediately after it. Since STDLIB_HAS_GETOPT isn't defined in CMake, it defaults to false, resulting in /* #undef STDLIB_HAS_GETOPT */.

How to fix it #

You have two straightforward options to get the expected output:

Option 1: Set STDLIB_HAS_GETOPT in CMakeLists.txt #

Add this line right after your check_function_exists call:

set(STDLIB_HAS_GETOPT ${HAVE_GETOPT})

This copies the value of HAVE_GETOPT into STDLIB_HAS_GETOPT. Now when CMake processes the second #cmakedefine line, it will see STDLIB_HAS_GETOPT is true and generate #define STDLIB_HAS_GETOPT 1.

Option 2: Use a preprocessor conditional instead of #cmakedefine #

If you don't want to add another variable to your CMake config, replace the second #cmakedefine line in config.h.in with:

#if @HAVE_GETOPT@
#define STDLIB_HAS_GETOPT @HAVE_GETOPT@
#endif

This will define STDLIB_HAS_GETOPT only when HAVE_GETOPT is true, and leave it undefined otherwise—exactly matching the behavior you want.

Either approach will produce your desired output:

#define HAVE_GETOPT 1
#define STDLIB_HAS_GETOPT 1

内容的提问来源于stack exchange，提问作者einpoklum