• Understand the issue: <resolv.h> defines _res macro that conflicts with user code variable names
• Locate the include of <resolv.h> at line 259 in httplib.h within #ifdef __linux__ block
• Add #undef _res after #include <resolv.h> to prevent macro conflicts
• Add inline comment explaining why #undef _res is necessary
• Verify the fix compiles correctly with a test case
• Run code review and codeql checker

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Problem

On Linux, when httplib.h is included, it includes <resolv.h> which defines a macro _res:

// From /usr/include/resolv.h
#define _res (*__res_state())

This causes compilation errors when user code uses _res as a variable name. For example:

#include <iostream>
#include <string>
#include "httplib.h"

int func(int &_res){  // Error: cannot declare pointer to 'int&'
    return 0;
}

int main(int argc, char *argv[])
{
    return 0;
}

The error occurs because the preprocessor expands _res to (*__res_state()), resulting in invalid syntax like int &(*__res_state()).

Solution

Add #undef _res after including <resolv.h> in httplib.h. This is safe because:

1. httplib.h does not use the _res macro internally
2. The resolver functions (res_init(), res_query(), etc.) work independently of this macro
3. This allows user code to use _res as a variable name without conflicts

Implementation

In httplib.h, after the #include <resolv.h> line (around line 259), add:

#ifdef __linux__
#include <resolv.h>
#ifdef _res
#undef _res
#endif
#endif

References

• Issue: Will get cannot declare pointer to error When include httplib.h #2278

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