Question

当我尝试使用extern函数执行marco时，我在Objective C中遇到了链接器问题。知道为什么吗？

标题文件
协助与设备版本进行比较

extern NSString* getOperatingSystemVerisonCode();

#if TARGET_OS_IPHONE // iOS
#define DEVICE_SYSTEM_VERSION                       [[UIDevice currentDevice]      systemVersion]
#else // Mac
#define DEVICE_SYSTEM_VERSION                       getOperatingSystemVerisonCode()
#endif

#define COMPARE_DEVICE_SYSTEM_VERSION(v)            [DEVICE_SYSTEM_VERSION compare:v options:NSNumericSearch]
#define SYSTEM_VERSION_EQUAL_TO(v)                  (COMPARE_DEVICE_SYSTEM_VERSION(v) == NSOrderedSame)
#define SYSTEM_VERSION_GREATER_THAN(v)              (COMPARE_DEVICE_SYSTEM_VERSION(v) == NSOrderedDescending)
#define SYSTEM_VERSION_GREATER_THAN_OR_EQUAL_TO(v)  (COMPARE_DEVICE_SYSTEM_VERSION(v) != NSOrderedAscending)
#define SYSTEM_VERSION_LESS_THAN(v)                 (COMPARE_DEVICE_SYSTEM_VERSION(v) == NSOrderedAscending)
#define SYSTEM_VERSION_LESS_THAN_OR_EQUAL_TO(v)     (COMPARE_DEVICE_SYSTEM_VERSION(v) != NSOrderedDescending)

.mm文件

NSString* getOperatingSystemVerisonCode()
{
    /*
     [[NSProcessInfo processInfo] operatingSystemVersionString]
     */
    NSDictionary *systemVersionDictionary =
    [NSDictionary dictionaryWithContentsOfFile:
     @"/System/Library/CoreServices/SystemVersion.plist"];

    NSString *systemVersion =
    [systemVersionDictionary objectForKey:@"ProductVersion"];
    return systemVersion;
}

链接器错误：

Undefined symbols for architecture x86_64:
  "_getOperatingSystemVerisonCode", referenced from:
      -[Manager isFeatureAvailable] in Manager.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Answer 1

问题不是由宏定义引起的。

getOperatingSystemVerisonCode()函数在＆＃34; .mm＆＃34;中定义。文件，因此编译为Objective- C ++ 。特别是，函数名称被破坏为C ++函数。但是当从（Objective-）C源引用时，预期会出现未编码的名称。

您有两种方法可以解决问题：

重命名＆＃34; .mm＆＃34;将文件转换为＆＃34; .m＆＃34;，以便将其编译为Objective-C文件。
在声明函数的头文件中，添加extern "C"声明以强制执行C链接，即使在（Objective-）C ++文件中也是如此：
```
#ifdef __cplusplus
extern "C" {
#endif

NSString* getOperatingSystemVerisonCode();

#ifdef __cplusplus
}
#endif
```

有关混合C和C ++的更多信息，请参阅示例

Combining C++ and C - how does #ifdef __cplusplus work?

与宏的extern功能

1 个答案: