C语言代码中调用C++代码的方法示例分享

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由于历史原因，以及不同开发人员的技术偏好，C语言和C++语言都有一些独有的非常有价值的项目，因而两种语言的互操作，充分利用前人造的轮子是一件非常有价值的事情。

C++代码调用C代码很简单，只要分别在包含的C头文件的开头和结尾加上如下的两个块：

  #ifdef __cplusplus  extern "C" {  #endif

和

  #ifdef __cplusplus  }  #endif

即可。

然而为了支持类、重载等更加高级的特性，在编译C++代码时，C++符号会被修饰。我们dump Linux平台加密库 libcrypto++ 的符号表，可以看到如下的内容：

  $ readelf -s /usr/lib/libcrypto++.so  Symbol table '.dynsym' contains 9607 entries:   Num: Value   Size Type Bind Vis  Ndx Name    0: 0000000000000000  0 NOTYPE LOCAL DEFAULT UND     1: 00000000001daa58  0 SECTION LOCAL DEFAULT 9     2: 0000000000000000  0 OBJECT GLOBAL DEFAULT UND _ZTIi@CXXABI_1.3 (2)    3: 0000000000000000  0 FUNC GLOBAL DEFAULT UND __errno_location@GLIBC_2.2.5 (3)    4: 0000000000000000  0 FUNC GLOBAL DEFAULT UND _ZSt18uncaught_exceptionv@GLIBCXX_3.4 (4)    5: 0000000000000000  0 FUNC GLOBAL DEFAULT UND _ZNSt8__detail15_List_node_base7_M_hookEPS0_@GLIBCXX_3.4.15 (5)    6: 0000000000000000  0 FUNC GLOBAL DEFAULT UND getservbyname@GLIBC_2.2.5 (6)    7: 0000000000000000  0 FUNC GLOBAL DEFAULT UND bind@GLIBC_2.2.5 (6)    8: 0000000000000000  0 FUNC GLOBAL DEFAULT UND _ZSt29_Rb_tree_insert_and_rebalancebPSt18_Rb_tree_node_baseS0_RS_@GLIBCXX_3.4 (4)    9: 0000000000000000  0 FUNC GLOBAL DEFAULT UND __longjmp_chk@GLIBC_2.11 (7)   10: 0000000000000000  0 OBJECT GLOBAL DEFAULT UND _ZTIh@CXXABI_1.3 (2)   11: 0000000000000000  0 OBJECT GLOBAL DEFAULT UND _ZTVSt9basic_iosIcSt11char_traitsIcEE@GLIBCXX_3.4 (4)   12: 0000000000000000  0 FUNC GLOBAL DEFAULT UND socket@GLIBC_2.2.5 (6)   13: 0000000000000000  0 FUNC GLOBAL DEFAULT UND _ZNSt14basic_ifstreamIcSt11char_traitsIcEED1Ev@GLIBCXX_3.4 (4)   . . . . . .   86: 0000000000000000  0 FUNC GLOBAL DEFAULT UND _ZNSo5writeEPKcl@GLIBCXX_3.4 (4)   87: 0000000000000000  0 FUNC GLOBAL DEFAULT UND malloc@GLIBC_2.2.5 (6)   88: 0000000000000000  0 FUNC GLOBAL DEFAULT UND _ZNSt9basic_iosIcSt11char_traitsIcEE4initEPSt15basic_streambufIcS1_E@GLIBCXX_3.4 (4)   89: 0000000000000000  0 FUNC GLOBAL DEFAULT UND _ZNSi5seekgElSt12_Ios_Seekdir@GLIBCXX_3.4 (4)   90: 0000000000000000  0 FUNC GLOBAL DEFAULT UND pthread_key_delete@GLIBC_2.2.5 (3)   91: 0000000000000000  0 FUNC GLOBAL DEFAULT UND shutdown@GLIBC_2.2.5 (6)   92: 0000000000000000  0 FUNC GLOBAL DEFAULT UND _ZSt15set_new_handlerPFvvE@GLIBCXX_3.4 (4)   93: 0000000000000000  0 FUNC GLOBAL DEFAULT UND pthread_getspecific@GLIBC_2.2.5 (3)   94: 0000000000000000  0 FUNC GLOBAL DEFAULT UND strcmp@GLIBC_2.2.5 (6)   95: 0000000000000000  0 FUNC GLOBAL DEFAULT UND strtol@GLIBC_2.2.5 (6)   96: 0000000000000000  0 FUNC GLOBAL DEFAULT UND ioctl@GLIBC_2.2.5 (6)   . . . . . .   186: 00000000002c5a80 142 FUNC GLOBAL DEFAULT 12 _ZN8CryptoPP6xorbufEPhPKhS2_m   187: 00000000002fd6d0  9 FUNC WEAK DEFAULT 12 _ZN8CryptoPP21InvertibleRSAFunction9BERDecodeERNS_22BufferedTransformationE   188: 00000000001ea840 73 FUNC GLOBAL DEFAULT 12 _ZN8CryptoPP13Base64Decoder22GetDecodingLookupArrayEv   189: 0000000000249760  6 FUNC WEAK DEFAULT 12 _ZThn8_N8CryptoPP13DL_SignerImplINS_25DL_SignatureSchemeOptionsINS_5DL_SSINS_13DL_Keys_ECDSAINS_4EC2NEEENS_18DL_Algorithm_ECDSAIS4_EENS_37DL_SignatureMessageEncodingMethod_DSAENS_6SHA256EiEES5_S7_S8_S9_EEED0Ev   190: 0000000000278b60 86 FUNC WEAK DEFAULT 12 _ZN8CryptoPP8Rijndael3DecD1Ev   191: 00000000001fd1f0  2 FUNC WEAK DEFAULT 12 _ZN8CryptoPP23DefaultEncryptorWithMAC8FirstPutEPKh   192: 000000000026a490 51 FUNC GLOBAL DEFAULT 12 _ZN8CryptoPP23FilterWithBufferedInputC2EPNS_22BufferedTransformationE   193: 0000000000285180  6 FUNC WEAK DEFAULT 12 _ZNK8CryptoPP8GCM_Base6IVSizeEv   194: 000000000032e830 510 FUNC WEAK DEFAULT 12 _ZN8CryptoPP18StandardReallocateItNS_20AllocatorWithCleanupItLb0EEEEENT0_7pointerERS3_PT_NS3_9size_typeES8_b   195: 00000000002a1790 185 FUNC WEAK DEFAULT 12 _ZSt18uninitialized_copyISt15_Deque_iteratorIyRKyPS1_ES0_IyRyPyEET0_T_S9_S8_   196: 0000000000355610 25 OBJECT WEAK DEFAULT 14 _ZTSN8CryptoPP11RSAFunctionE   . . . . . .

这与我们在源文件和头文件里看到的那些函数、类的声明定义都不一样。通过binutils的工具c++filt demangle这些符号可以让我们看到它们在代码里的样子：

  $ c++filt _ZTSN8CryptoPP11RSAFunctionE  typeinfo name for CryptoPP::RSAFunction  $ c++filt _ZN8CryptoPP18StandardReallocateItNS_20AllocatorWithCleanupItLb0EEEEENT0_7pointerERS3_PT_NS3_9size_typeES8_b  CryptoPP::AllocatorWithCleanup<unsigned short, false>::pointer CryptoPP::StandardReallocate<unsigned short, CryptoPP::AllocatorWithCleanup<unsigned short, false> >(CryptoPP::AllocatorWithCleanup<unsigned short, false>&, unsigned short*, CryptoPP::AllocatorWithCleanup<unsigned short, false>::size_type, CryptoPP::AllocatorWithCleanup<unsigned short, false>::size_type, bool)

那到底有没有办法在C代码中调用C++代码呢？方法当然是有的，而且还不止一种。

通过extern “C”调用

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