c++ - 使用外部库进行编译

我在编译来自github项目的基于C++的jwt库的示例代码(示例代码)时遇到问题:jwt-cpp
我克隆了它，并使用README文件中提供的步骤对其进行了编译，似乎很成功。

之后，我做了一个ldconfig。

现在，我正在尝试构建示例代码。

#include <iostream>
#include "jwt/jwt_all.h"
using json = nlohmann::json;

int main()
{
    // Setup a signer
    HS256Validator signer("secret!");

    // Create the json payload that expires 01/01/2017 @ 12:00am (UTC)
    json payload = {{"sub", "subject"}, {"exp", 1483228800}};

    // Let's encode the token to a string
    auto token = JWT::Encode(signer, payload);

    std::cout << token << std::endl;
}

我用终端上的命令编译它，它说:

g++ -std=c++11 \
 -I/usr/local/include \
 -I/usr/local/include \
 /usr/local/lib/libjwt.a \
 /usr/local/lib/libcrypto.a \
 sign.cpp -o sign

并导致以下错误:

/tmp/ccX4ghoR.o: In function `main':
sign.cpp:(.text+0x24e): undefined reference to
JWT::Encode(MessageSigner const&, nlohmann::basic_json<std::map,
std::vector, std::__cxx11::basic_string<char, std::char_traits<char>,
std::allocator<char> >, bool, long, unsigned long, double,
std::allocator, nlohmann::adl_serializer> const&,
nlohmann::basic_json<std::map, std::vector,
std::__cxx11::basic_string<char, std::char_traits<char>,
std::allocator<char> >, bool, long, unsigned long, double,
std::allocator, nlohmann::adl_serializer>)'
/tmp/ccX4ghoR.o: In function
`HS256Validator::HS256Validator(std::__cxx11::basic_string<char,
std::char_traits<char>, std::allocator<char> > const&)': sign.cpp:(.text._ZN14HS256ValidatorC2ERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE[_ZN14HS256ValidatorC5ERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE]+0x21): undefined reference to `EVP_sha256' sign.cpp:(.text._ZN14HS256ValidatorC2ERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE[_ZN14HS256ValidatorC5ERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE]+0x5f): undefined reference to `HMACValidator::HMACValidator(std::__cxx11::basic_string<char,
std::char_traits<char>, std::allocator<char> > const&, evp_md_st
const*, std::__cxx11::basic_string<char, std::char_traits<char>,
std::allocator<char> > const&)'
/tmp/ccX4ghoR.o:
(.rodata._ZTV14HS256Validator[_ZTV14HS256Validator]+0x20): undefined
reference to `HMACValidator::Verify(nlohmann::basic_json<std::map,
std::vector, std::__cxx11::basic_string<char, std::char_traits<char>,
std::allocator<char> >, bool, long, unsigned long, double,
std::allocator, nlohmann::adl_serializer> const&, unsigned char
const*, unsigned long, unsigned char const*, unsigned long) const'
/tmp/ccX4ghoR.o:
(.rodata._ZTV14HS256Validator[_ZTV14HS256Validator]+0x28): undefined
reference to `HMACValidator::toJson[abi:cxx11]() const'
/tmp/ccX4ghoR.o:
(.rodata._ZTV14HS256Validator[_ZTV14HS256Validator]+0x38): undefined
reference to `MessageValidator::Accepts(nlohmann::basic_json<std::map,
std::vector, std::__cxx11::basic_string<char, std::char_traits<char>,
std::allocator<char> >, bool, long, unsigned long, double,
std::allocator, nlohmann::adl_serializer> const&) const'
/tmp/ccX4ghoR.o:
(.rodata._ZTV14HS256Validator[_ZTV14HS256Validator]+0x40): undefined
reference to `HMACValidator::Sign(unsigned char const*, unsigned long,
unsigned char*, unsigned long*) const'
/tmp/ccX4ghoR.o: In function `HS256Validator::~HS256Validator()':
sign.cpp:
(.text._ZN14HS256ValidatorD2Ev[_ZN14HS256ValidatorD5Ev]+0x20):
undefined reference to `HMACValidator::~HMACValidator()'
/tmp/ccX4ghoR.o:
(.rodata._ZTI14HS256Validator[_ZTI14HS256Validator]+0x10): undefined
reference to `typeinfo for HMACValidator'
collect2: error: ld returned 1 exit status

我尝试了什么:

我已仔细阅读以下问题，以了解自己可能做错了什么:
How to use libraries和compile C file using external library from linux terminal，尽管第二个问题专门针对C而不是C++，但那里的问题和解决方案也适用于C++。

我还尝试了命令行编译的变体，例如，

g++ -std=c++11 \
 -I/usr/local/include \
 -I/usr/local/include \
 -L/usr/local/lib/ \
 -lcrypto -ljwt \
 sign.cpp -o sign

我熟悉一个事实，当我执行-lfoo时，链接程序尝试在-L选项提供的位置找到libfoo.a。

我已经确认，用于编译的选项包含它们应包含的内容。
对于-I选项:
我可以在jwt中看到openssl和/usr/local/include目录
对于-L选项:
我可以在libjwt.a上看到libssl.a，libcrypto.a，libssl.so，/usr/local/lib/等

题:
编译此示例时我做错了什么？

最佳答案

当我告诉您问题时，您会踢自己。

将源文件sign.cpp 放在程序中的库声明之前:

g++ -std=c++11 -I/usr/local/include -L/usr/local/lib sign.cpp  -lcrypto -ljwt -o sign

原因是unix链接程序不会在命令行中向后看来解决依赖关系。有一些命令行开关(例如--start-group和--end-group)可以调整此行为，但是上述操作现在可以使您不受阻碍。 More details here。

在上面的示例中，我自由删除了重复的INCLUDE路径参数。您甚至可能不需要-I/usr/local/include或-L/usr/local/lib部分，因为它通常已经在标准编译器路径中。