C++ 隐式进入/启动主要可执行错误

Question

我在 Mac 上运行 Hadoop 管道代码时遇到问题。这是我的 C++ 代码。

#include <algorithm>
#include <limits>
#include <stdint.h>
#include <string>

#include "Pipes.hh"
#include "TemplateFactory.hh"
#include "StringUtils.hh"
using namespace std;

class MaxTemperatureMapper : public HadoopPipes::Mapper {
public:
  MaxTemperatureMapper(HadoopPipes::TaskContext& context) {
  }
  void map(HadoopPipes::MapContext& context) {
    std::string line = context.getInputValue();
    std::string year = line.substr(15, 4);
    std::string airTemperature = line.substr(87, 5);
    std::string q = line.substr(92, 1);
    if (airTemperature != "+9999" &&
        (q == "0" || q == "1" || q == "4" || q == "5" || q == "9")) {
      context.emit(year, airTemperature);
    }
  }
};

class MapTemperatureReducer : public HadoopPipes::Reducer {
public:
  MapTemperatureReducer(HadoopPipes::TaskContext& context) {
  }
  void reduce(HadoopPipes::ReduceContext& context) {
    int maxValue = INT_MIN;
    while (context.nextValue()) {
      maxValue = std::max(maxValue, HadoopUtils::toInt(context.getInputValue()));
    }
    context.emit(context.getInputKey(), HadoopUtils::toString(maxValue));
  }
};

int main(int argc, char * argv[]) {

    int i=HadoopPipes::runTask(HadoopPipes::TemplateFactory<WordCountMap, WordCountReduce>()); // 运行任务
    return 0;
}

我的 makefile 是:

wordcount :wordcount.cpp
    g++ -Wall -I/Users/macbookpro/Documents/hadoop-2.7.5/include -L/Users/macbookpro/Documents/hadoop-2.7.5/lib/native -lhadooppipes -lhadooputils -lpthread -lcrypto -lssl -g -O2 -o $@

当我尝试编译源文件时，出现错误提示

Undefined symbols for architecture x86_64: "_main", referenced from: implicit entry/start for main executable ld: symbol(s) not found for architecture x86_64 clang: error: linker command failed with exit code 1 (use -v to see invocation) make: *** [wordcount] Error 1

我不知道这意味着什么，因为我已经有了一个 main 函数。

你能解决这个问题吗？

谢谢!

Answer 1

您没有将源文件传递给配方，避免这种情况的最简单方法是仅依靠一次性程序的内置 make 规则，您所需要的只是:

CPPFLAGS := -I/Users/macbookpro/Documents/hadoop-2.7.5/include -pthread
CXXFLAGS := -Wall -g -O2
LDFLAGS  := -L/Users/macbookpro/Documents/hadoop-2.7.5/lib/native -pthread 
LDLIBS   := -lhadooppipes -lhadooputils -lcrypto -lssl

wordcount:

%: %.cpp 的 makeimplicit 规则将处理剩下的事情。请注意，您错误地使用了 pthread，您需要将 pthread 选项传递给预处理器和链接器，而不是库。