c++ implicit entry/start for main executable error

Question

I have a problem in running Hadoop pipes code on Mac. This is my c++ code.

#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;
}

My makefile is:

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 $@

When I tried to compile the source file, I got an error saying that

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

I don't know what this means because I already have a main function.

could you solve this problem?

Thanks!

what happens if you just compile it directly without using makefile? — Joseph D., Apr 20 '18 at 04:10
if I just compile it using "g++ word count.cpp",there'll be another error:wordcount.cc:6:10: fatal error: 'hadoop/Pipes.hh' file not found #include "hadoop/Pipes.hh" — Joon, Apr 20 '18 at 08:23

score 1 · Accepted Answer · answered Apr 20 '18 at 05:44

You didn't pass the source file to the recipe, the easiest way to avoid this here is to just rely on the built-in make rule for one-shotting programs, all you need is:

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:

The make implicit rule for %: %.cpp will take care of the rest. Note that you're using pthread incorrectly, you need to pass the pthread option to the preprocessor and linker, not the lib.

c++ implicit entry/start for main executable error

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