Why can't the linker find main()?

Question

I'm getting the next error message:

 usr/lib/gcc/x86_64-linux-gnu/4.6/../../../x86_64-linux-gnu/crt1.o: In function `_start': (.text+0x20): undefined reference to `main' collect2: ld returned 1 exit status 

The only code I have is:

FILE *f = fopen("data/file.dat", "rb");
fseek(f, 0, SEEK_END);
long pos = ftell(f);
fseek(f, 0, SEEK_SET);

char *bytes = malloc(pos);
fread(bytes, pos, 1, f);
fclose(f);

Now, I come from a Java background but I've been Googling around, it says I might be missing a reference but I don't know what could it be, I even added the #include <stdio.h> and I read something about adding an extern but I have no idea where since I have no other files unless I need to refer to the .dat file?

EDIT I also tried in a point to cast the byte array (char*)malloc(pos); but it didn't help either.

EDIT 2 the whole code is using the NS-3 framework but everything compiled perfectly until I added these lines. It looks something like this:

#include "ns3/core-module.h"
#include "ns3/point-to-point-module.h"
#include "ns3/network-module.h"
#include "ns3/applications-module.h"
#include "ns3/wifi-module.h"
#include "ns3/mobility-module.h"
#include "ns3/csma-module.h"
#include "ns3/internet-module.h"

using namespace ns3;

NS_LOG_COMPONENT_DEFINE ("ThirdScriptExample");

int 
main (int argc, char *argv[])
{
      .....
//STARTS FILE READING
  FILE *f = fopen("data/Terse_Jurassic_10_14_18.dat", "rb");
  fseek(f, 0, SEEK_END);
  long pos = ftell(f);
  fseek(f, 0, SEEK_SET);

  char *bytes = (char*)malloc(pos);
  fread(bytes, pos, 1, f);
  fclose(f);

  Simulator::Stop (Seconds (10.0));

  pointToPoint.EnablePcapAll ("third");
  phy.EnablePcap ("third", apDevices.Get (0));
  csma.EnablePcap ("third", csmaDevices.Get (0), true);

  Simulator::Run ();
  Simulator::Destroy ();
  return 0;
}

The compiler error message is:

 [1888/1930] cxxprogram: -> build/scratch/data/data /usr/lib/gcc/x86_64-linux-gnu/4.6/../../../x86_64-linux-gnu/crt1.o: In function `_start': (.text+0x20): undefined reference to `main' collect2: ld returned 1 exit status Build failed -> task in 'data' failed (exit status 1): {task 43470800: cxxprogram -> data} 

I am extremely sure the NS-3 code (both the part I didn't add due to code lines and the the one after reading the file works because before adding the part to read the file everything worked perfectly.

Answer 1

The only code I have is

Eiher you didn't put this code into main function (which you are required to have), or your compiler/linker invocation is incorrect. It's impossible to say which, given the details you've supplied.

Also, your question has incorrect title: you don't have a problem reading a file, you have a problem linking your program (that intends to read a file).

Answer 2

Your program must contain one main() function.

#include <stdio.h>
#include <string.h>
int main(int argc, char *argv[])
{
    FILE *f = fopen("data/file.dat", "rb");
    fseek(f, 0, SEEK_END);
    long pos = ftell(f);
    fseek(f, 0, SEEK_SET);

    char *bytes = malloc(pos);
    fread(bytes, pos, 1, f);
    fclose(f);
    free(bytes);
    return 0;
}