Compilation problem in the standard x86_64 libraries

Question

Hi everyone,

I am having trouble compiling a program I have written. I have two different files with the same includes but only one generates the following error when compiled with g++

/usr/lib/gcc/x86_64-linux-gnu/4.4.1/../../../../lib/crt1.o: In function `_start':
/build/buildd/eglibc-2.10.1/csu/../sysdeps/x86_64/elf/start.S:109: undefined reference to `main'
collect2: ld returned 1 exit status

The files I am including in my header are as follows:

#include <google/sparse_hash_map>
using google::sparse_hash_map;

#include <ext/hash_map>
#include <math.h>
#include <iostream>
#include <queue>
#include <vector>
#include <stack>

using std::priority_queue;
using std::stack;
using std::vector;

using __gnu_cxx::hash_map;
using __gnu_cxx::hash; 

using namespace std;

Searching the internet for those two lines hasn't resulted in anything to help me. I would be very grateful for any advice. Thank you

Answer 1

You need a main function and you don't have one. If you do have a main function, show more code please.

Answer 2

It looks like main is not defined. Do you have one defined for your second program? Can you post more details about the source body that fails to link?

Answer 3

To build two separate programs you need both source files to define main() function.

To build a single program out of two source files - first compile each file with -c options (compile only) - you will get two .o files, then link these files together. Something like this:

$ g++ -Wall -pedantic -ggdb -O -c -o module0.o module0.cpp
$ g++ -Wall -pedantic -ggdb -O -c -o module1.o module1.cpp
$ g++ -Wall -pedantic -ggdb -O -o prog module0.o module1.o

to build binary prog from two source files.

If you need to link with some library, you'll have to point compiler to it's headers with -I and to objects with -L flags, then tell the linker to actually reference the library with -l.

Hope this helps.