Undefined reference to a user defined function

Question

I have seen the standard Undefined Reference to thread from this site but I do not believe it solves my problem. I am not putting header guards on my .cpp files, but still get an undefined reference to a user defined function. Here are my files:

(1) pth_funs.h

// hello from thread <pid>
void* hello(void* ptr);

(2) pth_funs.cpp

#include <stdio.h>

void* hello(void *ptr)
{
  char *message;
  int pid = (long) ptr;
  printf("Hello from thread %i\n", pid);
}

(3) structs.h

#ifndef STRUCTS_H
#define STRUCTS_H
struct grd_str {
  long nx;
  long ny;
  long natoms;
  char** atnames;
  double* xs;
  double* ys;
  double** fs;
  double** xyzs;
};
#endif

(4) fio.h

#ifndef FIO_H
#define FIO_H
#include <iostream>
#include <string.h>
#include "structs.h"

void read_grd(std::string, grd_str);

#endif

(5) fio.cpp

#include <string.h>
#include "structs.h"
#include "fio.h"

void read_grd( std::string fname, grd_str &grd)
{
  grd.nx = 10;
  grd.ny = 10;
}

(6) and finally, xdriver.cpp

#include <iostream> // needed for cout, endl, etc
using namespace std; // needed for cout, endl, etc
#include <pthread.h> // needed for pthreads
#include <string.h> // string handling

#include "pth_funs.h" // pthread function headers
#include "structs.h"
#include "fio.h"

int main(int argc, char** argv)
{
  // thread stuff
  int nthreads = 4;
  pthread_t rank[4];
  int iret[4];

  // file stuff
  string base_dir = "D:\\cygwin64\\home\\Robert\\code\\C\\form_reconstruction\\data\\";
  string fname;

  // topology stuff
  int nx, ny;
  double* xs;
  double* ys;
  double** fs;
  grd_str grd;

  for(long tid = 0; tid < nthreads; tid++)
  { iret[tid] = pthread_create( &rank[tid], NULL, hello, (void*) tid); }

  fname = base_dir;
  fname.append("adf\\adf.6.grd");
  cout << "Filename: " << fname << endl;

  read_grd(fname, grd);
}

I am compiling this using a Makefile which is as follows:

cc=g++
exe=create_grd.exe

flags=-pthread
hds= pth_funs.h fio.h structs.h
objs= pth_funs.o fio.o

all: create_grd.exe

create_grd.exe: xdriver.cpp $(hds) $(objs)
  $(cc) -o $(exe) $(objs) xdriver.cpp

pth_funs.o: pth_funs.cpp pth_funs.h
  $(cc) -c pth_funs.cpp $(flags)

fio.o: fio.cpp fio.h
  $(cc) -c fio.cpp $(flags)

clean:
  rm -rf *.o

However, upon compilation I get

g++ -c pth_funs.cpp -lpthread
g++ -c fio.cpp -lpthread
g++ -o create_grd.exe pth_funs.o fio.o xdriver.cpp -lpthread
/tmp/ccdaBayB.o: In function `main':
xdriver.cpp:(.text+0x16f): undefined reference to `read_grd(std::basic_string<char, std::char_traits<char>, std::allocator<char> >, grd_str)'
collect2: ld returned 1 exit status
make: *** [create_grd.exe] Error 1

but I have no idea why my main routine can't find read_grd since I believe I am properly defining it and including it. What am I doing wrong?

Answer 1

Your declaration and definition of read_grd do not have matching arguments. One takes a grd_str as its second argument, the other takes a grd_str&. Since xdriver.cpp includes fio.h, it sees and attempts to use the former function, but the linker can't find a definition for it anywhere. Chances are you want to change your declaration in fio.h to:

void read_grd(std::string, grd_str&);

Now the definition for this function is provided by fio.cpp.