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C program - Undefined reference error

Hi All,

I'm building my main program on SUSE using C, but the compiler returns the error below. Any suggestions? Thanks in advance.

Code:

undefined reference to `init_aacgm'

undefined reference to `convert_geo_coord'

Here's my code to which the errors are pointing to:

Code:

#include <stdio.h>

#include <string.h>



#define MAX_FNAME_SZ  200

#define LATEST_IGRF_YEAR  2000    

#define EARLIEST_IGRF_YEAR 1975    



extern int init_aacgm();

extern int convert_geo_coord();



static short int prev_year = 0;





long int cnvcoord_cover(year,in_lat,in_lon,height,out_lat,out_lon,m

short int year;

double in_lat; 

double in_lon; 

double height; 

double *out_lat;

double *out_lon;

short int mgflag;

{

  short int cur_year;

  char filename[MAX_FNAME_SZ];

  long int status;

  int err;

  char year_str[5];

  int order;

  short int mod_year;

  double out_lat_copy;

  double out_lon_copy;

  int int_mgflag;





  /* define constants */

  order = 10;  



  int_mgflag = mgflag;



  status = 0L;

  

  cur_year = year;

  if(cur_year < 50)

    cur_year = cur_year + 2000;



  if(cur_year < 1900)

      cur_year = cur_year + 1900;



  if(cur_year != prev_year) {



    

      if(filename == NULL) {

        fprintf(stderr,

        "aacgm_init_cover: missing environment variable: AACGM_DAT_

        status = -1L;

        return(status);

      }



  

      prev_year = cur_year;



      mod_year = cur_year % 5;



      if(mod_year <= 2)

        cur_year = cur_year - mod_year; 

      else

        cur_year = cur_year + 5 - mod_year;  



      

      if(cur_year > LATEST_IGRF_YEAR)

        cur_year = LATEST_IGRF_YEAR;



      if(cur_year < EARLIEST_IGRF_YEAR)



      err = init_aacgm(cur_year);

      if(err != 0) {

        fprintf(stderr,

        "cnvcoord_cover: init_aacgm error\n");

        status = -1L;

        return(status);

      }





  }





    err = convert_geo_coord(in_lat,in_lon,height,&out_lat_copy,

                                    &out_lon_copy,int_mgflag,order)

                                    

    if(err != 0) {

      status = -1L;

      *out_lat = 0.;

      *out_lon = 0.;

  }

  else {

      *out_lat = out_lat_copy;

      *out_lon = out_lon_copy;

  }





  return(status);

Undefined reference means the compiler cannot find the two functions in question. Do you recognise the two functions?

This is a problem with the linker - the compiler's come along and successfully turned your C source (.c) file into an object (.o) file, and now the linker's trying to turn your object file(s) into an executable file. I assume the two functions in question are defined by yourself in another file?

You have two options:

Compile, but do not link, both files by passing the -c option to gcc, and then link them afterwards:

Code:

$ gcc -c file1.c -o file1.o # compile first file $ gcc -c file2.c -o file2.o # compile second file $ gcc file1.o file2.o -o program # link
Get gcc to do the above work for you by just giving them both to gcc to organise compilation and linking:

Code:

$ gcc file1.c file2.c -o program # compile and link in one step

Either way, gcc has to be able to find all your symbols (i.e. see all the relevant object files) at link-time.

[shameless_plug] For more in-depth information, see the first few posts of my rather technical blog. [/shameless_plug];

Thank you both!

Compiles a-ok!