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Old 07-21-2010, 10:32 AM   #1
LQ Newbie
Registered: Jun 2010
Posts: 15

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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.
undefined reference to `init_aacgm'
undefined reference to `convert_geo_coord'
Here's my code to which the errors are pointing to:
#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) {
        "aacgm_init_cover: missing environment variable: AACGM_DAT_
         status = -1L;

      prev_year = cur_year;

      mod_year = cur_year % 5;

      if(mod_year <= 2)
         cur_year = cur_year - mod_year; 
         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) {
        "cnvcoord_cover: init_aacgm error\n");
         status = -1L;


     err = convert_geo_coord(in_lat,in_lon,height,&out_lat_copy,
     if(err != 0) {
      status = -1L;
      *out_lat = 0.;
      *out_lon = 0.;
   else {
      *out_lat = out_lat_copy;
      *out_lon = out_lon_copy;

Old 07-21-2010, 10:36 AM   #2
harry edwards
Registered: Nov 2007
Location: Lincolnshire, UK
Distribution: CentOS, Fedora, and Suse
Posts: 365

Rep: Reputation: 48
Undefined reference means the compiler cannot find the two functions in question. Do you recognise the two functions?
Old 07-21-2010, 10:55 AM   #3
Registered: Oct 2009
Posts: 467

Rep: Reputation: 139Reputation: 139
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:
    $ 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:
    $ 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];

Last edited by JohnGraham; 07-21-2010 at 10:57 AM.
Old 07-23-2010, 02:20 PM   #4
LQ Newbie
Registered: Jun 2010
Posts: 15

Original Poster
Rep: Reputation: 0
Thank you both!

Compiles a-ok!


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