moduler design with "extern keyword"

[asif_rahman0]

Can someone help me? just practicing with a program. It is compiling but not working when i run it. Here i paste it(three parts).
part 1

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////////////////////////
//file name is : m1.h
////////////////////////
#ifndef	__M1_H__
#define	__M1_H__

#include <stdlib.h>
#include <ctype.h>
#include <string.h>

extern char* LCase(char *str);
extern char* UCase(char *str);

#endif

part 2

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//////////////////////////
//file name is : m1.cpp
//////////////////////////
#include "m1.h"

char* UCase(char *str)
{
	int len=strlen(str);
	for(int i=0;i<len;i++){
		str[i]=toupper(str[i]);
	}
	return str;
}
char* LCase(char *str)
{
	int len=strlen(str);
	for(int i=0;i<len;i++){
		str[i]=tolower(str[i]);
	}
	return str;
}

part 3

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/////////////////////////
//file name is : main.h
/////////////////////////

#include <stdio.h>
#include "m1.h"

void main()
{
	printf("This is a test\n");
	printf("%s\n",UCase("This is a test of UCase"));
	printf("%s\n",LCase("This is a test of LCase"));
}

And when i run the program for *.EXE file then it shows the following error. i dont understand it.

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--------------------Configuration: main - Win32 Debug--------------------
Linking...
main.obj : error LNK2001: unresolved external symbol "char * __cdecl LCase(char *)" (?LCase@@YAPADPAD@Z)
main.obj : error LNK2001: unresolved external symbol "char * __cdecl UCase(char *)" (?UCase@@YAPADPAD@Z)
Debug/main.exe : fatal error LNK1120: 2 unresolved externals
Error executing link.exe.

main.exe - 3 error(s), 0 warning(s)

what is the reason? plz tell.

bye
rabbi

[Krzysztof Duleba]

You have to link m1.o with main.o (no idea how to do it in VC, but it should be easy).

[Kire Sopov]

Doesn't seem to have a problem in VS. I hope you meant "main.cpp" for the name of the file where the main() function is. I compiled it using VC++ 8.0 with default compiler settings. However, the program crashes during runtime. You should change the main file to something like the following:

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#include <stdio.h> 
#include "m1.h" 

void main() 
{
   char szUpper[] = "This is a test of UCase";
   char szLower[] = "This is a test of LCase";
   printf("This is a test\n"); 
   printf("%s\n",UCase(szUpper)); 
   printf("%s\n",LCase(szLower));

   // Writing to a char * declared like this:
   // char *pChar = "abcdefg";
   // is illegal
   // *pChar = 'x';   // <- runtime error
}