[SOLVED] confusion on variables in c

doughyi8u · 04-20-2015, 12:17 PM

I'm writing a program as an exercise out of a book. I keep getting an error that says "multiple definition of variable". I put the program w/ the compiler errors in a tar file and attached it.

Thanks for any help.

I'm having trouble attaching a file..can anyone point me in the right direction?

suicidaleggroll · 04-20-2015, 12:33 PM

How big is the program? If it's less than, say, a hundred lines you can just paste it between [code][/code] tags.

doughyi8u · 04-20-2015, 12:43 PM

Here's the pertinent files. Hopefullly this is enough.

Code:

#include "hotel.h" 


int
findfree()
{
	off_t offset;
	int j;
	ssize_t nread;

	for(j = 1; j <= NROOMS; j++)
	{
		offset = (j-1) * NAMELENGTH;
		if ( lseek(infile, offset, SEEK_SET) == -1) {
			perror("lseek");
			exit(1);
		} else {
			if ( (nread = read(infile, namebuf, NAMELENGTH)) <= 0) {
				perror("read?");
				exit(1);
			} else 
				namebuf[nread-1] = '\0';
		}
		if ( (isempty()) == 1) 
			return(j);

	}
	puts("No empty rooms");
	return -1;
}

#include "hotel.h"


int
freeroom(int roomno)
{
	off_t offset;
	ssize_t nread;

	offset = (roomno - 1) * NAMELENGTH;
	if ( lseek(infile, offset, SEEK_SET) == -1) {
		perror("lseek");
		exit(1);
	} else {
		if ( (nread = read(infile,namebuf, NAMELENGTH)) <= 0) {
			perror("read");
			exit(1);
		} else if (isempty())
				return -1;
		else {
			write(infile, empty, strlen(empty));
		}
	}
}

#include <string.h>
#include <stdio.h>
#include <fcntl.h>
#include <stdlib.h>
#include <unistd.h>

#define NAMELENGTH 21
#define NROOMS 10

#define DBUG

char *getoccupier(int);
int openfile();
int findfree();
int freeroom(int);
int isempty(); 

extern int infile;
extern char namebuf[NAMELENGTH];
const char *empty = "EMPTY";

Here's the errors I get when I try to compile this:
/tmp/cclQs7ZG.o:(.data+0x0): multiple definition of `empty'
/tmp/cc4F45dN.o:(.data+0x0): first defined here
/tmp/ccuGRWWA.o:(.data+0x0): multiple definition of `empty'
/tmp/cc4F45dN.o:(.data+0x0): first defined here
/tmp/ccQ7Tq4u.o:(.data+0x0): multiple definition of `empty'
/tmp/cc4F45dN.o:(.data+0x0): first defined here
/tmp/ccevNsmp.o:(.data+0x0): multiple definition of `empty'
/tmp/cc4F45dN.o:(.data+0x0): first defined here
collect2: error: ld returned 1 exit status

Hope this is enough.

johnsfine · 04-20-2015, 12:49 PM

You are defining (rather than just declaring) the variable empty in your .h file, then including that .h file in multiple .c files, causing the multiple definition error.

The correct way is to only declare the variable (using extern) in the .h file and define it in only one .c file.

doughyi8u · 04-20-2015, 02:33 PM

I'm having some trouble. I declared the variable in the .h file but am having trouble assigning to it.

hotel.h file:

Code:

extern const empty[6];

plain .c file:

Code:

empty = "EMPTY"; //this makes errors

How do I assign to this variable w/o errors?

suicidaleggroll · 04-20-2015, 02:41 PM

hotel.h:

Code:

extern const char *empty;

main.c:

Code:

const char *empty = "EMPTY";

millgates · 04-20-2015, 02:51 PM

Quote:

Originally Posted by doughyi8u

Code:

extern const empty[6];

In your .c file, you declared empty as char*. Here, however, you don't specify the type, so, depending on what standard you use, this will either default to int, or refuse to compile.

Quote:

Originally Posted by doughyi8u

[code]
plain .c file:

Code:

empty = "EMPTY"; //this makes errors

You can only "assign" to an array in initialization (in the .c file where it is defined).

doughyi8u · 04-20-2015, 02:54 PM

I tried that and am getting the following error:

Code:

/tmp/ccp52ey8.o:(.data+0x0): multiple definition of `empty'
/tmp/ccQeNdUZ.o:(.data+0x0): first defined here
collect2: error: ld returned 1 exit status
doughy@zoe:~/sourcery/c/usp/ch2/2_9$ exit

suicidaleggroll · 04-20-2015, 02:56 PM

Tried what?
The code I posted?

How exactly did you implement it?
That code I have listed for main.c should ONLY be in main.c, you should not re-declare the variable "empty" ANYWHERE else, just use it. Eg:

inc.h

Code:

#include <stdio.h>
extern const char *empty;
void other(void);

main.c

Code:

#include "inc.h"
const char *empty = "EMPTY";
int main(void) {
   printf("main: %s\n",empty);
   other();
   return 0;
}

other.c

Code:

#include "inc.h"
void other(void) {
   printf("other: %s\n",empty);
}

Code:

$ gcc -Wall *.c
$ ./a.out 
main: EMPTY
other: EMPTY

doughyi8u · 04-20-2015, 02:56 PM

tried your posted code....I mistakingly declared it in two files hence the error. Now I'm getting a seg fault.

suicidaleggroll · 04-20-2015, 03:02 PM

Compile with "-Wall -g -O0 -fbounds-check", then run in gdb and see if it sheds some light on where it's failing.

doughyi8u · 04-20-2015, 03:12 PM

I'm having trouble reading the compiler command...is that 0(zero)o?

millgates · 04-20-2015, 03:16 PM

That's an O (a letter) followed by a zero.

doughyi8u · 04-20-2015, 03:22 PM

I'm clueless as I've never used gdb. Any advice?

millgates · 04-20-2015, 03:24 PM

run

Code:

gdb programname

then

Code:

to run it inside the debugger, and when it crashes

Code:

bt

to get a backtrace.