Question

I’m working on my exam assignment. It’s due in almost 6 hours. Suddenly my program won’t compile anymore with this error message:

gcc -g -D DEBUG -c -o obj/stringops.o src/stringops.c
gcc -g -D DEBUG -c -o obj/arrayops.o src/arrayops.c
gcc -g -D DEBUG -c -o obj/fileops.o src/fileops.c
gcc -g -D DEBUG -c -o obj/builtins.o src/builtins/*.c
gcc -g -D DEBUG -c -o obj/tomashell.o src/tomashell.c
gcc -g -D DEBUG -o bin/tomashell \
                obj/stringops.o obj/arrayops.o obj/fileops.o obj/builtins.o \
                obj/tomashell.o
obj/tomashell.o: In function `n_processes':
/root/sc/tomashell/src/safefork.c:11: multiple definition of `h_meta'
obj/builtins.o:/root/sc/tomashell/src/builtins/history.c:4: first defined here
obj/tomashell.o: In function `n_processes':
/root/sc/tomashell/src/safefork.c:11: multiple definition of `h_meta_len'
obj/builtins.o:/root/sc/tomashell/src/builtins/history.c:4: first defined here
collect2: ld returned 1 exit status
make: *** [bin/tomashell] Error 1

In this file:

#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
#include <sys/errno.h>

extern int errno;

#define MAX_PROCESSES 6

static int n_processes(void)
{ // <- THIS IS LINE 11
    return system("exit `ps | wc -l`")/256;
}


pid_t safefork(void)
{
  static int n_initial = -1;

  if (n_initial == -1)
    n_initial = n_processes();
  else if (n_processes() >= n_initial+MAX_PROCESSES) {
    sleep(2);
    errno = EAGAIN;  return (pid_t)-1;
  }

  return fork();
}

Someone please help me or kill me. I don’t want to live in a world where this sort of error is possible.

Any ideas to what might be wrong?

builtins/history.c
builtins/history.h

Answer 1

Like the others have said, you should not “define” (either implicitly or explicitly allocate space for) a variable in your headers.

This example might help:

#ifndef HISTORY
#define HISTORY
...

/* Bad!  Don't actually DEFINE (allocate space for) variables in a header!
 * h_metablock* h_meta = NULL;
 * int h_meta_len = 0;
 */

/* Better: declare "extern", then define in exactly ONE module (e.g. "main.c") */
extern h_metablock* h_meta;
extern int h_meta_len;
...

The same, of course, goes for any globals you might define in your .c/.cpp files.

A global can only be “defined” exactly once.