c - implicit declaration of functions - c

Question

This is my header file, tree.h
#ifndef TREE_H_
#define TREE_H_
#if defined treeItem
extern int totalnode;
treeItem *addItem(treeItem *node, char *w);
void printInOrder(treeItem *node, FILE *output);
void freeTree(treeItem *node);
#endif
#endif
This is the main() in main.c which include tree.h
#include <stdio.h>
#include <ctype.h>
#include <string.h>
#include <stdlib.h>
#include "tree.h"
#define MAX 1024
extern int totalnode;
int main(int argc, char *argv[]){
FILE *input;
FILE *output;
char *filename;
char ch[MAX];
//extern int totalnode;
struct treeItem *element;
element = NULL;
int i;
if (argc > 2){
output = fopen(argv[1], "w");
for(i = 2; i < argc + 1; i++){
filename = argv[i];
input = fopen(filename, "r");
if(input != NULL){
while(getword(ch, MAX, input) != EOF)
if (isalpha(ch[0]))
element = addItem(element, ch);
}
}
printInOrder(element, output);
fprintf(output,"-------------- \n ");
fprintf(output,"%4d Total number of different words",totalnode);
freeTree(element);
fclose(input);
fclose(output);
}
else{
printf("There is no input file.\n");
}
return 0;
}
compiler says:
../main.c: In function 'main':
../main.c:57: warning: implicit declaration of function 'addItem'
../main.c:57: warning: assignment makes pointer from integer without a cast
../main.c:60: warning: implicit declaration of function 'printInOrder'
../main.c:64: warning: implicit declaration of function 'freeTree'
another error: Undefined symbols for architecture x86_64:
"_totalnode", referenced from:
_main in main.o
_addItem in tree.o
ld: symbol(s) not found for architecture x86_64
collect2: ld returned 1 exit status
If I put all code in the same .c file without using header file, it works. But for now, it doesn't work. How can I fix it?

Answer 1

The line
#if defined treeItem
and the matching #endif should be removed from tree.h
Remember that preprocessing occurs conceptually before (or as the very first step of) the real compilation.
In general, you could have obtained the preprocessed form of main.c with
gcc -C -E main.c > main.i
and then look (e.g. with a pager like less) inside main.i
I often remove the generated preprocessor directives with
gcc -C -E main.c | grep -v '^#' > main.i
gcc -Wall -c main.i
this gives error messages with line numbers referring inside main.i (not main.c or tree.h) which is sometimes useful to debug macros. Another useful option to gcc is -H: it shows every #include-d file

Answer 2

In order to use the tree.h file 'as-is', and have it define items as you expect, you will have to define treeItem:
#include <stdio.h>
#include <ctype.h>
#include <string.h>
#include <stdlib.h>
/* You will need to define 'treeItem' prior to including 'tree.h'. */
typedef /* the defintition here */ treeItem;
#include "tree.h"
#define MAX 1024
extern int totalnode;
If you have the liberty of editing the tree.h file, it would probably be better to remove the line:
#if defined treeItem
and one of the lines:
#endif

Answer 3

Because you have pre compiler directives which exclude the substantive body of tree.h unless treeItem is defined, you must #define treeItem prior to the inclusion of tree.h within main.c, this is due to the way that precompiler directives are processed. This will solve the immediate problem.
I see that in main.c you have the line: struct treeItem *element;
If I am interpreting correctly that you intended this to satisfy the #ifdef treeItem line in your header, then you are misunderstanding how these # lines are like a separate language to C/C++. These lines are called precompiler directives, or more casually macros. Macros are commands for the preprocesser which operates over text files prior to compilation, for the purposes of creating in memory a full source code that can be compiled into an object that can later be linked with others to form your program. The preprocessor's defines are different from definitions in C/C++, and they don't directly interact. This line about the struct is practically invisible to the preprocessor, it knows nothing about C/C++. You must define this treeItem using #define, prior to the #ifdef.
More aesthetically, your header file should not guard like this, because it is superfluous. The main.c module's #include "tree.h" line alone is enough to indicate the intention of including the substantive body of tree.h. You have an appropriate guard against repeated inclusions, but the second guard regarding the treeItem definition seems unnecessary and is the cause of this issue. This latter paragraph is simply reiterating what other's have said, and would be technically superfluous as well if it does not contribute to persuading you of this opinion, however avoiding hypocrisy the first paragraph is a solution to your issue that does not intrude on your decision regarding said opinion.