implicit declaration of functions? - c

Question

I'm still learning C and I understand that to get rid of most implicit declaration warnings, you add the prototype header at the beginning. But I'm confused as to what you do when you have outside methods being used in your code.
This is my code when I'm using the outside methods
#include <stdio.h>
#include <string.h>
int main(void)
{
int arrayCapacity = 10;
int maxCmdLength = 20;
int A[arrayCapacity];
int count = 0; /* how many ints stored in array A */
char command[maxCmdLength + 1];
int n;
while (scanf("%s", command) != EOF)
{
if (strcmp(command, "insert") == 0)
{
scanf("%d", &n);
insert (n, A, arrayCapacity, &count);
printArray(A, arrayCapacity, count);
}
else if (strcmp(command, "delete") == 0)
{
scanf("%d", &n);
delete(n,A,&count);
printArray(A, arrayCapacity, count);
}
else
{
scanf("%d", &n);
printArray(A, arrayCapacity, count);
}
}
return 0;
}
The methods printArray, insert, and delete are all in the form of: printArray.o, insert.o, delete.o
This is how I compiled my program: gcc -Wall insert.o delete.o printArray.o q1.c
and I get these warnings:
q1.c: In function âmainâ:
q1.c:20: warning: implicit declaration of function `insert'
q1.c:21: warning: implicit declaration of function `printArray'
q1.c:30: warning: implicit declaration of function `delete'
I've tried including this in headers but I get errors saying file or directory not found.
Any help appreciated.

Answer 1

Put them in a header file foo.h like so:
extern void printArray(int *A, int capacity, int count);
...
then include that file in your source
#include "foo.h"

Answer 2

You need to include the correct headers to get rid of such warnings.
If you get a "file not found" error, try to include them as
#include "myheader.h"
and put your header files in the same directory as your source code.
Generally speaking, #include "file" is for programmer-defined headers while #include <file> is or standard headers.

Answer 3

You should be able to just put in the function prototype at the top of the file like you do for other functions in the same file. The linker should take care of the rest.

Answer 4

Where did you get those .o files from? If you have written them yourself, then you should create the corresponding .h files. If you got these files from somewhere else, then you should search for the headers in the same place.

Answer 5

If all called functions are written before the main() function the compiler will know their name, return type and parameter signature and can match all three of these properties with each following function invocation.
Some programmers like to write a function signature first, and do the implementation at a later time.
The only time a function declaration is essential is when using co-routines: functionA invokes functionB which in turn invokes functionA.
Done as follows:
type a(...signatureOfA...)
/* compiler now knows about a() */
type b(...signatureOfB...)
{…
// implementation of b
a(…arguments for a…);
/* compiler knows about above */
…}
type a(...signatureOfA...)i
{…
// implementation of a
b(…arguments for b…);
/* compiler knows about above */
…}
int main()
{
a(… arguments for a…);
/* compiler knows */
return(code);
}