Given 2 files, for examples:
file1.c :
int main(){
f();
return 0;
}
file2.c:
void f(){
return;
}
Why I can't call f from file1.c like that?
Because first you need to tell the compiler (declare) that it exists somewhere:
void f(); //function declaration
int main()
{
f();
return 0;
}
Usually, though, it is better to put such declarations in a separate header file (e.g. file2.h) so that later you could include this file (e.g. #include "file2.h") instead of duplicating such declaration in every other file where you need this function.
The problem is that file1.c does not "know" that the function f exists. You need to use a prototype. The standard way is to put prototypes in header files and definitions in .c files.
It could look like this:
file1.c:
#include "file2.h"
int main(){
f();
return 0;
}
file2.h:
#ifndef FILE2_H
#define FILE2_H
void f();
#endif
file2.c:
#include "file2.h"
void f(){
return;
}
Related
Can anyone explain how to create a header file in C with a simple example from beginning to end.
foo.h
#ifndef FOO_H_ /* Include guard */
#define FOO_H_
int foo(int x); /* An example function declaration */
#endif // FOO_H_
foo.c
#include "foo.h" /* Include the header (not strictly necessary here) */
int foo(int x) /* Function definition */
{
return x + 5;
}
main.c
#include <stdio.h>
#include "foo.h" /* Include the header here, to obtain the function declaration */
int main(void)
{
int y = foo(3); /* Use the function here */
printf("%d\n", y);
return 0;
}
To compile using GCC
gcc -o my_app main.c foo.c
#ifndef MY_HEADER_H
# define MY_HEADER_H
//put your function headers here
#endif
MY_HEADER_H serves as a double-inclusion guard.
For the function declaration, you only need to define the signature, that is, without parameter names, like this:
int foo(char*);
If you really want to, you can also include the parameter's identifier, but it's not necessary because the identifier would only be used in a function's body (implementation), which in case of a header (parameter signature), it's missing.
This declares the function foo which accepts a char* and returns an int.
In your source file, you would have:
#include "my_header.h"
int foo(char* name) {
//do stuff
return 0;
}
myfile.h
#ifndef _myfile_h
#define _myfile_h
void function();
#endif
myfile.c
#include "myfile.h"
void function() {
}
header files contain prototypes for functions you define in a .c or .cpp/.cxx file (depending if you're using c or c++). You want to place #ifndef/#defines around your .h code so that if you include the same .h twice in different parts of your programs, the prototypes are only included once.
client.h
#ifndef CLIENT_H
#define CLIENT_H
short socketConnect(char *host,unsigned short port,char *sendbuf,char *recievebuf, long rbufsize);
#endif /** CLIENT_H */
Then you'd implement the .h in a .c file like so:
client.c
#include "client.h"
short socketConnect(char *host,unsigned short port,char *sendbuf,char *recievebuf, long rbufsize) {
short ret = -1;
//some implementation here
return ret;
}
I have a problem about a program. I bet that it has to do with the fact that I use static. Here is my t.h
static int cnt;
void f();
my main.c
#include <stdio.h>
#include "t.h"
void main()
{
cnt=0;
printf("before f : cnt=%d\n",cnt);
f();
printf("after f : cnt=%d\n",cnt);
}
and finally my f.c
#include "t.h"
void f()
{
cnt++;
}
The printf prints cnt=0 both times. How is this possible when I do cnt++? Any ideas?
Thanks in advance
In C, static means "Local to the module"
Take note, that the #include statements just pastes the header file in the including file.
therefore, you are creating two distinct symbols (happens to have the same logical name) in different modules.
f.c cnt is a different cnt then main.c
Note:
static in C has different meaning then its C++ counterpart.
and because C++ is C Compatible, static outside a class have the same meaning as in C
Edit:
In your case, you don't want a static you want a variable, but i guess you had problem with the Linker telling you about "ambiguous symbols".
I would suggest to declare an extern in the header file, and declare the actual variable in a module.
t.h
extern int cnt; // declaration of the variable cnt
main.cpp
#include
#include "t.h"
void main()
{
cnt=0;
printf("before f : cnt=%d\n",cnt);
f();
printf("after f : cnt=%d\n",cnt);
}
t.cpp
#include "t.h"
int cnt = 0; // actual definition of cnt
void f()
{
cnt++;
}
Data should not be defined in the header files.
In your example you will create a separate copy of that static variable in every compilation module which includes this .h file.
Don't define cnt in your header file. Instead, define it in f.c:
#include "t.h"
int cnt = 0;
void f(){
cnt++;
}
Then in main.c, add the following before the beginning of your main function:
extern int cnt;
I'm a beginner into Linking, lets say I have two .c files
file1.c is
#include <stdio.h>
#include "file2.c"
int main(int argc, char *argv[])
{
int a = function2();
printf("%d",a);
return 0;
}
and file2.c is
int function2()
{
return 2018;
}
when I compiled, there is a linker error which is multiple definition of function2, but I only define function once in file2.c?
You should create a header file, "file2.h", with:
int function2(void);
and a file "file2.c" with the function: "file2.h" with:
#include "file2.h"
int function2(void)
{
return 2018;
...
}
Then in your main you have to include the header with:
#include "file2.h"
Keep care that all those files should be in the same folder to avoid any link problem
Try something like this:
#include <stdio.h>
#include "file2.h"
int main(int argc, char *argv[])
{
int a = function2();
printf("%d",a);
return 0;
}
file2.h:
extern int function2(void);
and file2.c is
#include "file2.h"
int function2(void)
{
return 2018;
}
And then link it together.
The statement #include "file2.c" effectively incorporates the contents of file2.c into file1.c. Then file1.c is compiled as if it contains:
int function2()
{
return 2018;
}
Those lines define function2; they tell the compiler “Here is function2, create code for it.” Because those lines effectively appear in both file1.c and file2.c, your program has two copies of function2.
Instead, you should create file2.h that contains:
int function2();
That line tells the compiler “There exists a function called function2, but its definition is somewhere else.”
Then, in file1.c, use #include "file2.h" instead of #include "file2.c". This will tell the compiler, while file1.c is being compiled, what it needs to know to compile a call to function2. The compiler will have the declaration it needs, but it will not have the definition, which is not needed in file1.c.
Also, in file2.c, insert #include "file2.h". Then file2.c will contain both a declaration of function2 (from file2.h) and a definition of function2 (from the actual lines in file2.c). The purpose of this is so the compiler can see both the declaration and the definition while it is compiling file2.c, so it can warn you if there is a typographical error that makes them incompatible.
Additionally, in C, you should use int function2(void) rather than int function2(). For historic reasons, the latter leaves the parameters unspecified. Using (void) tells the compiler there are no parameters.
Why we don't use extern when using function from one .c file in another .c file , but we must do extern for variables case? Is it related to linker?
Functions are extern qualified by default (unless you change it to internal with static). For example,
int func(void) {
}
extern int func2(void) {
}
Both func and func2 are external. The extern keyword is optional for external functions.
Actually, function names act just like variable names, but function prototypes are extern by default.
From cpprerefence:
If a function declaration appears outside of any function, the identifier it introduces has file scope and external linkage, unless static is used or an earlier static declaration is visible.
you can create a .hfile,declare functions you want to use in the other .c files and #include the .hfile in the other .c files.
Demo program,
one.c
#include "one.h"
void func1() //defination
{
//code
}
one.h
void func1(); //declaration
main.c
#include <stdio.h>
#include "one.h"
int main()
{
func1();
}
Then compile program in Gcc Linux : gcc main.c one.c
Yes, Let consider you have one .c file as process.c and you declared it in process.h . Now if you want to use the function from process.c to suppose tools.c then simply #include "process.h" in tools.c and use ther function. The process.h and process.c file should be in your project.
process.c file
#include<stdio.h>
#include<conio.h>
#include "process.h"
unsigned int function_addition(unsigned int a, unsigned int b)
{
unsigned int c = 0;
c = a + b;
return c;
}
process.h:
<bla bla bla >
unsigned int function_addition(unsigned int a, unsigned int b);
<bla bla bla >
tools.c file:
#include<stdio.h>
#include<conio.h>
#include "process.h"
my_tools()
{
unsigned int X = 1, Y = 9, C = 0;
C = function_addition(X,Y);
}
All these files are in one project.
I'm getting started with C programming. I currently have a large file that contains a lot of functions. I would like to move these functions to a separate file so that the code is easier to read. However, I can't seem to figure out how to properly include/compile and can't find an example in any online tutorials that I've found. Here's a simplified example:
#include <stdlib.h>
#include <stdio.h>
void func1(void) {
printf("Function 1!\n");
}
void func2(void) {
printf("Function 2!\n");
}
int main(void) {
func1();
func2();
return 0;
}
How do you move C functions into a separate file? FYI: I'm using gcc.
Update: These answers are very helpful, thank you. Now it seems that my simplified example is not good enough because I realized the reason my program failed to compile is because I'm using a global variable in my functions.
#include <stdlib.h>
#include <stdio.h>
int counter = 0;
void func1(void) {
printf("Function 1!\n");
counter++;
}
int main(void) {
func1();
return 0;
}
Moving these functions to an external file doesn't work because they need to reference this global variable:
#include <stdlib.h>
#include <stdio.h>
#include "functions.c"
int counter = 0;
int main(void) {
func1();
counter = 100;
return 0;
}
How can I get around this issue?
Okay. Here we go.
Your main.c file
#include <stdlib.h>
#include <stdio.h>
#include "functions.h"
int main(void) {
func1();
func2();
return 0;
}
Your functions.h file
void func1(void);
void func2(void);
Your functions.c file
#include "functions.h"
void func1(void) {
printf("Function 1!\n");
}
void func2(void) {
printf("Function 2!\n");
}
Compile it with:
gcc -o main.exe main.c functions.c
The most common way is to place your function prototypes in a header file and your function implementations in a source file. For example:
func1.h
#ifndef MY_FUNC1_H
#define MY_FUNC1_H
#include <stdio.h>
// declares a variable
extern int var1;
// declares a function
void func1(void);
#endif
func1.c
#include "func1.h"
// defines a variable
int var1 = 512;
// defines a function
void func1(void) {
printf("Function 1!\n");
}
func2.h:
#ifndef MY_FUNC2_H
#define MY_FUNC2_H
#include <stdio.h>
void func2(void);
#endif
func2.c:
#include "func1.h" // included in order to use var1
#include "func2.h"
void func2(void) {
printf("Function 2 with var1 == %i\n", var1);
}
main.c:
#include <stdio.h>
#include "func1.h"
#include "func2.h"
int main(void) {
var1 += 512;
func1();
func2();
return 0;
}
You would then compile using the following:
gcc -c -o func1.o func1.c
gcc -c -o func2.o func2.c
gcc -c -o main.o main.c
gcc -o myprog main.o func1.o func2.o
./myprog
I only placed one function in each source/header pair for illustration. You could create just one header which includes the prototypes for all of the source files, or you could create multiple header files for each source file. The key is that any source file which will call the function, needs to include a header file which includes the function's prototype.
As a general rule, you only want a header file included once, this is the purpose of the #ifndef #define #endif macros in the header files.
First you have to learn the difference between a declaration and definition. A declaration tells the compiler that something, like a function, exists. A definition is, for the case of functions, the actual function implementation.
So what you do is move the definition to another file, but add a declaration in the file where the function is to be called. You then build both files together, and the compiler and linker will take care of the rest.
You can do something like this.
/* func1.c */
void func1(void) {
printf("Function 1!\n");
}
/* func2.c */
void func2(void) {
printf("Function 2!\n");
}
/* main.c */
#include "func1.c"
#include "func2.c"
int main ( void )
{
func1();
func2();
return 0;
}