I have this folder structure:
I wrote a simple function to test the folder structure, for seen if all header file but when I compile with the make command, I have this error:
warning: function 'stampa' has internal linkage but is not defined
I the lagrange.h file I have this:
#ifndef LAGRANGE_H
static void stampa(int i);
#endif /* LAGRANGE_H */
and in the lagrange.c file I have this:
#include <stdlib.h>
#include <stdio.h>
#include <math.h>
#include <stdio.h>
void stampa(int i){
printf("%d", i);
}
in the end, int main.c I simply call the function stampa passing them a number.
Explanation of the error:
The compilation error occurs because:
You have declared a function with static keyword (i.e. with internal linkage) inside lagrange.h
And you have included this file lagrange.h in main.c file different from lagrange.c, where the function is not defined.
So when the compiler compiles main.c file, it encounters the static declaration without any associated definition, and raises logically the error. The error message is explicit.
Solution:
In your case the solution is to remove the static keyword, because static means that the function can be called only in the .c file where it is defined, which is not your case.
Moreover, a good practice may be to declare any static function in the same .c file where it is defined, and not in .h file.
Related
below is the code:
//test.h
...
extern int globalVariable;
...
//test.c
#include "test.h"
...
int globalVariable = 2020;
...
//main.c
#include <stdio.h>
#include "test.h"
int main()
{
printf("Value is %d", globalVariable);
}
let's say in a scenario, there are hundreds of variables are declared in test.h and globalVariable is just one of them.
since there are two many variables, I easily makes a typo error in test.c as:
#include "test.h"
int globalVariables = 2020; //extra 's' in the name which contradicts the declaration of its counterpart in test.h
if I compile(only compile,not linking them) test.c, test.h and main.c, it compiles and shows no error. the unresolved error will only occur when linker involved in the linking stage.
But in a large application, I might just write some modules without the need of linking all existing to an executable file, so it would be better the compiler throw an error in the compile stage to indicate the error so I can correct them asap, so how can I let the compiler force the source file implement definition for a header file?
You could also use the preprocessor
test.h:
#ifndef TEST_C_IMPLEMENTATION
#define DEFINE_AND_INIT_VARIABLE(type, name, value) \
extern type name;
#else
#define DEFINE_AND_INIT_VARIABLE(type, name, value) \
type name = value;
#endif
DEFINE_AND_INIT_VARIABLE(int, globalVariable, 2020);
test.c:
#define TEST_C_IMPLEMENTATION
#include "test.h"
This technique can be taken even further - there are small utility libraries that are shipped as a single include file; you're just to set a macro in one of the translation units to force the implementation to be compiled in there.
The declaration extern int globalVariable; says that the variable exists somewhere, but not necessarily in the current translation unit. So any source file that includes the header containing this declaration will know that the variable exists without needing the full definition.
When you then get to the linking stage is when you'll get the error regarding glovalVariable being undefined. Since the variables is declared in test.h, convention would dictate that the definition would be in test.c. Upon inspecting that file, you would then find that no such variable exists and could then either add it or find the typo and fix it.
I'm a beginner learning c. I know that use of word "static" makes a c function and variable local to the source file it's declared in. But consider the following...
test.h
static int n = 2;
static void f(){
printf("%d", n);
}
main.c
#include <stdio.h>
#include "test.h"
int main()
{
printf("%d", n);
f();
return 0;
}
My expected result was that an error message will throw up, since the function f and variable n is local to test.h only? Thanks.
But instead, the output was
2
2
EDIT:
If it only works for a compilation unit, what does that mean? And how do I use static the way I intended to?
static makes your function/variable local to the compilation unit, ie the whole set of source code that is read when you compile a single .c file.
#includeing a .h file is a bit like copy/paste-ing the content of this header file in your .c file. Thus, n and f in your example are considered local to your main.c compilation unit.
Example
module.h
#ifndef MODULE_H
#define MODULE_H
int fnct(void);
#endif /* MODULE_H */
module.c
#include "module.h"
static
int
detail(void)
{
return 2;
}
int
fnct(void)
{
return 3+detail();
}
main.c
#include <stdio.h>
#include "module.h"
int
main(void)
{
printf("fnct() gives %d\n", fnct());
/* printf("detail() gives %d\n", detail()); */
/* detail cannot be called because:
. it was not declared
(rejected at compilation, or at least a warning)
. even if it were, it is static to the module.c compilation unit
(rejected at link)
*/
return 0;
}
build (compile each .c then link)
gcc -c module.c
gcc -c main.c
gcc -o prog module.o main.o
You have included test.h in main.c.
Therefore static int n and static void f() will be visible inside main.c also.
When a variable or function is declared at file scope (not inside any other { } brace pair), and they are declared static, they are local to the translation unit they reside in.
Translation unit is a formal term in C and it's slightly different from a file. A translation unit is a single c file and all the h files it includes.
So in your case, the static variable is local to the translation unit consisting of test.h and main.c. You will be able to access it in main.c, but not in foo.c.
Meaning that if you have another .c file including test.h, you'll get two instances of the same variable, with the same name. That in turn can lead to all manner of crazy bugs.
This is one of many reasons why we never define variables inside header files.
(To avoid spaghetti program design, we should not declare variables in headers either, unless they are const qualified.)
I have a weird problem in my Geany project. The project is extremely simple and contains 3 files all in the same directory: main.c, foo.h and foo.c.
Compiler error:
In file included from main.c:1:0:
foo.h:4:12: warning: ‘bar’ used but never defined
static int bar(void);
^
/tmp/cc0zCvOX.o: In function `main':
main.c:(.text+0x12): undefined reference to `bar'
Compilation failed.
collect2: error: ld returned 1 exit status
What is going wrong?
main.c:
#include "foo.h"
int main(int argv, char* argc[])
{
bar();
return 0;
}
foo.h:
#ifndef _FOO_H_
#define _FOO_H_
static int bar(void);
#endif // _FOO_H_
foo.c:
#include "foo.h"
#include <stdio.h>
static int bar(void)
{
printf("Hello World\n");
return 1;
}
If a function is declared as static, the function is in file scope, means the scope of the function is limited to the translation unit only (in this case, the source file). Other functions which are present in the same compilation unit can call the functions, but no functions present outside the compilation unit can see the definition (presence) or call the function.
Related: From C11 standard document, chapter , linkage of identifiers
If the declaration of a file scope identifier for an object or a function contains the storage class specifier static, the identifier has internal linkage.(30)
and, footnote (30),
A function declaration can contain the storage-class specifier static only if it is at file scope;
Solution: Remove the static in the function definition and declaration.
FWIW, there is not much meaning of putting the forward declaration of a static function in a header file. Anyway, the static function can not be called from other source files.
What are advantages and disadvantages of both approaches?
Source vs. header implementation
Function definition inside source file
Header file sourcefunction.h contains declaration only.
#ifndef SOURCEFUNCTION_H
#define SOURCEFUNCTION_H
void sourcefunction(void);
#endif // SOURCEFUNCTION_H
Source file sourcefunction.c contains definition
#include "sourcefunction.h"
#include <stdio.h>
void sourcefunction(void) { printf(" My body is in a source file\n"); }
Function definition inside header file
Header file headerfunction.h contains definition which is the declaration at the same time.
#ifndef HEADERFUNCTION_H
#define HEADERFUNCTION_H
#include <stdio.h>
void headerfunction(void) { printf(" My body is in a header file\n"); }
#endif // HEADERFUNCTION_H
No source file is needed.
Consumer
File main.c
#include "sourcefunction.h"
#include "headerfunction.h"
int main(void) {
sourcefunction();
headerfunction();
return 0;
}
Why compile many source files?
We have to compile all source files and remember about them during linking.
gcc -c sourcefunction.c
gcc -c main.c
gcc main.o sourcefunction.o
Make can handle file managing but why even bother?
Is separation of interface and implementation always an issue?
It is obvious reason for big projects and teamwork. The designer specifies the interface. The programmers implement functionality.
What about smaller projects and non-formal approach?
Is removing definition from header files always preventing from linker errors?
Let's assume my program is using another module that defines the function with the same name sourcefunction().
#include "sourcefunction.h"
#include "sourcefunction1.h"
#include "headerfunction.h"
int main(void) {
headerfunction();
sourcefunction();
return 0;
}
Different function interface
File sourcefunction1.h
#ifndef SOURCEFUNCTION1_H
#define SOURCEFUNCTION1_H
int sourcefunction(void);
#endif // SOURCEFUNCTION1_H
File sourcefunction1.c
#include "sourcefunction1.h"
#include <stdio.h>
int sourcefunction(void) { int a = 5; return a; }
By compiling main.c, I get a nice compiler error
sourcefunction1.h:4:5: error: conflicting types for 'sourcefunction'
showing me the location of error.
Same function interface
File sourcefunction1.h
#ifndef SOURCEFUNCTION1_H
#define SOURCEFUNCTION1_H
void sourcefunction(void);
#endif // SOURCEFUNCTION1_H
File sourcefunction1.c
#include "sourcefunction1.h"
#include <stdio.h>
void sourcefunction(void) { int a = 5; printf("%d",a); }
Compiler does not mind multiple declarations. I get ugly linker error.
Can header implementation serve as library?
jschultz410 says
If you are writing a library and all your function definitions are in headers, then other people who do segment their development into multiple translation units will get multiple definitions of your functions if they are needed in multiple translation units
Lets' have
File consumer1.c
#include "headerfunction.h"
void consume1(void) { headerfunction(); }
File consumer2.c
#include "headerfunction.h"
void consume2(void) { headerfunction(); headerfunction();}
File twoConsumers.c
extern void consume1(void);
extern void consume2(void);
int main(void) {
consume1();
consume2();
return 0;
}
Let's compile sources.
gcc -c consumer1.c
gcc -c consumer2.c
gcc -c twoConsumers.c
So far, so good. Now, linking.
gcc consumer1.o consumer2.o twoConsumers.o
Linker error: multiple definition of 'headerfunction', of course.
But I can make my library function static.
File headerfunction.h, afterwards.
#ifndef HEADERFUNCTION_H
#define HEADERFUNCTION_H
#include <stdio.h>
static void headerfunction(void) { printf(" My body is in a header file\n"); }
#endif // HEADERFUNCTION_H
It hides the definition from other translation units.
I shouldn't answer this, but I will.
This can create duplicate definitions unless you really only have a single .c file in your project (unwise). Even the header guards won't prevent files the headers from being included multiple times if those multiple times are with different .c files. When the .obj files are linked together, there will be conflicts.
If only the function declaration and not definition is in the header, then only changes to the interface (the function name, parameters or return type) require recompiling dependencies. However, if the entire definition is in the header, then any change to the function requires recompiling all .c and .h files that depend on it, which, in a larger project, can create a lot of unnecessary recompiling.
It's not the convention. Libraries will not use this convention, so you'll be stuck dealing with their header file structure. Other developers will not use this convention, so you can create confusion or annoyance there.
I was told that a function defined as static in one .c file is not accessible from other files. But in the following program, I can access the static void show() function from another file. Is my understanding of static functions in C wrong?
a.h (first file):
static void show()
{
printf("I am in static show function in a.c");
}
b.c (another file):
#include"a.h"
void main()
{
show();
}
Remember that #includes work by copy-and-pasting the content of the included file. So in your example, after the #include has been processed, you get this:
static void show()
{
printf("I am in static show function in a.c");
}
void main()
{
show();
}
So clearly main can see show.1
The solution is to not #include .c files. In general, you should only #include header (.h) files. Your static functions shouldn't be declared or defined in the header file, so main will not be able to see it.
1. However, you now actually have two definitions of the show function, one in a.c and one in b.c. For static functions, this isn't a problem, but for non-static functions you would get a linker error.
static keyword changes the linkage specification to Internal Linkage.
A function marked as static will only be visible in that Translation Unit(TU).
Perhaps, You have same named symbols available in that particular TU, where you access the function. The how part of it can be only answered after you show us the code.
EDIT:
When you define a static function in header file, A copy of the same function gets created in every Translation Unit where you include it.Each instance of such a function is treated as a separate function(address of each function is different) and each instance of these functions have their own copies of static local variables & string literals.
Clearly, this will work but this might as well increase the size of your generated binary.
The other answers are correct, but it's not quite accurate to say that the static function is not accessible from another file. It is possible to access the function through a function pointer. It would be more accurate to say that the name of the function is not accessible in another translation unit.
Remember that converting C source code to an executable program consists of conceptual stages, including:
preprocessing (in which #include directives are replaced with the contents of the included file
compilation (which processes one translation unit at a time)
linking (in which the translation units are put together into the final program)
Suppose we have three files. foo.h:
typedef void (*void_function_p)(void);
extern void_function_p foo(void);
foo.c:
#include "foo.h"
#include <stdio.h>
static void baz(void) {
printf("worked!\n");
}
void_function_p foo(void) {
return baz;
}
bar.c:
#include "foo.h"
#include <stdio.h>
int main(void) {
(*foo())();
return 0;
}
This program compiles and prints "worked!" when it runs.
There are two translation units here. One is the code in the preprocessed foo.c (which, because of how #include works also includes the code in foo.h and stdio.h). The other is the code in the preprocessed bar.c (which, again, has its own copy of the code in foo.h and stdio.h).
By having the function foo return a pointer to the static function baz, we are able to call baz from the main function.
Now, consider what happens if we modify main to look like this:
int main(void) {
(*foo())();
baz();
return 0;
}
This code will result in a linker error because the name baz in this translation unit cannot be linked to the definition of baz in the other translation unit.
This is the first advantage of static functions: another programmer cannot accidentally access our baz function from another translation unit.
Now, consider what happens if we modify bar.c to look like this:
#include "foo.h"
#include <stdio.h>
static void baz(void) {
printf("still works!");
}
int main() {
(*foo())();
baz();
return 0;
}
This code will compile, and print "worked!" followed by "still works!"
This is the second advantage of static functions: we've defined two functions (in different translation units) with the same name.
If you try to put both static definitions in the same translation unit, you will get a compiler error about defining baz twice.
As a final note, if you take the program as it now stands and remove all the statics, it will result in a linker error because baz has been defined twice (with external linkage), which is not permitted.