I write C library and I don't know how make some functions invisible for user (which are used to implement functionality). There is any way in C? And my second question is: when I include some library in .h or .c file (let's see math.h) is there any problem when user declare the same header in his program (which use my library of course)?
And how about structure? Is there any key word?
how to make some functions invisible ?
do not declare the function in the header file of your library
define the function as static in the source file of your library
Example:
your-lib.h
#ifndef YOUR_LIB_H
#define YOUR_LIB_H
void do_public_stuff(void);
#endif YOUR_LIB_H
your-lib.c
#include "your-lib.h"
// this functions is invisible
static void do_private_stuff(void)
{
// ...
}
// this functions is visible
void do_public_stuff(void)
{
// ...
}
user-code.c
#include "your-lib.h"
int main(void)
{
do_public_stuff();
return 0;
}
A usual way is to make implementation functions static that is to declare them with storage specifier static and place them in modules with code. So neither header will have references to these functions.
You can use the keyword static as the other answers suggested.
But sometimes, the library itself will consist of several source- and object-files, and there might be functions, which have to be used throughout the whole library, but shall not be exported after linking.
Simple example:
Your library has 3 objects: lib1.o lib2.o util.o
Located in util.o are several helper-functions, which are used by lib1.o and lib2.o. You cannot use static here, because the functions would not be visible to them and linking would fail.
What you can do:
Make the helper-functions non-static and provide the linker with a version script, which only exports the needed symbols:
gcc -shared lib1.o lib2.o util.o -o mylib.so -Wl,--version-script=map
The file map could have the following content:
{
global:
bar;
baz;
local: *;
};
This would only export the symbols bar and baz (which are the relevant API-functions for this sample lib), while omitting all other symbols from the symbol table.
Related
Let's suppose that I write some functions in a file, that we'll call foo.c.
This is foo.c:
int d;
typedef int bar;
int foo(int a, int b){
...
}
void boo(){
...
}
char *test(){
...
}
Now, boo is a function used only inside foo.c, while foo(), test(), d, and Bool will need to be able to get called inside other files.
In order to do that, I know that I have to create a foo.h file and write it like this:
extern int d;
extern typedef int bar;
int foo(int a, int b);
char *test();
then #include "foo.h" in the foo.c file, and whenever I want to use the types and functions defined in foo.c, I have to include both foo.hand foo.cin the file in which I wanna use foo.c functions and types.
So foo.c in the end would look like this:
#include "foo.h"
int d;
typedef int bar;
int foo(int a, int b){
...
}
void boo(){
...
}
char *test(){
...
}
Here are my questions.
Q1. Is this how you actually do it? Since foo.h is already included in foo.c, wouldn't it be sufficient to include only foo.c in the file in which I want to use its functions? Can't I just directly define the functions inside of the foo.c file, not using the foo.h file at all?
Q2. Do you actually have to put the extern in front of typedefs in the foo.h file?
Q3. Let's say that in foo.c I use some standard C libraries like string.h and math.h . Where should I include them? Directly in the foo.c file, in the foo.h file or in both? Are #ifndef instructions necessary? If so, how do you use them correctly?
Q4. After writing the foo.c and foo.h file, am I all ready to go? Like, I don't need to compile them or anything, right? I can just #include them wherever I need just like that?
Q5. Somewhere else I've read that if I want to use a custom library these are the steps that I need to follow:
define the interface (foo.h)
write foo.c #include ing foo.h
creating an object file like this gcc -o foo.o -c foo.c
including foo.h in the program in which I want to use foo.c functions
linking the object file like this gcc my_proj.c foo.o
Are these steps actually necessary? Because I haven't seen them mentioned anywhere else. Why do I only need to include foo.h in the file in which I want to use foo.c functions? What exactly is an object file?
Thanks for your time and sorry if this is a bit lengthy
Q1. Is this how you actually do it? Since foo.h is already included in foo.c, wouldn't it be sufficient to include only foo.c in the file in which I want to use its functions?
You just don't include .c files. In your case, foo.c and the other files are separate compilation units which get linked together in the end.
Q2. Do you actually have to put the external in front of typedefs in the foo.h file?
No, typedefs don't need extern.
Q3. Let's say that in foo.c I use some standard C libraries like string.h and math.h . Where should I include them? Directly in the foo.c file, in the foo.h file or in both? Are #ifndef instructions necessary? If so, how do you use them correctly?
If you need these files in the .h as well, you include them there (e. g. for types used in function prototypes). If you need them only in your .c, include them there.
Q4. After writing the foo.c and foo.h file, am I all ready to go? Like, I don't need to compile them or anything, right? I can just #include them wherever I need just like that?
You can compile them in order to get something callable. If you don't compile your program, you cannot use it. Or do I understand you wrong?
Q5. Somewhere else I've read that if I want to use a custom library these are the steps that I need to follow:
[snip]
Indeed, this is one way to go. The steps 1, 2 and 4 cannot be omitted for obvious reasons. But you can execute step 3 and 5 together by doing
gcc my_proj.c foo.c
This compiles the given files and then links them together in one call.
Why do I only need to include foo.h in the file in which I want to use foo.c functions?
That's because the resulting object file contains information for the linker about which function it needs from other object files.
What exactly is an object file?
It is what results from compiling one source file. If you link several object files together, you get a running executable.
In other words: An object file is the compiled version of a source file. It "provides" the identifiers needed by other object files, and it "requires" other identifiers provided by other files. Linking them together means that the required and the provided objects are connected in an appropriate way so that the program can run.
Example: You have a foo.c which defines the functions foo and test. Then you have a main.c which makes use of these functions and provides a function main. In the end, they are linked together and combined with the startup code which is needed to start a program and which calls main(). The points in main() where foo() and test() are called respectively are marked in a special way so that the linker can put the actual call address there.
Q1. Is this how you actually do it?
No
Since foo.h is already included in foo.c, wouldn't it be sufficient to
include only foo.c in the file in which I want to use its functions?
You should only include .h files; both in the file that defines them, and in the ones that use them. As an extra, that include should be the first one in the file that defines the functions, but in the files that use them, it should go after the rest (standard headers, other packages' headers, ...); the reason for this is to detect errors easily.
Can't I just directly define the functions inside of the foo.c file,
not using the foo.h file at all?
Usually, no. Only static inline functions should do that.
Q2. Do you actually have to put the extern in front of typedefs in
the foo.h file?
No: typedef int foobar_t;
Q3. Let's say that in foo.c I use some standard C libraries like
string.h and math.h . Where should I include them?
In the file that needs them (foo.c). Include in every file only the headers that it needs. No more; no less.
Directly in the foo.c file, in the foo.h file or in both?
foo.c
Are #ifndefinstructions necessary? If so, how do you use them
correctly?
Yes:
// foo.h
#ifndef FOO_H
#define FOO_H
Here go all the contents of foo.h
#endif /* foo.h */
Q4. After writing the foo.c and foo.h file, am I all ready to go?
Like, I don't need to compile them or anything, right? I can
just #include them wherever I need just like that?
You need to compile foo.c into foo.o (object file) and then you probably would like to do a static library (.a) or a dynamic one. You can include it wherever you want, but you will need to prepare your Makefile to do so.
Q5. Somewhere else I've read that if I want to use a custom library
these are the steps that I need to follow:
define the interface (foo.h)
write foo.c #include ing foo.h
creating an object file like this gcc -o foo.o -c foo.c
including foo.h in the program in which I want to use foo.c functions
linking the object file like this gcc my_proj.c foo.o
Are these steps actually necessary?
Yes
Because I haven't seen them mentioned anywhere else. Why do I only
need to include foo.h in the file in which I want to
use foo.c functions?
Read more about compiling and linking.
What exactly is an object file?
A compiled file. Many of them are usually linked together to produce the executable file.
I build two libraries called templib1.a and templib2.a
And some global variables and function names in templib1.a and templib2.a are the same; when I try to compile my c code with these two libraries, it failed.
Is there any method except namespace for solving this problem? Do I need to re-build the libraries? Thanks.
=======================================================================
I build libraries as following
build templib1.a from aaa.c bbb.c ddd.c aaa.h; there is global variable int var_a, int var b; and functions void func_a(), void func_b() in aaa.c and bbb.c respectively
build templib2.a from aaa.c bbb.c eee.c aaa.h; there is global variable int var_a, int var b; and functions void func_a(), void func_b() in aaa.c and bbb.c respectively
And I my c code like
gcc $(INC) $(TEMPLIB1_DIR)/templib1.a $(TEMPLIB2_DIR)/temp2lib.a -o run
and it shows
"multiple definition of var_a
temp2lib.a : first defined here"
"multiple definition of var_b
temp2lib.a : first defined here"
Judging by the names of your libs, object files and variables, this is only a toy lib. When you write a real lib, you have to think about its design:
Under Unix, it is customary to name your lib libsomething.a. Your libs should therefore be called libtemp1.a and libtemp2.a, so that you can easily link them with -ltemp1 and -ltemp2.
If you export all symbols, you'll probably export too much and therefore clutter the namespace available for link symbols.
Think about which symbols you should export. The user of the lib needs to know which symbols she can use, so there's probably a header file accompanying the lib. This is the lib's interface. You need to export only the symbols in that interface. Start by making as many global variables and functions static, i.e. local to one compilation unit (object file). If you still export too many symbols, consider controlling your exports manually.
C doesn't have packages or namespaces. It is good practice to prefix all exported symbols with a short prefix, e.g. temp1_var_a, temp1_initi and so on.
Choose meaningful names for your symbols. A global variable shouldn't be called var_a. Use this rule of thumb: Small scope, short names. Big scope, longer names.
Don't be shy to rebuild your libs if you are their only user.
If your want your functions and variables accessible from outside your libs, rename them and include prefix.
If those functions and variables are for internal use only, use the static keyword:
What does "static" mean?
I would like to know , if I have source file like this
#include <stdio.h>
int main()
{
printf("Hello world");
}
as I know header files contains only prototypes of functions. If it so how can my source file get the function printf ? If I don't include the source file where it has been declared?
thank you
The question is not clear, but there are 2 things which happen before a program gets created,
Compiling (requires prototypes / declarations)
Linking (requires definitions).
Header information is needed for knowing prototypes. Even this would compile fine:
int printf ( const char * format, ... );
int main()
{
printf("Hello world");
}
On linking there will be no issues because the printf function is found in the C standard library, so on linking it will look into the standard directories (of the compiler where the library is kept - bin/lib folder) and link the function.
The source only needs to know the prototype. The problem a programmer will have in this case:
int my_printf ( const char * format, ... );
int main()
{
my_printf("Hello world");
}
The above will compile, but when linking my_printf your code will not have a definition so it will give an error on linking.
Header file has the definitions declarations of the functions ( stdio.h has definition declaration for printf ). The actual function exists in the libraries and gets linked when you compile the code.
When it comes to using libraries, you include the header of the library in your code and you instruct the linker to link using the code files of that library(usually object files). For the standard libraries, the IDE usually instructs the linker to link to them by default.
Assuming you're using gcc as a compiler the standard libraries are linked by default and that is where the function definitions lie. If you'd like to see exactly which libraries are being linked you can pass gcc the -v option which will cause it to dump information about the default options it will use including the library paths and default libraries and object files that will be linked in.
If you give the -Wl,--verbose option, gcc will pass the --verbose to the linker which will dump exactly where it's looking for libraries, including both failed and successful searches
gcc -v foo.c -Wl,--verbose
The header file stdio.h would declare printf as an extern function, i.e., it is defined elsewhere. The compiler is happy as long as functions you use have a declaration. The linker is the one that resolves these dependencies.
A very useful thing to do when you start asking good questions like this is to play with some linker commands.
Assuming you're on *nix, once you have your executable file (lets call it foo), do:
ldd foo
You should see a list of libraries that were linked with while creating foo.
libc.so should be one among those. It contains the definition for printf among other things!
You can refer to this link to learn more
How do you have a header file for a function and the implementation of that function in different files? Also, how do you have main in yet another file and call this function?
The advantage is so that this function will then be an independent component which can be reused, right?
This is best illustrated by an example.
Say we want a function to find the cube of an integer.
You would have the definition (implementation) in, say, cube.c
int cube( int x ) {
return x * x * x;
}
Then we'll put the function declaration in another file. By convention, this is done in a header file, cube.h in this case.
int cube( int x );
We can now call the function from somewhere else, driver.c for instance, by using the #include directive (which is part of the C preprocessor) .
#include "cube.h"
int main() {
int c = cube( 10 );
...
}
Finally, you'll need to compile each of your source files into an object file, and then link those to obtain an executable.
Using gcc, for instance
$ gcc -c cube.c #this produces a file named 'cube.o'
$ gcc -c driver.c #idem for 'driver.o'
$ gcc -o driver driver.c cube.c #produces your executable, 'driver'
Actually you can implement any function in header files for better performance(when implementing libraries for example) as long are not referenced to a specific object(actually it won't compile that).
By the way even with that way, you have separate interface and implementation ;)
Of course you will have include gurads in you header files to avoid "multiple definition" errors.
In C/C++, non-inline functions should be defined only once. If you put function defination
in header files, you will get "multiple defination" link error when the header file is included more than once.
I'm a C newbie and I was just trying to write a console application with Code::Blocks. Here's the (simplified) code:
main.c:
#include <stdio.h>
#include <stdlib.h>
#include "test.c" // include not necessary for error in Code::Blocks
int main()
{
//t = test(); // calling of method also not necessary
return 0;
}
test.c:
void test() {}
When I try to build this program, it gives the following errors:
*path*\test.c|1|multiple definition of `_ test'|
obj\Debug\main.o:*path*\test.c|1|first defined here|
There is no way that I'm multiply defining test (although I don't know where the underscore is coming from) and it seems highly unlikely that the definition is somehow included twice. This is all the code there is.
I've ruled out that this error is due to some naming conflict with other functions or files being called test or test.c. Note that the multiple and the first definition are on the same line in the same file.
Does anyone know what is causing this and what I can do about it? Thanks!
You actually compile the source code of test.c twice:
The first time when compiling test.c itself,
The second time when compiling main.c which includes all the test.c source.
What you need in your main.c in order to use the test() function is a simple declaration, not its definition. This is achieved by including a test.h header file which contains something like:
void test(void);
This informs the compiler that such a function with input parameters and return type exists. What this function does ( everything inside { and } ) is left in your test.c file.
In main.c, replace #include "test.c" by #include "test.h".
A last point: with your programs being more complex, you will be faced to situations when header files may be included several times. To prevent this, header sources are sometimes enclosed by specific macro definitions, like:
#ifndef TEST_H_INCLUDED
#define TEST_H_INCLUDED
void test(void);
#endif
The underscore is put there by the compiler and used by the linker. The basic path is:
main.c
test.h ---> [compiler] ---> main.o --+
|
test.c ---> [compiler] ---> test.o --+--> [linker] ---> main.exe
So, your main program should include the header file for the test module which should consist only of declarations, such as the function prototype:
void test(void);
This lets the compiler know that it exists when main.c is being compiled but the actual code is in test.c, then test.o.
It's the linking phase that joins together the two modules.
By including test.c into main.c, you're defining the test() function in main.o. Presumably, you're then linking main.o and test.o, both of which contain the function test().
You shouldn't include other source files (*.c) in .c files. I think you want to have a header (.h) file with the DECLARATION of test function, and have it's DEFINITION in a separate .c file.
The error is caused by multiple definitions of the test function (one in test.c and other in main.c)
I had similar problem and i solved it following way.
Solve as follows:
Function prototype declarations and global variable should be in test.h file and you can not initialize global variable in header file.
Function definition and use of global variable in test.c file
if you initialize global variables in header it will have following error
multiple definition of `_ test'|
obj\Debug\main.o:path\test.c|1|first defined here|
Just declarations of global variables in Header file no initialization should work.
Hope it helps
Cheers
Including the implementation file (test.c) causes it to be prepended to your main.c and complied there and then again separately. So, the function test has two definitions -- one in the object code of main.c and once in that of test.c, which gives you a ODR violation. You need to create a header file containing the declaration of test and include it in main.c:
/* test.h */
#ifndef TEST_H
#define TEST_H
void test(); /* declaration */
#endif /* TEST_H */
If you have added test.c to your Code::Blocks project, the definition will be seen twice - once via the #include and once by the linker. You need to:
remove the #include "test.c"
create a file test.h which contains the declaration:
void test();
include the file test.h in main.c
If you're using Visual Studio you could also do "#pragma once" at the top of the headerfile to achieve the same thing as the "#ifndef ..."-wrapping. Some other compilers probably support it as well ..
.. However, don't do this :D Stick with the #ifndef-wrapping to achieve cross-compiler compatibility. I just wanted to let you know that you could also do #pragma once, since you'll probably meet this statement quite a bit when reading other peoples code.
Good luck with it
Ages after this I found another problem that causes the same error and did not find the answer anywhere. I thought to put it here for reference to other people experiencing the same problem.
I defined a function in a header file and it kept throwing this error. (I know it is not the right way, but I thought I would quickly test it that way.)
The solution was to ONLY put a declaration in the header file and the definition in the cpp file.
The reason is that header files are not compiled, they only provide definitions.