I'm writing an embedded C program in eclipse with gcc and can't see how to get around a certain issue. I have a typedef in a header file which is protected by an include guard. Since I use this typedef in multiple headers, I need to include it in other headers, however when I try to compile, no matter what I do, one of the headers cannot see the typedef and complains of an unknown type name.
I believe this illustrates the problem:
header a.h:
#ifndef _a_h
#define _a_h
typedef enum {
USBD_OK = 0,
USBD_BUSY,
USBD_FAIL,
}USBD_Status;
#endif
header b.h:
#ifndef _b_h
#define _b_h
#include "a.h"
extern USBD_Status USB_getStatus(void);
#endif
header c.h:
#ifndef _c_h
#define _c_h
#include "a.h"
extern USBD_Status USBD_Sync(void);
#endif
This always seems to result in the error "unknown type name 'USBD_Status'" as whichever header is compiled second cannot see the typedef. Removing the include guard from a.h results in the complaint that USBD_Status is being redeclared.
edit:
I have double checked all include paths, all includes, all file names and all include guards - there are no duplicates or typos.
It could be that you have another header that uses the same header guard name.
You could add some code to the top of your a.h that does this:
#ifdef _a_h_
#error this header is already defined
#endif
This way you can track down everywhere that a.h is included and see where any oddities may occur.
As stated in the comments, your above example works so the problem must lie somewhere else ...
Related
I've inherited some code* which declares and defines a struct in a header file (a_A.h). This struct is in the top file of an include hierarchy tree which symbolically looks like:
file: t_T.h (#includes "c_C.h") //defines a struct
file: c_C.h (#includes "h_H.h")
file: h_H.h (#includes "a_C.h")
file: a_C.h (#includes <stdio.h>)
Each header has the appropriate header guards and appear to be non-recursive when looked at as a flat collection of files. However, files c_C.h and a_C.h reside in the same library. While h_H.h resides in a different library. From a library perspective this symbolically appears as:
t_T.h (includes a file from Lib_C)
Lib_C: (includes a file from Lib_H)
Lib_H (includes a file from Lib_C)
which is recursive and is the likely cause of redefinition problems when I compile the code (the linker complains that the struct in file a_C.h is redefined).
1) Did I correctly identify the issue?
2) If so, why? I am guessing that linked objects in a library appear flat to the linker (i.e. they've lost their hierarchy context). And if guessed somewhat correctly then:
3) Should I consider header guards to be limited to the scope of their respective library?
Below is the error statement from the problems window:
symbol "ov5642_1280x960_RAW" redefined: first defined in "./TakePhoto.obj"; redefined in "./ArduCam/ov5642_Config.obj"
Header in ./TakePhoto:
#ifndef TAKEPHOTO_H
#define TAKEPHOTO_H
#include "ov5642_Config.h"
#include "HAL_ArduCAM.h"
...
#endif /* TAKEPHOTO_H_ */
Header in ./ArduCAM/ov5642_Config.h:
#ifndef ARDUCAM_OV5642_CONFIG_H_
#define ARDUCAM_OV5642_CONFIG_H_
#include "HAL_ArduCAM.h"
#include "ov5642_Sensor_Values.h"
....
#endif /* ARDUCAM_OV5642_CONFIG_H_ */
Header in HAL_ArduCAM
#ifndef HAL_ArduCAM_h
#define HAL_ArduCAM_h
#include <stdint.h>
#include "driverlib.h"
....
#endif /* HAL_ArduCAM_h */
ov5642_Sensor_Values.h has the following
#ifndef ARDUCAM_OV5642_SENSOR_VALUES_H_
#define ARDUCAM_OV5642_SENSOR_VALUES_H_
#include <stdint.h>
const struct sensor_reg ov5642_1280x960_RAW[] =
{
{0x3103,0x93},
{0x3008,0x02},
{0x3017,0x7f},
.....
#endif /* ARDUCAM_OV5642_SENSOR_VALUES_H_ */
It seems that the contents of OV5642_Sensor_Values is copied twice, once for TakePhoto and once again for ovV5642_Config despite their header guards. My original thought was that there was a recursive dependencies but didn't find it.
Ok, I've made an example pasted below. There are five files in this example, three files (bar.h, foo.h, foo.c reside in a library), the other two files (foo1.h, foo1.c) do not. Notice that foo.h includes bar.h and that foo1 includes both foo.h and bar.h.
I am conjecturing that the guard headers of bar.h are not preserved when the pre-processor copies into foo.h. Thus when foo1 includes bar.h and foo.h the symbol redefinition arises. So to answer my own question, no, it is not a library issue. Not preserving the header guard seems the likely cause of my problem.
They are pasted below.
/*
* bar.h
*
*/
#ifndef ARDUCAM_BAR_H_
#define ARDUCAM_BAR_H_
#include <stdint.h>
typedef struct regStruct {
uint16_t reg;
uint8_t val;
} regStruct;
const struct regStruct regArray[] =
{
{0x3103,0x03},
{0x3104,0x03},
{0x3008,0x82},
{0xffff,0xff},
};
const struct regStruct tinyArray[] =
{
{0x3106,0x03},
{0x3003,0x82},
{0xffff,0xff},
};
#endif /* ARDUCAM_BAR_H_ */
/*
* foo.h
*
*
*/
#ifndef ARDUCAM_FOO_H_
#define ARDUCAM_FOO_H_
#include <stdint.h>
#include <stdio.h>
#include "bar.h" //including this file causes redefinition
typedef struct Init_Parameters {
//! Select sensor resolution
//! options.
//! \n Valid values are:
//! - \b big
//! - \b small
//! - \b tiny
uint8_t size;
} Init_Parameters;
uint8_t Sensor_Init(Init_Parameters *param);
typedef enum {
small=0,
big,
tiny
} select_size;
#endif /* ARDUCAM_FOO_H_ */
/*
* foo.c
*
*
*/
#include "foo.h"
uint8_t Sensor_Init(Init_Parameters *param)
{
switch(param->size)
{
case big:
break;
case small:
break;
case tiny:
break;
}
return 0x01;
}
/*
* foo1.h
*
* Created on: Feb 28, 2019
* Author: jnadi
*/
#ifndef FOO1_H_
#define FOO1_H_
#include "foo.h"
#include "bar.h"
#endif /* FOO1_H_ */
/*
* foo1.c
*
* Created on: Feb 28, 2019
* Author: jnadi
*/
#include "foo1.h"
void Camera_Init(){
Init_Parameters setParams; //create instance
setParams.size=big;
Sensor_Init(&setParams);
}
The physical locations of header files affect C source compilation only via the include-file search path. Headers have to be in one of the directories that are searched, and if there is more than one with the same name, then the search path order determines which is used. The association, if any, between a given header and a library is not known to the compiler, and it does not affect compilation, unless indirectly via the search path.
Your claim that
the linker complains that the struct in file a_C.h is redefined
(emphasis added) makes sense only if by "the struct" you mean an object of a structure type. The linker may complain if a variable with external linkage is defined in more than one translation unit, which will likely happen if a header contains a definition of that variable (as opposed to merely a declaration).
If instead the issue is that the structure type is redefined, then that would be diagnosed by the compiler, not the linker, and it would tend to contradict your conclusion that
Each header has the appropriate header guards
. Proper header guards are exactly what would prevent such a type redefinition issue.
1) Did I correctly identify the issue?
Not with any specificity, for sure. That there are header dependencies in both directions between headers associated with different libraries reflects poor design, but it would not inherently cause compilation or linking to fail.
2) If so [...]
N/A
3) Should I consider header guards to be limited to the scope of their
respective library?
No. Header guards are relevant at the compilation stage only, and C compilation knows nothing about libraries. All header files processed by the compiler in the course of translating a single translation unit are on equal footing. In fact, the main risk in this area is in the opposite direction: a collision of header guards.
That answers the question as posed. As for what the true nature of your build issue may be, you have not given us enough information to do more than speculate. My own speculations are already conveyed above.
Using the five files posted above and commenting out an #includes bar.h in foo.h, I believe I found the answer to my problem.
The simple answer is that header guards are not preserved once it is included into the header of another file.
When bar.h is included into another header file, its header guards are superseded by the header guards of its new host (foo.h). Thus symbol redefinition issues arise when foo1.h includes both bar.h and foo.h
Header guards only prevent a .h file from including its contents twice or more within one top-level translation unit. They are to deal with cases like this, where two or more headers need to include the same set of common definitions:
// A.h
struct A { int x, y, z; };
// B.h
#include "A.h"
struct B { struct A aye; float f, g; };
// C.h
#include "A.h"
struct C { struct A aye; long l, m; };
// main.c
#include "B.h"
#include "C.h" // error, redefinition of struct A
But each translation unit starts out with a clean macro environment, so if you include a header file in two different top-level translation units, that header's declarations are made visible (once) to each. And that's what you want. (Think about standard library headers. You wouldn't want stdio.h not to declare printf in bar.c just because there existed foo.c in the same project that also included stdio.h.)
Now, your problem is that ov5642_Sensor_Values.h defines a data object (not a type), ov5642_1280x960_RAW, and this header is included into two different top-level translation units (.c source files). Each translation unit produces an object file containing a definition of ov5642_1280x960_RAW, and you get a multiple definition error from the linker when you try to combine them.
The bug causing this problem is not that ov5642_Sensor_Values.h's header guards are ineffective. The bug is that ov5642_Sensor_Values.h should not be making any global definitions. Header files should only declare things (with rare exceptions that you shouldn't worry about until you encounter them).
To fix the bug, change ov5642_Sensor_Values.h to declare ov5642_1280x960_RAW but not define it, like this:
#ifndef ARDUCAM_OV5642_SENSOR_VALUES_H_
#define ARDUCAM_OV5642_SENSOR_VALUES_H_
#include <stdint.h>
#include "sensor_reg.h"
extern const struct sensor_reg ov5642_1280x960_RAW[];
#endif
and create a new file named ov5642_Sensor_Values.c that contains the initialized definition:
#include "ov5642_Sensor_Values.h"
extern const struct sensor_reg ov5642_1280x960_RAW[] =
{
{0x3103,0x93},
{0x3008,0x02},
{0x3017,0x7f},
.....
};
and add that file to your link.
thank you everyone especially John, Zwoi. I was up against a deadline (this was inherited code) but was able to calm myself down enough to figure out what John was saying. I moved the struct definition into a c file and used an extern pointer in the header which is similar to zwoi's declaration. I am also relieved that what I did matched zwoi's example (thank you so much!).
extern const struct sensor_reg * ov5642_1280x960_RAW;
I'm trying to figure out, how to use C headers with #ifndef and #include.
Lets say I have these two header files:
headerA.h:
#ifndef HEADERA_H
#define HEADERA_H
#include "headerB.h"
typedef int MyInt;
TFoo foo;
... some other structures from headerB.h ...
#endif
headerB.h
#ifndef HEADERB_H
#define HEADERB_H
#include "headerA.h"
typedef struct foo{
MyInt x;
} TFoo;
#endif
headerA.c
#include "headerA.h"
... some code ...
headerB.c
#include "headerB.h"
... some code ...
When compiling headerB.c, it says
In file included from headerB.h,
from headerB.c:
headerA.h: error: unknown type name ‘MyInt’
I think, it's becouse when headerB.h is compiling, it defines HEADERB_H and then, when headerA.h wants to include headerB.h, the #ifndef HEADERA_H is false = skips including.
What is the best practice here? I just read, that best practice is to do all #include directives in header files, but in this situation it looks like a problem.
EDIT: Ok, sorry for misleading you. This is just and example from bigger project with more files.
You have a circular dependency. Header file headerA.h depends on headerB.h which depends on headerA.h and so on and on.
You need to break that dependency, for example by not including headerB.h in headerA.h. It's not needed (nothing in headerA.h needs anything from headerB.h).
If you have to include headerB.h (as stated in your recent edit) then you first should reconsider how you use your header files, and what definition you place where. Perhaps move the definition of MyInt to headerB.h? Or have more header files, like one for type-aliases (like your MyInt which I personally see no use for), one for structures and one for variable declarations?
If that's not possible then you could try by changing the order of definitions and the include, like
#ifndef HEADERA_H
#define HEADERA_H
// Define type alias first, and other things needed by headerB.h
typedef int MyInt;
// Then include the header file needing the above definitions
#include "headerB.h"
TFoo foo;
... some other structures from headerB.h ...
#endif
Just drop the line
#include "headerB.h"
from file headerA.h
My Files are
main.c
#include"A.h"
#include"B.h"
A.c
#include"A.h"
B.c
#include"B.h"
I have a file with a couple of structures that I have defined that I am supposed to use in all the files i.e A.c , B.c, main.c and even the header files for A and B.
Hence I have
A.h and B.h both have
#include"struct.h"
Now, I see that in my main.c
I will have multiple declaration for both the structures, how do I get rid of this problem. What shall I change in my structure?
Thanks
Use include guards.
aheader.h:
#ifndef AHEADER_H
#define AHEADER_H
// ... rest of header here
#endif
bheader.h:
#ifndef BHEADER_H
#define BHEADER_H
// ... rest of header here
#endif
You can use a guard as such,
#ifndef MY_STRUCT
#define MY_STRUCT
#include "struct.h"
#endif
If you want to selectively take care of which parts should not be duplicated
Wrap the header files in include guards., like this:
#ifndef MYHEADER_H
#define MYHEADER_H
// your definitions
#endif
Each header file should have its own guard with an unique name. The above preprocessor directives translated to english say something like: "If MYHEADER_H is not defined, then define it and paste the contents until #endif directive." This guarantees that a single header is included only once inside a single translation unit.
Simply use so called header guard to be sure of including "struct.h" only once:
// struct.h
#ifndef STRUCT_H
#define STRUCT_H
struct ...{
}
#endif
So I have a header file let's say "header.h" which is protected as follows:
#ifndef __HEADER1_H
#define __HEADER1_H
//type and function def
typedef struct
{
float r; //process noise
float k; //process gain
}state_t;
int get_state(state_t* state, float b);
#endif
Now I have two other headers which I defined as follows:
#ifdef __HEADER2_H
#include "header.h"
//function def
#endif
Second header:
#ifdef __HEADER3_H
//function
//the reason it is done this way is for cnditional compiling such that if the caller
//defines __HEADER3_H t this file won't be included.
#include "header.h"
#endif
Now as I suspected the compiler complained that types and functions defined in header.h were not detected in the source implementation of header2 and header3. So I included header.h in the source files as well. Now the linker is complaining functions that are defined in header.h are multiply defined.
My understanding was since the header.h is protected by ifndef it will only be included once so I don't see the problem.
here is the error that I am getting:
Symbol get_state multiply defined(by kalman.o and dsp.o)
Is there any chance that I am doing something unusally wrong?
#ifndef __HEADER1_H
#define __HEADER_H
The problem is your guard (__HEADER_H) is different from what you are checking for (__HEADER1_H). Make these both the same value.
The typical "guard" for a header file is:
myheader.h:
#ifndef _MYHEADER
#define _MYHEADER
<do stuff>
#endif
Optionally where myheader.h is included, you can do:
#ifndef _MYHEADER
#include "myheader.h"
#endif
This is optional and basically is only to improve compile performance.
I have a plain C code with *.c and *.h files in the workspace.
I have a header file 1.h declaring some structure as
struct my1
{
int a;
..
..
}my_t;
But when i try to declare a variable of type struct my1 in another header file 2.h as follows:-
struct my1 variable1;
It gives error at this declaration point.
Looks like my1 is undefined here in 2.h file.
In file 1.h I need to include 2.h, so in file 2.h I cannot include 1.h, for fear of recursive inclusion.
My question is:-
What do i need to declare to resolve the compilation error in this case?
This whole thing made me think about further questions about header file inclusions.
How are the header files included, in what order, which header file first then which one?
Will recursive inclusion of header files cause errors one file including other and other including first?
Could not post the actual code snippets for some security reason, so sorry if the question somewhat poses some readability problems.
You should start by putting an inclusion lock in all your .h files (this is called an include guard):
#ifndef ONE_H
#define ONE_H
//rest of header
#endif //ONE_H
That way you can include it multiple times.
Second:
typedef struct my1 { int a; .. .. }my_t;
You need a typedef in C (not in C++)
The headers are included in the order of inclusion.
If you compile a file abc.c which starts with:
#include "a.h"
#include "b.h"
then a.h will be included first, then b.h.
You could think of it, as if you paste the code in the file. It is included at that point.
It's like the folks said before.
I just want to add that sometimes even #ifdef won't help you.
//file1.h
#ifndef F1
#define F1
#include "file2.h"
struct file1st {
struct file2st *ptr;
};
#endif
//file2.h
#ifndef F2
#define F2
#include "file1.h"
struct file2st {
struct file1st *ptr;
};
#endif
//main.c
#include "file1.h"
#include "file2.h"
/*
This will give you an error of **struct file1st not defined**
Let's see why:
1) file1.h is included
2) file1.h includes file2.h before it declares anything
3) the definition of struct file2st occurs and it uses struct file1st which isn't declared yet
*/
int main(int argc, char* argv[]){
struct file1st st1;
struct file2st st2;
return 0;
}
The way to work this out is:
//file1.h
#ifndef F1
#define F1
struct file2st;//just declare, it will be defined later.
struct file1st {
struct file2st *ptr; // ok, compiler KNOWS the size of struct file2st*(pointer)
struct file2st file2Var;// NOT ok, compiler doesn't know sizeof(struct file2st)
};
#endif
//file2.h
#ifndef F2
#define F2
#include "file1.h"
struct file2st {
struct file1st *ptr;
};
#endif
Header files are included in the order of include directives. Once the compiler sees an include directive it opens the file to include and simply inserts all of its contents into the including file.
If the included file has include directives inside, the same is done for them. This process continues until all of the include directives have been processed.
Only after that the compilation is started.
That's why if any file is included more than once (A included B and C; both B and C include D for example) you'll often see compiler complaining about redefinitons. To resolve this add inclusion locks (aka include guards) - the ifdef directives.
//file Header1
#ifndef Header1Guard
#define Header1Guard
// all the header text here
#endif
I second the guard suggestion.
I religiously use the following header template:
#ifndef HELLOWORLD_H_
#define HELLOWORLD_H_
// Header stuff here.
#endif // HELLOWORLD_H_
When the compiler see's an #include, it simply replaces that line with the contents of the header file (minus any processed directives in the header). So, that means you can include the file in as many places as you like without risking recursive includes.
Every header file is included in every translation unit (source file) in which there is an include directive for it. This is intended, and will happen even with inclusion guards -- every translation unit that uses your struct needs to know how that struct is defined so that it can be laid out in memory the same way throughout all the translation units of your app. The inclusion guards just prevent it from being included multiple times within one translation unit. Include files will be included in the order you include them within that translation unit (and they'll be recursively included if include files include other files... as others have said). The order of translation units being compiled is up to you (or your IDE) to specify to the compiler. It shouldn't matter what that order is, however, since every translation unit is completely independent until it gets to the linking phase of the build process.
It's been a while since I worked with C, but I think what you want to do is forward define my1.
In 2.h, try putting this near the top:
struct my1;
Sorry, I can't answer your other two questions.
// Header1.h
typedef struct tagHeader1
{
} Header1;
// Header2.h
struct Header1;
// Header2.c
#include "Header1.h"
Note: This only works for pointers (and in c++, references). If you have a reference to a complete object, the compiler will need to know about it.