I am getting this following error,
I have a "A.c" file in which I have included a "b.h" file, which has a "c.h" file.
Now this c.h has structures which are getting used and they are all int.
the structures are used in the following way:
In "c.h" file
struct abc{
int a;<---- error
};
In "b.h"
struct def{
struct abc;
};
and I have used struct def in file "A.c" file.
Please, help me know what wrong have I done.
You probably have some nesting error, a missing ; or something that confuses the compiler.
I would recommend trying to get hold of the preprocessor output, so you can see what the compiler sees, once the #includes have been executed.
Related
I have static library file(lib_XXX.a) with global variable defined in it. I am trying to access the global variable in my executable(exe_XXX.o).
Linker error is coming. Any help would be thankful.
Languae : c
OS : Ubuntu gcc compiler
Sample as follows
exe_xxx.o module has 2 files resource.h and main.c
resource.h code as follows :
#ifndef RESOURCE_H
#define RESOURCE_H
#define APL
extern const StructTest g_AplObjDef;
const StructTest g_AplObjDef = {
abc, def, ghi,
....
};
#endif //APL
main.c code as follows:
#include "resource.h"
....
....
....
lib_xxx.a has another main.c in it. Its sample code as follows:
#include "resource.h"
int main()
{
#if defined(APL)
fun1(g_AplObjDef);
#endif
}
I suspect the reason is because resource.h included in both the main.c files.
I couldn't way to get rid of this. Can anyone help ?
Error details:
/lib_XXX.a(lib_XXX_a-main.o):(.data.rel.ro.local+0x40): `g_AplObjDef' が重複して定義されています
/exe_xxx-main.o:(.data.rel.ro.local+0x260): ここで最初に定義されています
Above error is in Japanese.. 1st line says "Duplicate is defined". 2nd line says "Here it is defined"
This part:
const StructTest g_AplObjDef = {
abc, def, ghi,
....
};
is a definition, and should not be in a header. Move it to
a .c file.
The reason for this is that header files are textually inserted, so if a header has a definition, and is included from multiple translation units, the symbol will be defined multiple times, which is an error.
Here you are defining a variable in the header
const StructTest g_AplObjDef =
You should only declare, which you did the line before.
This definition should go into a code file, accessing it will be possible by the knowledge provded by the declaration in the header. But the definition in the header will be done in each code file which includes it, which causes the redundant definition mentioned in the error message.
Moving the definition (as you now have it in the header, including the {...} into the libs code file should help.
Note that having two main() will probably get you into trouble, I only focus this answer on the double definition of the variable.
I'm new to linux.
I'm practice syscall and I want to copy struct to user.
So I write a syscall which is using copy_to_user().
But When I write and compile test.c to test my syscall appear some error.
in linux/sched.h has defined:
struct pacct_struct {
int ac_flag;
long ac_exitcode;
unsigned long ac_mem;
cputime_t ac_utime, ac_stime;
unsigned long ac_minflt, ac_majflt;
};
and then I write a program that use this struct(test.c)
#include "linux/sched.h"
#include <stdlib.h>
int main(){
struct pacct_struct *ts;
ts = (struct pacct_struct *)malloc(sizeof(struct pacct_struct));
return 0;
}
and gcc show the follow error message:
test.c:6:44: error: invalid application of 'sizeof' to incomplete type 'struct pacct_struct'
I wonder to know if it's fine to use kernel struct by include header file.
If so, do I miss something? How to make pacct_struct become a 'complete type'?
thanks.
addition:
I check preprocessor with gcc -E
I found that seems no "struct pacct_struct" being included.
Next, I going to view sched.h that I included which was created by "make headers_install" after I compile the kernel.
The file only contains some cloning flags such like "#define CSIGNAL 0x000000ff"
So, I tried to include original source file of sched.h in dir "/usr/src/linux-2.6.39.4/include/linux", but it continue showing same error 'incomplete type'.
Then I check preprocessor again.
I still can't find 'struct pacct_struct' even I include original header file
Anything in sched.h after #ifdef __KERNEL__ is disappear, What happened?
I have a.h:
#include "b.h" // XX
struct A {
int asdf;
struct B *b;
};
extern struct A aaa;
Also b.h
#include "a.h"
struct B {
int asdf;
int ghjk;
}
static inline void sth() { aaa.asdf*** }
It gives following errors:
b.h:17:6: error: ‘aaa’ undeclared (first use in this function)
But if I remove line XX, the compile succeeds.
Is it OK to do that?
The compile might work if you remove that line, but the program won't link.
You have to define aaa somewhere.
Your header files must be surrounded by #ifndef HEADER_H... header guards.
Having two header files that include each other is a sign of bad program design.
The presence of extern anywhere in your code is a sign of bad program design, with very few exceptions.
There are a few cases where you can place a (static inline) function definition inside a header file, but most of the time that means bad program design.
Summary: really bad program design leads to really bad programs.
Ok, I have a pretty good understanding of how header files are to be used in C programs but I keep having this problem that I can't seem to get around. Here is the situation:
Device.c contains #include "Device.h"
Device.h contains a definition for a struct and the following...
#ifndef DEVICE_H
#define DEVICE_H
#include "SubDevice.h"
typedef struct {
subDevice * subDevice1;
subDevice * subDevice2;
} device;
#endif
SubDevice.c contains #include SubDevice.h
SubDevice.h contains the definition of the subDevice struct and the following...
#ifndef SUBDEVICE_H
#define SUBDEVICE_H
#include "Device.h"
typedef struct{
int MyInt;
double MyDouble;
}subDevice;
#endif
The problem is that I get a compiler error in Device.h in my struct definition. At the line subDevice * subDevice1; it says syntax error before subDevice.
Is the problem that I am including the header file of the other file in each file? I thought the #ifndef - #define statements would prevent that from causing a problem...
It's not a header problem. There is a problem on one typedef you have in SubDevice.h. So, do change:
typedef struct{
int MyInt;
double MyDouble;
}
To:
typedef struct{
int MyInt;
double MyDouble;
} subDevice;
Your problem should be gone.
You must put a semicolon after the declaration of a struct, and your typedef is missing the second parameter.
EDIT to reflect OP's edit
You're including DEVICE_H, which is an macro.
You need to include "device.h":
#include "device.h"
The problem is that you have a circular reference.
I guess your main file looks something like this:
#include "Subdevice.h"
#include "Device.h"
int main() {}
Because of how the C preprocessor works (and how you use the include guards), the contents of Device.h will be read before Subdevice.h, meaning that the subDevice struct has yet to be defined when you define the device struct.
Remove the #include "Device.h" from Subdevice.h and it should compile OK.
You have "typedef struct {...}" in your subdevice.h, when you should have "typedef struct {...} subDevice;"
The line "#include DEVICE_H" also looks somewhat wrong. Did you mean #include "device.h"?
What you seem to be missing is a good understanding of the compilation model used by C (and C++) compilers.
Compiling a C or C++ source file can be separated in three stages:
Preprocessing
Compiling
Linking
Preprocessing
During preprocessing, the compiler (or separate preprocessor) goes over the source file and performs macro expansion and #include replacements.
All #include directives are replaced by the contents of the referenced file.
All macro definitions are processed and where a macro invocation occurs in the text, it is substituted with the replacement text. This is a completely textual process.
After preprocessing, there are no preprocessor directives (lines starting with a #) left in the source code
Compiling
During compilation, the compiler interprets the source code according to the rules of the C (or C++) language and translates the code to object code.
Linking
During the linking phase, all object files are combined together and linked with the relevant libraries to create an executable.
The source code presented here has two problems:
The subdevice.h file contains a spurious #include DEVICE_H directive. This should be reported by the preprocessor/compiler, because DEVICE_H does not resolve to a filename in quotes or angle-brackets.
There is a missing semi-colon (;) (and a missing typedef name) at the end of the typedef in subdevice.h. As a result, the compiler tries to combine this with the next lines of code it sees (the typedef struct in device.h) and gets terribly confused. This triggers the syntax error message.
One good way of debugging your code with preprocessor errors is to gcc -E <files>, so that you can actually see what would your program look like after that preprocessor is done with it.
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.