In a.h I have
#ifndef A_H
#define A_H 1
const int global_val = 10;
#endif
And in a.c I have
#include "a.h"
int someFunc() {
return global_val;
}
But the compiler (gcc) throws an error: 'global_int' undeclared when compiling a.c. How can it possibly be undeclared when I've included the header file?
You should have two files.
A.h
#ifndef A_H
#define A_H 1
extern const int global_val;
#endif
and
A.c
#include "A.h"
const int global_val = 10;
I found the problem. I deleted all my .o, .gch, and executable files and the code started compiling again. My guess is that the precompiled headers (which I don't even remember making) were tricking the compiler.
Instead of closing the question, I decided to leave this answer it case it helps someone.
Related
I'm working on recompiling a C project and I'm not sure how do I fix this problem in a right way.
Here is a situation -
a.h
#ifndef A_H
#define A_H
typedef int INT;
// other variables and function definition
#endif
b.h
#ifndef B_H
#define B_H
typedef int INT;
// other variables and function definition
#endif
main.c
#include "a.h"
#include "b.h"
int main()
{
INT i = 10;
return 0;
}
The error I get in Linux with gcc:
In file included from ./main.c,
./b.h:<linenumber>: error: redefinition of typedef ‘INT’
a.h.h:<linenumber>: note: previous declaration of ‘INT’ was here
I have to include both headers due to other variables and functions. I haven't written this code, but this seems to compile in my Solaris environment which is strange. What can I do to fix this ?
Probably the native compiler on Solaris accepts that you can redefine a typedef (probably provided that the new typedef is identical to the previous one which is the case here).
I'd introduce another header file mytypes.h like this:
mytypes.h
#ifndef MYTYPES_H
#define MYTYPES_H
typedef int INT;
#endif
Include mtypes.h whereever INT is used, possibly even in main.c:
a.h
#ifndef A_H
#define A_H
#include "mytypes.h" // can be removed if INT is not used in a.h
// other variables and function definition
#endif
b.h
#ifndef B_H
#define B_H
#include "mytypes.h" // can be removed if INT is not used in b.h
// other variables and function definition
#endif
main.c
#include "a.h"
#include "b.h"
#include "mytypes.h" // not really necessary because it's already included
// via a.h and b.h, but still good practice
int main()
{
INT i = 10;
return 0;
}
If you are allowed to change the library code or compiler options then Michael Walz's answer is the way to go
In the unfortunate case that it's not changeable then it can be worked around by renaming before including the header then undefine it
#define INT INT_A
#include "a.h"
#undef INT
#define INT INT_B
#include "b.h"
#undef INT
Now just use INT_A for all the interfaces in a.h instead of INT. Same to INT_B in b.h
I have two source files, main.c and datamgr.c - and two header files, config.h and datamgr.h
The testing system we're using expects these files, and only these files.
main.c:
#include "datamgr.h"
#include "config.h"
int main() {
custom_type a = 1;
a = foo();
return 0;
}
datamgr.c:
#include "datamgr.h"
#include "config.h"
custom_type foo() {
custom_type a = 1;
return a;
}
datamgr.h:
#ifndef DATAMGR_H
#define DATAMGR_H
#include <stdio.h>
#include <stdlib.h>
#include <inttypes.h>
custom_type foo();
#endif
config.h:
#ifndef CONFIG_H
#define CONFIG_H
#ifndef SET_MAX_TEMP
#error "Max temperature not set."
#endif
#ifndef SET_MIN_TEMP
#error "Max temperature not set."
#endif
typedef custom_type uint16_t
#endif
Now, the problem is that I can only define SET_MAX_TEMP and SET_MIN_TEMP in main.c, but both main.c and datamgr.c need both the header files. So if I leave them undefined in datamgr.c I get a compiler error. However, if I do define them in datamgr.c and later overwrite them in main.c, I get a different compiler error.
Please, any assistance as to how to get this horrible setup to work would be greatly appreciated.
You can pass these defines directly while compiling:
gcc -DSET_MAX_TEMP -DSET_MIN_TEMP <your files>
In datamgr.c do:
#define SET_MAX_TEMP
#define SET_MIN_TEMP
#include "datamgr.h"
#include "config.h"
#undef SET_MAX_TEMP
#undef SET_MIN_TEMP
In a comment, you said:
Because main.c is the file that our testing system uses to implement the test scenarios.
In that case, make sure that the testing system defines those macros in the command line of the compiler for every file being compiled.
im having trouble linking .h and .c files, i've also read some threads regarding this problem and all of them is a bit vague and still i can't fully grasp the concept of it, and im having a lot of linking problems, Say i have b.c and b.h which i will use in a.c, and im confused whether to include b.h both a.c and b.c cuz b.c itself needs to know the structure defined in b.h, i have some function which has its prototype in b.h and is defined in b.c which also use the structure in b.h, im am not including b.h in b.c cuz as what i know b.h is more like an interface to a.c which will use the functions in b.c... Here a more clear example
b.h file
typedef struct{
int x, y;
}myStruct;
void funct1(myStruct);
void funct2(myStruct);
b.c file
void funct1(myStruct x)
{
//do something
}
void funct2(myStruct y)
{
//do something
}
a.c file
#include "b.h"
int main()
{
myStruct x;
funct1(x);
funct2(y);
return 0;
}
Executed the command in cygwin: gcc b.c a.c -g
Now the confusing part, i have a linking error wherein when b.c is compiled it can't detect the structure and the prototypes in b.h. Cuz all i know is that b.h is used to link b.c from a.c but when both .c is compiled it seems that b.c can't find its strucutre and prototypes,
Why didn't i include b.h in b.c?
Answer: Cuz as what i know, b.h is already included in a.c and when i include it again in b.c, i'll be doing double inclusions <--- thats what i learn so far and i know there is #ifdef but it seems it won't work, maybe i still don't know how to use it, if you know please feel free to discuss this.
If you have any idea as to how to go about this feel free to tell me some.
there is a #ifdef directive but i can't seem to have any idea how to do this.
NOTE: ASSUME THAT ALL ABOVE CODES IS SYNTACTICALLY CORRECT if there are any misspelled word please ignore, i'm only after the inclusions between .h and .c
You do indeed need to #include b.h in b.c. Each file is compiled separately before the linker takes over, so it doesn't matter that you have included b.h in a.c, because b.c is compiled by itself and has no idea about the contents of b.h unless you include it.
Here's an example of a #include guard
// some_header_file.h
#ifndef SOME_HEADER_FILE_H
#define SOME_HEADER_FILE_H
// your code
#endif
When some_header_file.h is included anywhere, everything in between the #ifndef and the #endif will be ignored if SOME_HEADER_FILE_H has been defined, which will happen on the first time it is included in the compilation unit.
It is common practice to name the #define after the name of the file, to ensure uniqueness within your project. I like to prefix it with the name of my project or namespace as well, to reduce the risk of clashes with other code.
NOTE: The same header file CAN be included multiple times within your project even with the above include guard, it just can't be included twice within the same compilation unit. This is demonstrated as follows:
// header1.h
#ifndef HEADER_H
#define HEADER_H
int test1 = 1;
#endif
// header2.h
#ifndef HEADER_H
#define HEADER_H
int test2 = 2;
#endif
Now let's see what happens when we try to include the above two files. In a single compilation unit:
// a.cpp
#include "header1.h"
#include "header2.h"
#include <iostream>
int main()
{
std::cout << test1;
std::cout << test2;
};
This generates a compiler error because test2 is not defined - it is ignored in header2.h because HEADER_H is already defined by the time that is included. Now if we include each header in separate compilation units:
// a.cpp
#include "header2.h"
int getTest2()
{
return test2;
};
// b.cpp
#include "header1.h"
#include <iostream>
int getTest2(); // forward declaration
int main()
{
std::cout << test1;
std::cout << getTest2();
};
It compiles fine and produces the expected output (1 and 2), even though we are including two files which both define HEADER_H.
You need to include b.h in all files that uses the structures that are defined in b.h. So you need to put a #include <b.h> in both files. To avoid that b.h is loaded several times, you need the directives #ifdef. In your case:
b.h
#ifndef B_H
#define B_H
typedef struct{
int x, y;
}myStruct;
void funct1(myStruct);
void funct2(myStruct);
#endif
and b.c:
#include "b.h"
void funct1(myStruct x)
{
//do something
}
void funct2(myStruct y)
{
//do something
}
Proper coding would have you include b.h in b.c.
Here is a header guard that should work:
#ifndef B_H_INCLUDED
#define B_H_INCLUDED
//header file
#endif
Put your declarations where the comment is, and include everywhere you need to.
EDIT
The way I understand it, is that gcc compiles b.c first, because a.c depends on b.c. But when it compiles b.c first, b.h has not yet been included.
You need to #include b.h in b.c. It is not just an interface for a.c, b.c needs to know the same definitions for its own code as well. Your reason for not including b.h in b.c is wrong. Each .c file is compiled separately from every other .c file. When the compiler is done with a.c, it starts over fresh with b.c. It does not matter that a.c included b.h, because b.c has no concept that a.c even exists. The purpose of a header guard is to prevent a .h file from being processed repeat times if it is included multiple times while compiling a given .c file. Without the guard, declarations would get compiled multiple times, causing errors about multiple declarations of existing symbols.
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.
For some reason, I'm getting multiple declarations of content within my header file even though I'm using header guards. My example code is below:
main.c:
#include "thing.h"
int main(){
printf("%d", increment());
return 0;
}
thing.c:
#include "thing.h"
int increment(){
return something++;
}
thing.h:
#ifndef THING_H_
#define THING_H_
#include <stdio.h>
int something = 0;
int increment();
#endif
When I attempt to compile this, GCC says that I have multiple definitions of the something variable. ifndef should make sure that this doesn't happen, so I'm confused why it is.
The include guards are functioning correctly and are not the source of the problem.
What happens is that every compilation unit that includes thing.h gets its own int something = 0, so the linker complains about multiple definitions.
Here is how you fix this:
thing.c:
#include "thing.h"
int something = 0;
int increment(){
return something++;
}
thing.h:
#ifndef THING_H_
#define THING_H_
#include <stdio.h>
extern int something;
int increment();
#endif
This way, only thing.c will have an instance of something, and main.c will refer to it.
You have one definition in each translation unit (one in main.c, and one in thing.c). The header guards stop the header from being included more than once in a single translation unit.
You need to declare something in the header file, and only define it in thing.c, just like the function:
thing.c:
#include "thing.h"
int something = 0;
int increment(void)
{
return something++;
}
thing.h:
#ifndef THING_H_
#define THING_H_
#include <stdio.h>
extern int something;
int increment(void);
#endif
The header guards will stop the file from being compiled more than once in the same compilation unit (file). You are including it in main.c and thing.c, so it will be compiled once in each, leading to the variable something being declared once in each unit, or twice in total.
try to avoid defining variables globally.
use functions like increment() to modify and read its value instead.
that way you can keep the variable static in the thing.c file, and you know for sure that only functions from that file will modify the value.
The variable something should be defined in a .c file, not
in a header file.
Only structures, macros and type declarations for variables and function prototypes
should be in header files. In your example, you can declare the type of something as extern int something in the header file. But the definition of the variable itself should be in a .c file.
With what you have done, the variable something will be defined
in each .c file that includes thing.h and you get a
"something defined multiple times" error message when GCC tries to link
everything together.
what ifndef is guarding is one .h included in a .c more than once. For instance
thing. h
#ifndef
#define
int something = 0;
#endif
thing2.h
#include "thing.h"
main.c
#include "thing.h"
#include "thing2.h"
int main()
{
printf("%d", something);
return 0;
}
if I leave ifndef out then GCC will complain
In file included from thing2.h:1:0,
from main.c:2:
thing.h:3:5: error: redefinition of ‘something’
thing.h:3:5: note: previous definition of ‘something’ was here