shared global variables in C - c

How can I create global variables that are shared in C? If I put it in a header file, then the linker complains that the variables are already defined. Is the only way to declare the variable in one of my C files and to manually put in externs at the top of all the other C files that want to use it? That sounds not ideal.

In one header file (shared.h):
extern int this_is_global;
In every file that you want to use this global symbol, include header containing the extern declaration:
#include "shared.h"
To avoid multiple linker definitions, just one declaration of your global symbol must be present across your compilation units (e.g: shared.cpp) :
/* shared.cpp */
#include "shared.h"
int this_is_global;

In the header file write it with extern.
And at the global scope of one of the c files declare it without extern.

In the header file
header file
#ifndef SHAREFILE_INCLUDED
#define SHAREFILE_INCLUDED
#ifdef MAIN_FILE
int global;
#else
extern int global;
#endif
#endif
In the file with the file you want the global to live:
#define MAIN_FILE
#include "share.h"
In the other files that need the extern version:
#include "share.h"

You put the declaration in a header file, e.g.
extern int my_global;
In one of your .c files you define it at global scope.
int my_global;
Every .c file that wants access to my_global includes the header file with the extern in.

If you're sharing code between C and C++, remember to add the following to the shared.hfile:
#ifdef __cplusplus
extern "C" {
#endif
extern int my_global;
/* other extern declarations ... */
#ifdef __cplusplus
}
#endif

There is a cleaner way with just one header file so it is simpler to maintain.
In the header with the global variables prefix each declaration with a keyword (I use common) then in just one source file include it like this
#define common
#include "globals.h"
#undef common
and any other source files like this
#define common extern
#include "globals.h"
#undef common
Just make sure you don't initialise any of the variables in the globals.h file or the linker will still complain as an initialised variable is not treated as external even with the extern keyword. The global.h file looks similar to this
#pragma once
common int globala;
common int globalb;
etc.
seems to work for any type of declaration. Don't use the common keyword on #define of course.

There is a more elegant way to create global variables.
Just declare the variables as static inside a ".c" source file and create set/get functions.
The example below I use to override malloc, realloc and free functions during memory allocation tests.
Example:
memory-allocator.h
#ifndef MEMORY_ALLOCATOR_H_
#define MEMORY_ALLOCATOR_H_
#include <stddef.h>
void std_set_memory_allocators(void *(*malloc)(size_t size),
void *(realloc)(void *ptr, size_t size),
void (*free)(void *ptr));
void std_set_reset_allocators();
void *std_malloc(size_t size);
void *std_realloc(void *ptr, size_t size);
void std_free(void *ptr);
#endif // MEMORY_ALLOCATOR_H_
memory-allocator.c
#include "memory-allocator.h"
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
typedef struct {
void *(*malloc)(size_t size);
void *(*realloc)(void *ptr, size_t size);
void (*free)(void *ptr);
} StdMemoryAllocator;
StdMemoryAllocator memory_allocators = {&malloc, &realloc, &free};
void std_set_memory_allocators(void *(*malloc)(size_t size),
void *(realloc)(void *ptr, size_t size),
void (*free)(void *ptr)) {
memory_allocators.malloc = malloc;
memory_allocators.realloc = realloc;
memory_allocators.free = free;
}
void std_set_reset_allocators() {
memory_allocators.malloc = malloc;
memory_allocators.realloc = realloc;
memory_allocators.free = free;
}
void *std_malloc(size_t size) {
return memory_allocators.malloc(size);
}
void *std_realloc(void *ptr, size_t size) {
return memory_allocators.realloc(ptr, size);
}
void std_free(void *ptr) {
memory_allocators.free(ptr);
}
The struct static struct StdMemoryAllocator_s memory_allocators is started automatically when the application starts, and it point to the default C memory allocators.

Related

Is there any way to access static global variable defined in main.c and modify it in another file?

For example:
In main.c
static glob_var;
I want to modify the value of glob_var in another file say file1.c
Making an variable static makes its identifier inaccessible from other translation unit (what usually means other C-file). You can either
Make the variable non-static.
//main.c
int glob_var;
//file1.c
extern int glob_var;
Note that the declaration should be put to a header file.
Keep it static and add a helper function for access.
//main.c
static int glob_var;
void SetGlobVar(int val) {
glob_var = val;
}
//file1.c
void SetGlobVar(int);
void foo(void) {
SetGlobVar(42);
}
Note that the declaration of SetGlobVar() should be put to a header file.
The comments and other answer address the question of how to access and modify a static global". The following is offed as an alternative to using static for this purpose...
When needing to create a variable that is global and can be changed among several translation units I believe it is more idiomatic to use the externstorage class. This is typically done by:
Declaring the extern variable in a header file
Defining the extern variable one-time-only in a .c file that #includess the header file.
Access and modify the extern class variable from any translation unit that #includes the header file in which the extern is declared.
Example:
in some.h:
void modify_glob_var(int val);
...
extern int glob_var;//declare extern variable
in main.c
#include "some.h"
...
int glob_var = 0;// define extern variable
...
modify_glob_var(10);//access and modify extern variable
in some_other.c
include "some.h"
...
void modify_glob_var(int val)
{
glob_var = val;//value of extern glob_var is changed here
}

need a workaround for a "multiple definition" error

A toy code illustrating my problem is as follows:
stuff.h:
#ifndef STUFF
#define STUFF
int a;
int testarr[]={1,2,3};
#endif
fcn.h:
#include "stuff.h"
int b[]={5,6,7};
void fcn();
main.h:
#include "stuff.h"
#include <stdio.h>
fcn.c:
#include "main.h"
void fcn() {
printf("Hello\n");
}
main.c:
#include "main.h"
#include "fcn.h"
int main() {
fcn();
printf("HI\n");
}
An attempt to compile fails with:
/g/pe_19976/fcn_2.o:(.data+0x40): multiple definition of `testarr'
/g/pe_19976/main_1.o:(.data+0x40): first defined here
After doing some reading, I realize that defining the array testarr in the header file is a problem. But the thing is, in my real code, several files need access to testarr and it needs to have the same assigned values everywhere. I guess I could put it in main.h (?) but even if that would work, in my real code it logically belongs in stuff.h. How do I solve this conundrum?
BTW, based on something else I found, I tried defining testarr as extern but got the same problem.
When you put a variable definition into a header file, any .c file that includes it will have a copy of that variable. When you then attempt to link them, you get a multiple definition error.
Your header files should contain only a declaration of the variable. This is done using the extern keyword, and with no initializer.
Then in exactly one .c file, you put the definition along with an optional initializer.
For example:
main.c:
#include "main.h"
#include "fcn.h"
int a;
int testarr[]={1,2,3};
int main() {
fcn();
printf("HI\n");
}
stuff.h:
#ifndef STUFF
#define STUFF
extern int a;
extern int testarr[];
#endif
fcn.h:
#include "stuff.h"
extern int b[];
void fcn();
fcn.c:
#include "main.h"
int b[]={5,6,7};
void fcn() {
printf("Hello\n");
}
It is not clear why you are using so many global variables. The array
int testarr[]={1,2,3};
is defined as many times as there are compilation units (in your example there are at least two compilation units) that include the corresponding header.
Declare the array in a header like
extern int testarr[3];
and define it in a cpp module.
int testarr[]={1,2,3};
The same is valid for other global variables that have external linkage.
As for this remark
BTW, based on something else I found, I tried defining testarr as
extern but got the same problem.
Then the array with the specifier extern shall not be initialized in a header. Otherwise it is a definition of the array.

How to define variables in a header file, C

I want to declare and define (with a default value) a variable in a .h file.
When I do that I get
/tmp/cc19EVVe.o:(.data+0x0): multiple definition of `car_name'
/tmp/cc3twlar.o:(.data+0x0): first defined here
collect2: error: ld returned 1 exit status
How do I achieve my goal? Namely, to declare and define with default values a variable in a .h file and use that variable in multiple .c files?
Here is the A.h file
char * car_name = "Volkswagen";
void execute();
Here are the first file that uses the variable car_name defined in A.h: (The file is called execute.c)
#include "A.h"
#include <stdio.h>
#include <string.h>
void execute(){
int len = sizeof(car_name) + 2;
char car_name_with_new_line[len];
strncat(car_name_with_new_line, car_name, sizeof(car_name));
strncat(car_name_with_new_line, "\n", 1);
printf(car_name_with_new_line);
}
That's the other .c file: (It's called main.c)
#include "A.h"
int main(int argc, char ** argv){
execute();
return 0;
}
The answer is simple: Define your variables in exactly one compilation unit (.c file). Declare them in the header file associated with that .c file.
foo.h
extern char *g_name; // Declare that g_name exists
foo.c
#include "foo.h"
char *g_name; // Define g_name in one place
static char *m_private; // Private to foo.c, not "exported" via foo.h
main.c
#include "foo.h"
void somefunc(void)
{
// use g_name
}
1) define the variable in a single file, do not add a static modifier
2) place an extern statement for that variable in the header file.
then only one instance of the variable exists anyone that includes the header file can access the variable.
Note: it is poor programming practice to have global variables.
Good programming practice is to write accessor functions and hide the variable within a file. similar to the following:
static int myVariable = 0;
void setMyVariable( int myVariableParm )
{
myVariable = myVariableParm;
}
int getMyVariable( void )
{
return myVariable;
}

extern without type

If the syntax of extern is
extern <type> <name>;
how do I extern if I have an unnamed, single use struct:
struct {
char **plymouthThemes;
char *plymouthTheme;
} global;
I've tried
extern global;
without any type, and it doesn't work.
Or, do I have to name the struct?
You need to name your struct and put it in a .h file or included the definition by hand in every source file that uses global. Like this
///glob.h
struct GlobalStruct
{
///char** ...
///
};
///glob.cpp
#include "glob.h"
struct GlobalStruct global;
///someOtherFile.cpp
#include "glob.h"
extern struct GlobalStruct global;
If you do not want to name a struct there's common method:
--- global.h: (file with global struct definition):
#ifdef GLOBAL_HERE /* some macro, which defined in one file only*/
#define GLOBAL
#else
#define GLOBAL extern
#endif
GLOBAL struct {
char **plymouthThemes;
char *plymouthTheme;
} global;
---- file1.c (file where you want to have global allocated)
#define GLOBAL_HERE
#include "global.h"
---- file2.c (any oher file referencing to global)
#include "global.h"
The macro GLOBAL is conditionally defined so its usage will prepend a definition with "extern" everywhere except source where GLOBAL_HERE is defined. When you define GLOBAL_HERE then variable gets non-extern, so it will be allocated in output object of this source.
There's also short trick definition (which set in single .c file where you allocate globals):
#define extern
which cause preprocessor to remove extern (replace with empty string). But do not do it: redefining standard keywords is bad.
The idea is that you need to declare only one but still need to define the variable in each other file that uses it. The definition includes both the type (in your case a header define structure - which therefore need include) and the extern keyword to let know the compiler the declaration is in a different file.
here is my example
ext.h
struct mystruct{
int s,r;
};
ext1.c
#include "ext.h"
struct mystruct aaaa;
main(){
return 0;
}
ext2.c
#include "ext.h"
extern struct mystruct aaaa;
void foo(){
aaaa;
}
ext3.c
#include "ext.h"
extern struct mystruct aaaa;
void foo2(){
aaaa;
}

error: expected ')' before '*' token

I have this include file (memory .h)
#ifndef MEMORY_H
#define MEMORY_H
#ifdef __cplusplus
extern "C" {
#endif
typedef struct mmemory {
int* cells;
int* current_cell;
int cells_number;
} memory;
void memory_init(memory* mymemory, int size);
void step_left(memory* mymemory, int steps);
void step_right(memory* mymemory, int steps);
void cell_inc(memory* mymemory, int quantity);
void print_cell(memory* mymemory);
void get_char(memory* mymemory);
#ifdef __cplusplus
}
#endif
#endif /* MEMORY_H */
And this implementation file (memory.c)
#include <stdlib.h>
#include "memory.h"
void
memory_init (memory* mymemory, int size)
{
mymemory->cells = (int*) malloc (sizeof (int) * size);
mymemory->cells_number = size;
mymemory->current_cell = (int*) ((mymemory->cells_number / 2) * sizeof (int));
}
... //other function definitions follow
When I try to compile memory.c I get this error for each and every function definition
src/memory.c:5: error: expected ')' before '*' token
where line 5 is the function definition for memory_init()
Can someone please tell me why I'm getting this error?
Because the system memory.h is shadowing your memory.h, causing the #include to succeed without declaring your types. Several possible fixes:
Rename your file -- probably for the best in any case, to reduce potential confusion.
Include your file via a prefix subdirectory (e.g., #include <myproj/memory.h>).
Move your file into the same directory as the source file, allowing the #include precedence rules for filenames wrapped in " to take effect.
Ensure that your C pre-processor include path options place your project header path prior to the system header paths.
This answer is really late, but I encountered a similar problem.
I think your problem is related to a typo in your .h file where you declare a struct mmemory. If you remove that extra 'm' it should work.
In your code you have defined like this for memory.h
#ifndef MEMORY_H
#define MEMORY_H
...
...
#endif
In case any of your other files which you use in your project is having the same #define i.e MEMORY_H then you can get this error.
Solution:
#ifndef XYZ_MEMORY_H
#define XYZ_MEMORY_H
...
...
#endif

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