Global Variables in C

First the disclaimers;
Global variables are bad. Global variables are evil. Global variables are, and will continue to be much like a 6-ft chainsaw, extremely dangerous to use, can regularly be replaced with a safer tool/technique, but are sometimes the proper tool for the job (like cutting down 12-ft diameter trees). None-the-less, on the rare occasion that a global variable is the proper technique, you'll need to know how to define and use them...the subject of the remainder of this post.

The following example will be used to explain techniques for defining and using globals, we will speak to it for the remainder of the post.

package1.h

1 #ifndef PACKAGE1_H
2 #define PACKAGE1_H
3 static const float GlobFloat = 3.14159;
4 static double GlobDouble = 1.23456;
5 extern unsigned GlobUnsigned;
6 //char GlobChar='a';
7
8 void func1(void);
9 #endif

package1.c

1 #include "package1.h"
2 #include
3
4 static int PackageGlobalInt = 1;
5 unsigned GlobUnsigned = 12345;
6
7 void func1() {
8 printf("(%s:%d) PackageGlobalInt(%d)\n",__FILE__,__LINE__,PackageGlobalInt++);
9 printf("(%s:%d) GlobFloat(%f)\n",__FILE__,__LINE__,GlobFloat);
10 printf("(%s:%d) GlobDouble(%lf)\n",__FILE__,__LINE__,GlobDouble++);
11 printf("(%s:%d) GlobUnsigned(%u)\n",__FILE__,__LINE__,GlobUnsigned++);
12 printf("(%s:%d) GlobDouble(%lf)\n",__FILE__,__LINE__,GlobDouble++);
13 printf("(%s:%d) GlobUnsigned(%u)\n",__FILE__,__LINE__,GlobUnsigned++);
14 }
15

package2.h

1 #ifndef PACKAGE2_H
2 #define PACKAGE2_H
3 #include "package1.h"
4
5 extern long GlobLong;
6 void func2(void);
7 #endif

package2.c

1 #include "package2.h"
2 #include
3
4 static int PackageGlobalInt = 2;
5 long GlobLong = 987654321;
6
7 void func2() {
8 printf("(%s:%d) PackageGlobalInt(%d)\n",__FILE__,__LINE__,PackageGlobalInt++);
9 printf("(%s:%d) GlobFloat(%f)\n",__FILE__,__LINE__,GlobFloat);
10 printf("(%s:%d) GlobDouble(%lf)\n",__FILE__,__LINE__,GlobDouble++);
11 printf("(%s:%d) GlobUnsigned(%u)\n",__FILE__,__LINE__,GlobUnsigned++);
12 printf("(%s:%d) GlobLong(%li)\n",__FILE__,__LINE__,GlobLong++);
13 printf("(%s:%d) GlobDouble(%lf)\n",__FILE__,__LINE__,GlobDouble++);
14 printf("(%s:%d) GlobUnsigned(%u)\n",__FILE__,__LINE__,GlobUnsigned++);
15 printf("(%s:%d) GlobLong(%li)\n",__FILE__,__LINE__,GlobLong++);
16 }
17

main.c

1 #include
2 #include "package1.h"
3 #include "package2.h"
4
5 int main() {
6 printf("(%s:%d) main process initializing\n",__FILE__,__LINE__);
7
8 func1();
9 func2();
10
11 printf("(%s:%d) GlobFloat(%f)\n",__FILE__,__LINE__,GlobFloat);
12 printf("(%s:%d) GlobDouble(%lf)\n",__FILE__,__LINE__,GlobDouble);
13 printf("(%s:%d) GlobUnsigned(%u)\n",__FILE__,__LINE__,GlobUnsigned);
14 printf("(%s:%d) GlobLong(%li)\n",__FILE__,__LINE__,GlobLong);
15
16
17 printf("(%s:%d) main process terminating\n",__FILE__,__LINE__);
18 }
19


Globals are a form of interface, so rightly are typically defined in a header file. Package 1 attempts to define 3 global variables. You will soon see that it was only successful in defining one. Before we get into the details, we should review some preliminaries.

#includes

As you should already be aware, header files serve little purpose other than organization of code. When encountering a #include directive, the precompiler does little more than copy-n-pasting the contents into the sourcing file. This is significant in our later discussion.

Static Variables

Static variables cannot be accessed outside a translation unit. For all intents and purposes, a translation unit is a C source file. This, along with what we know about #includes will explain why only one of our intended global variable declarations was successful.

Back To Our Example

The contents of package1.h on lines 3-5 are three attempts to declare a global variable. Note, lines 3-4 define the variables as static, line 5 defining it as external linkage. Why are the first two variables defined as static??? Because if I don't, I get a linker error "multiple definition of . . ." error. Pounding on the keyboard, slowly beating the linker into submission is accomplished by declaring the variable as static. Finally, the linker shuts up; we must have fixed it, right? Wrong!

So, why doesn't lines 3-4 work? More importantly, how do we know it doesn't work? Well, we can confirm that line 4 didn't work by comparing the results of lines package1.c:10, package1.c:12, package2.c:10, package2.c:13, and main.c:12. Note that on package 1 & 2 functions, we are incrementing the contents of GlobDouble. If we were successful in defining a global variable, we should see the variable initially assigned 1.23456, incremented 4 times with a final result of 5.23456.


~/StorageClasses$ ./main | grep GlobDouble
(package1.c:10) GlobDouble(1.234560)
(package1.c:12) GlobDouble(2.234560)
(package2.c:10) GlobDouble(1.234560)
(package2.c:13) GlobDouble(2.234560)
(main.c:12) GlobDouble(1.234560)

Hmmmph, we didn't accomplish what we intended, did we? The static storage class, coupled with the precompiler copy-n-paste behavior resulted in 3 independent variable declarations; one in package1.c, one in package2.c, and one in main.c. Each instance is unique to each translation unit, so incrementing one only affects the declaration in that translation unit.

Line 5 however successfully defined a global variable. How? Well, line 5 simply states that we are declaring a variable, to be defined elsewhere. Declaration simply defines a reference to variable that will be defined elsewhere (in this case, line 5 in package1.c). Translation units: package1.c, package2.c and main.c will each declare a reference to GlobUnsigned, but the single definition of the variable resides in package1.c.

We can confirm that all is well by means of:


~/StorageClasses$ ./main | grep GlobUnsigned
(package1.c:11) GlobUnsigned(12345)
(package1.c:13) GlobUnsigned(12346)
(package2.c:11) GlobUnsigned(12347)
(package2.c:14) GlobUnsigned(12348)
(main.c:13) GlobUnsigned(12349)



It is worth noting that lines 3-4 in package1.h have identical results as line 4 in both package1.c and package2.c. All instances are similar in the matter that their scope is limited to the translation unit in which they are declared.