How Do I Use Extern to Share Variables Between Source Files?

Using extern is only of relevance when the program you're building consists of multiple source files linked together, where some of the variables defined, for example, in source file file1.c need to be referenced in other source files, such as file2.c.

It is important to understand the difference between defining a variable and declaring a variable:

• A variable is declared when the compiler is informed that a variable exists (and this is its type); it does not allocate the storage for the variable at that point.

• A variable is defined when the compiler allocates the storage for the variable.

You may declare a variable multiple times (though once is sufficient); you may only define it once within a given scope. A variable definition is also a declaration, but not all variable declarations are definitions.

Best way to declare and define global variables

The clean, reliable way to declare and define global variables is to use a header file to contain an extern declaration of the variable.

The header is included by the one source file that defines the variable and by all the source files that reference the variable. For each program, one source file (and only one source file) defines the variable. Similarly, one header file (and only one header file) should declare the variable. The header file is crucial; it enables cross-checking between independent TUs (translation units — think source files) and ensures consistency.

Although there are other ways of doing it, this method is simple and reliable. It is demonstrated by file3.h, file1.c and file2.c:

file3.h

extern int global_variable;  /* Declaration of the variable */

file1.c

#include "file3.h"  /* Declaration made available here */
#include "prog1.h"  /* Function declarations */

/* Variable defined here */
int global_variable = 37;    /* Definition checked against declaration */

int increment(void) { return global_variable++; }

file2.c

#include "file3.h"
#include "prog1.h"
#include <stdio.h>

void use_it(void)
{
    printf("Global variable: %d\n", global_variable++);
}

That's the best way to declare and define global variables.

The next two files complete the source for prog1:

The complete programs shown use functions, so function declarations have crept in. Both C99 and C11 require functions to be declared or defined before they are used (whereas C90 did not, for good reasons). I use the keyword extern in front of function declarations in headers for consistency — to match the extern in front of variable declarations in headers. Many people prefer not to use extern in front of function declarations; the compiler doesn't care — and ultimately, neither do I as long as you're consistent, at least within a source file.

prog1.h

extern void use_it(void);
extern int increment(void);

prog1.c

#include "file3.h"
#include "prog1.h"
#include <stdio.h>

int main(void)
{
    use_it();
    global_variable += 19;
    use_it();
    printf("Increment: %d\n", increment());
    return 0;
}

• prog1 uses prog1.c, file1.c, file2.c, file3.h and prog1.h.

The file prog1.mk is a makefile for prog1 only. It will work with most versions of make produced since about the turn of the millennium. It is not tied specifically to GNU Make.

prog1.mk

# Minimal makefile for prog1

PROGRAM = prog1
FILES.c = prog1.c file1.c file2.c
FILES.h = prog1.h file3.h
FILES.o = ${FILES.c:.c=.o}

CC      = gcc
SFLAGS  = -std=c11
GFLAGS  = -g
OFLAGS  = -O3
WFLAG1  = -Wall
WFLAG2  = -Wextra
WFLAG3  = -Werror
WFLAG4  = -Wstrict-prototypes
WFLAG5  = -Wmissing-prototypes
WFLAGS  = ${WFLAG1} ${WFLAG2} ${WFLAG3} ${WFLAG4} ${WFLAG5}
UFLAGS  = # Set on command line only

CFLAGS  = ${SFLAGS} ${GFLAGS} ${OFLAGS} ${WFLAGS} ${UFLAGS}
LDFLAGS =
LDLIBS  =

all:    ${PROGRAM}

${PROGRAM}: ${FILES.o}
    ${CC} -o $@ ${CFLAGS} ${FILES.o} ${LDFLAGS} ${LDLIBS}

prog1.o: ${FILES.h}
file1.o: ${FILES.h}
file2.o: ${FILES.h}

# If it exists, prog1.dSYM is a directory on macOS
DEBRIS = a.out core *~ *.dSYM
RM_FR  = rm -fr

clean:
    ${RM_FR} ${FILES.o} ${PROGRAM} ${DEBRIS}