What is extern in C?

C has a keyword extern. For example:

#include <stdio.h>
int main() {
  extern int x;
  printf("x = %d\n", x);

Note the extern on the x. What does this do? Let’s compile it to find out:

$ clang main.c
Undefined symbols for architecture x86_64:
  "_x", referenced from:
      _main in main-18e659.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

To understand this error, we need to understand extern. extern is a keyword which can be applied to declarations. For examples:

extern int x;
extern char * errstr;

To understand extern, we must first understand a distinction between declaration, definition, and initialization. Take this example:

int x = 5;

The above line does three things: it says that x exists and has type int; it allocates memory for x (enough for an int), and finally it gives that memory the initial value 5. These three parts are called declaration, definition, and initialization: the variable declaration asserts the existence of the variable with a given type, the variable definition allocates memory for that variable, and the variable initialization gives that memory an initial value. We often declare, define, and initialize a variable all together, as above. However, we do not have to do all three of these things at once!

declares? defines? initializes?
int x = 5; yes yes yes, 5
int x; yes yes no
extern int x; yes no no

Marking a C variable extern declares the variable without defining it. That is, no memory is allocated for it at that point in the program. Something elsewhere has to define the variable.

That “something elsewhere” is left to the linker to find. Notice that the error message from clang is not a compiler error, it is a linker error. When running clang main.c, you run the compiler, then the linker. The compilation stage compiled successfully, but the link stage failed because it failed to find _x. We can see that the compilation stage succeeds by compiling without linking, using the -c flag:

$ clang -c main.c
$ ls
main.c	main.o

We can then see that the link stage fails by running the link stage on the main.o:

$ clang main.o
Undefined symbols for architecture x86_64:
  "_x", referenced from:
      _main in main.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Object files, like main.o, have a symbol table. This table describes the things defined in the object file, as well as things that the object file expects from elsewhere. We can see this symbol table with the nm tool:

$ nm main.o
0000000000000000 T _main
                 U _printf
                 U _x

The T _main says that main.o defines one symbol, _main. The U _printf and U _x says that main.o declares, but does not define, the symbols _printf and _x. Thus, main.o expects the linker to find _printf and _x. The clang linker successfully finds _printf by linking with the C standard library. However, the linker does not find _x anywhere, and thus it complains.

To fix the error, we need to define _x somewhere. We can do that in a separate object file, like this:

// x.c
int x = 5;  // define and initialize x
$ clang -c x.c
$ nm x.o
0000000000000000 D _x
$ clang main.o x.o
$ ./a.out
x = 5
Tagged #extern, #c, #semantics, #programming.

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