extern -- C Keyword

Declare storage class

extern indicates  that a C  element belongs to the  external storage class.
Both variables  and functions  may be  declared to be  extern. Use  of this
keyword  tells the  C compiler  that  the variable  or function  is defined
outside of  the present file  of source code.  All  functions and variables
defined outside of functions are implicitly extern unless declared static.

When a  source file references  data that are  defined in another  file, it
must declare  the data  to be  extern, or the  linker will return  an error
message of the form:

    undefined symbol name

For example, the following declares the array tzname:

    extern char tzname[2][32];

When a function calls a function  that is defined in another source file or
in a library,  it should declare the function to  be extern. In the absence
of a  declaration, extern functions  are assumed to return  ints, which may
cause serious  problems if the  function actually returns  a 32-bit pointer
(such as on the 68000 or i8086 LARGE model), a long, or a double.

For  example,  the function  malloc  appears  in a  library  and returns  a
pointer; therefore, it should be declared as follows:

    extern char *malloc();

If you do  not do so, the compiler assumes  that malloc returns an int, and
generate the error message

    integer pointer pun

when you attempt to use malloc in your program.

See Also

auto,
C keywords,
pun,
register,
static,
storage class
ANSI Standard, §6.5.1

© 2012 by Stephen A. Ness