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Full Discussion: Extern variable.
Top Forums Programming Extern variable. Post 302219549 by spirtle on Tuesday 29th of July 2008 12:17:46 PM
Old 07-29-2008
So, the gcc linker appears not to insist on the extern qualifier.
It looks in fact like the compler and linker is happy that a1 is declared and tacitly assumes the extern and the definition -- recall the difference in C between definition of a variable, which tells the compiler to allocate memory for it, and the declaration, which tells the compiler what type it is. The distinction is apparent in global variables, where
Code:
int foo = 3;

is both definition and declaration, but
Code:
extern int foo;

is just a declaration -- it says that a1 is of type int but memory for it is allocated elsewhere.

Note that
  1. If you define a1 in bothe file1.c and file2.c, e.g.
    Code:
    int a1 = 2;

    then the linker does complain
  2. If you compile bothe files in the same compilation unit, e.g.
    Code:
    gcc -o file12.o -c file1.c file2.c

    then the compiler complains.

The a1 variable in file2.c is protected by the lexical scope of the function next and so is completely different to the global int a1.
 

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COMBINE(1)																COMBINE(1)

NAME
combine - combine sets of lines from two files using boolean operations SYNOPSIS
combine file1 and file2 combine file1 not file2 combine file1 or file2 combine file1 xor file2 _ file1 and file2 _ _ file1 not file2 _ _ file1 or file2 _ _ file1 xor file2 _ DESCRIPTION
combine combines the lines in two files. Depending on the boolean operation specified, the contents will be combined in different ways: and Outputs lines that are in file1 if they are also present in file2. not Outputs lines that are in file1 but not in file2. or Outputs lines that are in file1 or file2. xor Outputs lines that are in either file1 or file2, but not in both files. "-" can be specified for either file to read stdin for that file. The input files need not be sorted, and the lines are output in the order they occur in file1 (followed by the order they occur in file2 for the two "or" operations). Bear in mind that this means that the operations are not commutative; "a and b" will not necessarily be the same as "b and a". To obtain commutative behavior sort and uniq the result. Note that this program can be installed as "_" to allow for the syntactic sugar shown in the latter half of the synopsis (similar to the test/[ command). It is not currently installed as "_" by default, but you can alias it to that if you like. SEE ALSO
join(1) AUTHOR
Copyright 2006 by Joey Hess <joey@kitenet.net> Licensed under the GNU GPL. moreutils 2012-04-09 COMBINE(1)
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