Combining Variable Declarations (GNU C Language Manual)

20.1.2 Combining Variable Declarations

When multiple declarations have the same keywords and basetype, you can combine them using commas. Thus,

keywords basetype
 decorated-variable-1 [= init1],
 decorated-variable-2 [= init2];

is equivalent to

keywords basetype
 decorated-variable-1 [= init1];
keywords basetype
 decorated-variable-2 [= init2];

Here are some simple examples:

int a, b;
int a = 1, b = 2;
int a, *p, array[5];
int a = 0, *p = &a, array[5] = {1, 2};

In the last two examples, a is an int, p is a pointer to int, and array is an array of 5 ints. Since the initializer for array specifies only two elements, the other three elements are initialized to zero.