More on Storage Association

Fortran allows for a special, unnamed common block, called the blank COMMON block. It is declared as

 
common // var_1, var_2, ..., var_n

Blank common block is useful when most, or a large number of routines, need to share same variables. Some special features of the blank common block are

• contained data is automatically static,and
• it cannot be initialized in a BLOCK DATA statement;
• the compiler is more permissive (e.g. it allows the blank common block to be invoked with a different number of variables in different program units).

In F77 several entities (scalar or array) can be associated with the same memory location, using

 
equivalence ( var_1, var_2, ..., var_n )

All the variables in the list are stored in the same memory locations, more exactly, their storage spaces start at the same address in the memory.

EQUIVALENCE is usually used in conjunction with large common blocks, to identify the parts of the block for the current procedure. For example, a common block /VERY_LONG/ can store 28 real arrays, containing a total of 15,421 real elements

 
common /very_long/ a(7311), b(121), ..., z(1272)

Suppose in a procedure only the second array, B(121) is used; we can use the declaration

 
real b(121) 
common /very_long/ blk(15421) 
equivalence (b, blk(7312))

(we employed F77 syntax on purpose).

Sometimes the results of EQUIVALENCE statements are hard to understand. For example

 
real x, y(2), z 
complex c 
equivalence (x,y,c), (y(2), z)
c = cmplx(1.0,2.0)

has the effect of setting y(1) x REAL(c) = 1.0 and y(2) z AIMAG(c) = 2.0.