16. Block data

A block data subprogram exists for one purpose: to give initial values to named COMMON blocks (§16). It is the only place a named common block may be initialized — a DATA statement in a main program or ordinary subprogram cannot touch common.

      BLOCK DATA TABLES
      COMMON /PHYS/ G, C
      DATA   G, C /9.81, 2.998E8/
      END

• Begins with BLOCK DATA, optionally named (BLOCK DATA TABLES). The name, if given, must be unique across the program.
• It is nonexecutable — it contains only specification statements: IMPLICIT, PARAMETER, DIMENSION, COMMON, SAVE, EQUIVALENCE, type statements, and DATA. No executable code.
• Only named common may be initialized here (not blank common).
• A program may have several block data subprograms, but a given common block may be initialized in only one of them.

The main program (and any subprogram) then declares the same COMMON block and sees the initial values:

      PROGRAM SIM
      COMMON /PHYS/ G, C        ! G and C already hold 9.81 and 2.998E8
      ...
      END

Two finer rules (§16.2): if you initialize any entity of a common block, the block data should list all of the block's entities (so its full layout is established); and a program may contain at most one unnamed block data subprogram.

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forterp notes. A BLOCK DATA correctly seeds named common read by other units. The at-most-one-unnamed rule is enforced — a second unnamed BLOCK DATA is a hard error (otherwise the two would collide and one block's initialization would be silently lost). The "specify all entities" rule is not enforced: a block data may initialize just a prefix of a block (harmless; the rest stays uninitialized). Note BLOCK DATA is obsolescent in modern Fortran (replaced by module initialization), but it is the standard mechanism in F77. See Appendix D.