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4.4 Common Blocks

A common block is a statically allocated section of memory which can be referred to by several source files. It may contain several variables. I believe Fortran is the only language with this feature.

A N_BCOMM stab begins a common block and an N_ECOMM stab ends it. The only field that is significant in these two stabs is the string, which names a normal (non-debugging) symbol that gives the address of the common block. According to IBM documentation, only the N_BCOMM has the name of the common block (even though their compiler actually puts it both places).

The stabs for the members of the common block are between the N_BCOMM and the N_ECOMM; the value of each stab is the offset within the common block of that variable. IBM uses the C_ECOML stab type, and there is a corresponding N_ECOML stab type, but Sun's Fortran compiler uses N_GSYM instead. The variables within a common block use the ‘V’ symbol descriptor (I believe this is true of all Fortran variables). Other stabs (at least type declarations using C_DECL) can also be between the N_BCOMM and the N_ECOMM.