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.