union u_tag { int u_int; float u_float; char* u_char; } an_u;
This code generates a stab for a union tag and a stab for a union
variable. Both use the N_LSYM
stab type. If a union variable is
scoped locally to the procedure in which it is defined, its stab is
located immediately preceding the N_LBRAC
for the procedure's block
start.
The stab for the union tag, however, is located preceding the code for
the procedure in which it is defined. The stab type is N_LSYM
. This
would seem to imply that the union type is file scope, like the struct
type s_tag
. This is not true. The contents and position of the stab
for u_type
do not convey any information about its procedure local
scope.
# 128 is N_LSYM
.stabs "u_tag:T23=u4u_int:1,0,32;u_float:12,0,32;u_char:21,0,32;;",
128,0,0,0
The symbol descriptor ‘T’, following the ‘name:’ means that the stab describes an enumeration, structure, or union tag. The type descriptor ‘u’, following the ‘23=’ of the type definition, narrows it down to a union type definition. Following the ‘u’ is the number of bytes in the union. After that is a list of union element descriptions. Their format is ‘name:type, bit offset into the union, number of bytes for the element;’.
The stab for the union variable is:
.stabs "an_u:23",128,0,0,-20 # 128 is N_LSYM
‘-20’ specifies where the variable is stored (see Stack Variables).