stabs.texinfo

早期freebsd实现

        N_GSYM, NIL, NIL, NIL

21 .stabs "g_foo:G2",32,0,0,0
22      .global _g_foo
23      .data
24 _g_foo:
25      .byte 99
@end example

The address of the variable represented by the @code{N_GSYM} is not contained
in the @code{N_GSYM} stab.  The debugger gets this information from the
external symbol for the global variable.

@node Register variables
@section Global register variables 

@table @strong
@item Directive:
@code{.stabs}
@item Type:
@code{N_RSYM}
@item Symbol Descriptor:
@code{r}
@end table

The following source line defines a global variable, @code{g_bar}, which is
explicitly allocated in global register @code{%g5}.  

@example
2  register int g_bar asm ("%g5");
@end example

Register variables have their own stab type, @code{N_RSYM}, and their own
symbol descriptor, @code{r}.  The stab's value field contains the number of
the register where the variable data will be stored.  Since the
variable was not initialized in this compilation unit, the stab is
emited at the end of the object file, with the stabs for other
uninitialized globals (@code{bcc}).

@example
@exdent @code{N_RSYM} (64): register variable

.stabs "@var{name}:
        @var{descriptor}
        @var{type-ref}",
       N_RSYM, NIL, NIL,
       @var{register}
 
133 .stabs "g_bar:r1",64,0,0,5
@end example


@node Initialized statics
@section Initialized static variables 

@table @strong
@item Directive:
@code{.stabs}
@item Type:
@code{N_STSYM}
@item Symbol Descriptors:
@code{S} (file scope), @code{V} (procedure scope)
@end table

Initialized static variables are represented by the @code{N_STSYM} stab
type.  The symbol descriptor part of the string field shows if the
variable is file scope static (@samp{S}) or procedure scope static
(@samp{V}). The source line

@example
3  static int s_g_repeat = 2; 
@end example

@noindent
yields the following code.  The stab is located immediately preceding
the storage for the variable it represents.  Since the variable in
this example is file scope static the symbol descriptor is @samp{S}.

@example
@exdent @code{N_STSYM} (38): initialized static variable (data seg w/internal linkage)

.stabs "@var{name}:
        @var{descriptor}
        @var{type-ref}",
       N_STSYM,NIL,NIL,
       @var{address}
        
26 .stabs "s_g_repeat:S1",38,0,0,_s_g_repeat
27      .align 4
28 _s_g_repeat:
29      .word 2
@end example


@node Un-initialized statics
@section Un-initialized static variables

@table @strong
@item Directive:
@code{.stabs}
@item Type:
@code{N_LCSYM}
@item Symbol Descriptors:
@code{S} (file scope), @code{V} (procedure scope)
@end table

Un-initialized static variables are represented by the @code{N_LCSYM}
stab type.  The symbol descriptor part of the string shows if the
variable is file scope static (@samp{S}) or procedure scope static
(@samp{V}).  In this example it is procedure scope static.  The source
line allocating @code{s_flap} immediately follows the open brace for the
procedure @code{main}.

@example
20 @{
21      static float s_flap;
@end example

The code that reserves storage for the variable @code{s_flap} precedes the
body of body of @code{main}.  

@example
39      .reserve _s_flap.0,4,"bss",4
@end example

But since @code{s_flap} is scoped locally to @code{main}, its stab is
located with the other stabs representing symbols local to @code{main}.
The stab for @code{s_flap} is located just before the @code{N_LBRAC} for
@code{main}.

@example
@exdent @code{N_LCSYM} (40): uninitialized static var (BSS seg w/internal linkage)

.stabs "@var{name}:
        @var{descriptor}
        @var{type-ref}",
        N_LCSYM, NIL, NIL,
        @var{address}

97 .stabs "s_flap:V12",40,0,0,_s_flap.0
98 .stabs "times:1",128,0,0,-20
99 .stabn 192,0,0,LBB2                  # N_LBRAC for main.
@end example

@c ............................................................

@node Parameters
@section Parameters 

@table @strong
@item Directive: 
@code{.stabs}
@item Type:
@code{N_PSYM}
@item Symbol Descriptor:
@code{p}
@end table

Procedure parameters are represented by the N_PSYM stab type.  The
following source lines show the parameters of the main routine.

@example
17 main (argc, argv)
18      int argc;
19      char* argv[];
20 @{
@end example

The N_PSYM stabs describing parameters to a function directly follow
the N_FUN stab that represents the procedure itself.  The N_FUN stab
immediately follows the code of the procedure it describes.  Following
the N_PSYM parameter stabs are any N_LSYM stabs representing local
variables.

@example
@exdent <36> N_FUN - describing the procedure main

94 .stabs "main:F1",36,0,0,_main

@exdent <160> N_PSYM - parameters
@exdent .stabs "name:sym_desc(value_param)type_ref(int)", N_PSYM,
@exdent NIL, NIL, frame_ptr_offset

95 .stabs "argc:p1",160,0,0,68
    
@exdent <160> N_PSYM - parameter
@exdent .stabs "name:sym_desc(value_param)type_def(20)=ptr_to type_def(21)=
@exdent ptr_to type_ref(char)

96 .stabs "argv:p20=*21=*2",160,0,0,72
@end example

The type definition of argv is interesting because it defines two new
types in terms of an existing one.  The array argv contains character
pointers.  The type of the array name is a pointer to the type the
array holds. Thus the type of argv is ptr to ptr to char.  The stab
for argv contains nested type_definitions.  Type 21 is ptr to type 2
(char) and argv (type 20) is ptr to type 21.
 
@node Aggregate Types
@chapter Aggregate Types 

Now let's look at some variable definitions involving complex types.
This involves understanding better how types are described.  In the
examples so far types have been described as references to previously
defined types or defined in terms of subranges of or pointers to
previously defined types.  The section that follows will talk about
the various other type descriptors that may follow the = sign in a
type definition.

@menu
* Arrays::
* Enumerations::
* Structure tags::
* Typedefs::
* Unions::
* Function types::
@end menu

@node Arrays
@section Array types 

@table @strong
@item Directive:
@code{.stabs}
@item Types:
@code{N_GSYM}, @code{N_LSYM}
@item Symbol Descriptor:
@code{T}
@item Type Descriptor:
@code{ar}
@end table

As an example of an array type consider the global variable below.

@example
15 char char_vec[3] = @{'a','b','c'@};
@end example

Since the array is a global variable, it is described by the N_GSYM
stab type.  The symbol descriptor G, following the colon in stab's
string field, also says the array is a global variable.  Following the
G is a definition for type (19) as shown by the equals sign after the
type number.  

After the equals sign is a type descriptor, ar, which says that the
type being defined is an array.  Following the type descriptor for an
array is the type of the index, a null field, the upper bound of the
array indexing, and the type of the array elements.

The array definition above generates the assembly language that
follows.

@example
@exdent <32> N_GSYM - global variable
@exdent .stabs "name:sym_desc(global)type_def(19)=type_desc(array)
@exdent index_type_ref(int);NIL;high_bound(2);element_type_ref(char)";
@exdent N_GSYM, NIL, NIL, NIL

32 .stabs "char_vec:G19=ar1;0;2;2",32,0,0,0
33      .global _char_vec
34      .align 4
35 _char_vec:
36      .byte 97
37      .byte 98
38      .byte 99
@end example

@node Enumerations
@section Enumerations 

@table @strong
@item Directive:
@code{.stabs}
@item Type:
@code{N_LSYM}
@item Symbol Descriptor:
@code{T}
@item Type Descriptor:
@code{e}
@end table

The source line below declares an enumeration type.  It is defined at
file scope between the bodies of main and s_proc in example2.c.
Because the N_LSYM is located after the N_RBRAC that marks the end of
the previous procedure's block scope, and before the N_FUN that marks
the beginning of the next procedure's block scope, the N_LSYM does not
describe a block local symbol, but a file local one.  The source line:

@example
29 enum e_places @{first,second=3,last@};
@end example

@noindent
generates the following stab, located just after the N_RBRAC (close
brace stab) for main.  The type definition is in an N_LSYM stab
because type definitions are file scope not global scope.

@display
    <128> N_LSYM - local symbol
    .stab "name:sym_dec(type)type_def(22)=sym_desc(enum)
           enum_name:value(0),enum_name:value(3),enum_name:value(4),;",
           N_LSYM, NIL, NIL, NIL
@end display

@example
104 .stabs "e_places:T22=efirst:0,second:3,last:4,;",128,0,0,0
@end example

The symbol descriptor (T) says that the stab describes a structure,
enumeration, or type tag.  The type descriptor e, following the 22= of
the type definition narrows it down to an enumeration type.  Following
the e is a list of the elements of the enumeration.  The format is
name:value,. The list of elements ends with a ;.

@node Structure tags
@section Structure Tags

@table @strong
@item Directive:
@code{.stabs}
@item Type:
@code{N_LSYM}
@item Symbol Descriptor:
@code{T}
@item Type Descriptor:
@code{s}
@end table

The following source code declares a structure tag and defines an
instance of  the structure in global scope. Then a typedef equates the
structure tag with a new type.  A seperate stab is generated for the
structure tag, the structure typedef, and the structure instance.  The
stabs for the tag and the typedef are emited when the definitions are
encountered.  Since the structure elements are not initialized, the
stab and code for the structure variable itself is located at the end
of the program in .common.

@example
6  struct s_tag @{
7    int   s_int;
8    float s_float;
9    char  s_char_vec[8];
10   struct s_tag* s_next;
11 @} g_an_s;
12 
13 typedef struct s_tag s_typedef;
@end example

The structure tag is an N_LSYM stab type because, like the enum, the
symbol is file scope.  Like the enum, the symbol descriptor is T, for
enumeration, struct or tag type.  The symbol descriptor s following
the 16= of the type definition narrows the symbol type to struct.

Following the struct symbol descriptor is the number of bytes the
struct occupies, followed by a description of each structure element.
The structure element descriptions are of the form name:type, bit
offset from the start of the struct, and number of bits in the
element.


@example
   <128> N_LSYM - type definition 
   .stabs "name:sym_desc(struct tag) Type_def(16)=type_desc(struct type) 
        struct_bytes
        elem_name:type_ref(int),bit_offset,field_bits;
        elem_name:type_ref(float),bit_offset,field_bits;
        elem_name:type_def(17)=type_desc(dynamic array) index_type(int);NIL;
        high_bound(7);element_type(char),bit_offset,field_bits;;",
        N_LSYM,NIL,NIL,NIL

30 .stabs "s_tag:T16=s20s_int:1,0,32;s_float:12,32,32;
           s_char_vec:17=ar1;0;7;2,64,64;s_next:18=*16,128,32;;",128,0,0,0
@end example
 
In this example, two of the structure elements are previously defined
types.  For these, the type following the name: part of the element
description is a simple type reference.  The other two structure
elements are new types.  In this case there is a type definition
embedded after the name:.  The type definition for the array element
looks just like a type definition for a standalone array.  The s_next
field is a pointer to the same kind of structure that the field is an
element of.  So the definition of structure type 16 contains an type
definition for an element which is a pointer to type 16. 

@node Typedefs
@section Typedefs

@table @strong
@item Directive:
@code{.stabs}
@item Type:
@code{N_LSYM}
@item Symbol Descriptor:
@code{t}
@end table

Here is the stab for the typedef equating the structure tag with a
type.

@display
    <128> N_LSYM - type definition 
    .stabs "name:sym_desc(type name)type_ref(struct_tag)",N_LSYM,NIL,NIL,NIL
@end display

@example
31 .stabs "s_typedef:t16",128,0,0,0
@end example

And here is the code generated for the structure variable.

@display
    <32> N_GSYM - global symbol 
    .stabs "name:sym_desc(global)type_ref(struct_tag)",N_GSYM,NIL,NIL,NIL
@end display