stabs.texinfo

早期freebsd实现

@item Types:
@code{N_LBRAC}, @code{N_RBRAC}
@end table

The program's block structure is represented by the @code{N_LBRAC} (left
brace) and the @code{N_RBRAC} (right brace) stab types.  The following code
range, which is the body of @code{main}, is labeled with @samp{LBB2:} at the
beginning and @samp{LBE2:} at the end.  

@example
37 LBB2:
38      sethi %hi(LC0),%o1
39      or %o1,%lo(LC0),%o0
40      call _printf,0
41      nop
42 .stabn 68,0,6,LM3
43 LM3:
44 LBE2:
@end example

The @code{N_LBRAC} and @code{N_RBRAC} stabs that describe the block
scope of the procedure are located after the @code{N_FUNC} stab that
represents the procedure itself.  The @code{N_LBRAC} uses the
@code{LBB2} label as the code address in its value field, and the
@code{N_RBRAC} uses @code{LBE2}.

@example
50 .stabs "main:F1",36,0,0,_main
@end example

@example
   .stabn N_LBRAC, NIL, NIL, @var{left-brace-address}
   .stabn N_RBRAC, NIL, NIL, @var{right-brace-address}
@end example

@example
51 .stabn 192,0,0,LBB2
52 .stabn 224,0,0,LBE2
@end example

@node Simple types
@chapter Simple types

@menu
* Basic types:: Basic type definitions
* Range types:: Range types defined by min and max value 
* Bit-ranges:: Range type defined by number of bits
@end menu

@node Basic types
@section Basic type definitions

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

The basic types for the language are described using the @code{N_LSYM} stab
type.  They are boilerplate and are emited by the compiler for each
compilation unit.  Basic type definitions are not always a complete
description of the type and are sometimes circular.  The debugger
recognizes the type anyway, and knows how to read bits as that type.

Each language and compiler defines a slightly different set of basic
types.  In this example we are looking at the basic types for C emited
by the GNU compiler targeting the Sun4.  Here the basic types are
mostly defined as range types.  


@node Range types
@section Range types defined by min and max value 

@table @strong
@item Type Descriptor:
@code{r}
@end table

When defining a range type, if the number after the first semicolon is
smaller than the number after the second one, then the two numbers
represent the smallest and the largest values in the range.
  
@example
4  .text
5  Ltext0:

.stabs "@var{name}:
        @var{descriptor}  @r{(type)}
        @var{type-def}=
        @var{type-desc}
        @var{type-ref};
        @var{low-bound};
        @var{high-bound};
       ",
       N_LSYM, NIL, NIL, NIL

6  .stabs "int:t1=r1;-2147483648;2147483647;",128,0,0,0
7  .stabs "char:t2=r2;0;127;",128,0,0,0
@end example

Here the integer type (@code{1}) is defined as a range of the integer
type (@code{1}).  Likewise @code{char} is a range of @code{char}.  This
part of the definition is circular, but at least the high and low bound
values of the range hold more information about the type.

Here short unsigned int is defined as type number 8 and described as a
range of type @code{int}, with a minimum value of 0 and a maximum of 65535.

@example
13 .stabs "short unsigned int:t8=r1;0;65535;",128,0,0,0
@end example

@node Bit-ranges
@section Range type defined by number of bits

@table @strong
@item Type Descriptor:
@code{r}
@end table

In a range definition, if the number after the second semicolon is 0,
then the number after the first semicolon is the number of bits needed
to represent the type.

@example
.stabs "@var{name}:
        @var{desc}
        @var{type-def}=
        @var{type-desc}
        @var{type-ref};
        @var{bit-count};
        0;
       ",
       N_LSYM, NIL, NIL, NIL

17 .stabs "float:t12=r1;4;0;",128,0,0,0
18 .stabs "double:t13=r1;8;0;",128,0,0,0
19 .stabs "long double:t14=r1;8;0;",128,0,0,0
@end example

Cosmically enough, the @code{void} type is defined directly in terms of
itself.

@example
.stabs "@var{name}:
        @var{symbol-desc}
        @var{type-def}=
        @var{type-ref}
       ",N_LSYM,NIL,NIL,NIL

20 .stabs "void:t15=15",128,0,0,0
@end example


@node Example
@chapter A Comprehensive Example in C 

Now we'll examine a second program, @code{example2}, which builds on the
first example to introduce the rest of the stab types, symbol
descriptors, and type descriptors used in C.
@xref{Example2.c} for the complete @file{.c} source,
and @pxref{Example2.s} for the @file{.s} assembly code.
This description includes parts of those files.

@section Flow of control and nested scopes 

@table @strong
@item Directive:
@code{.stabn}
@item Types:
@code{N_SLINE}, @code{N_LBRAC}, @code{N_RBRAC} (cont.)
@end table

Consider the body of @code{main}, from @file{example2.c}.  It shows more
about how @code{N_SLINE}, @code{N_RBRAC}, and @code{N_LBRAC} stabs are used.  

@example
20 @{
21      static float s_flap;
22      int times;
23      for (times=0; times < s_g_repeat; times++)@{
24        int inner;
25        printf ("Hello world\n");
26      @}
27 @};
@end example

Here we have a single source line, the @samp{for} line, that generates
non-linear flow of control, and non-contiguous code.  In this case, an
@code{N_SLINE} stab with the same line number proceeds each block of
non-contiguous code generated from the same source line.

The example also shows nested scopes.  The @code{N_LBRAC} and
@code{N_LBRAC} stabs that describe block structure are nested in the
same order as the corresponding code blocks, those of the for loop
inside those for the body of main.

@noindent
This is the label for the @code{N_LBRAC} (left brace) stab marking the
start of @code{main}.
 
@example
57 LBB2:
@end example

@noindent
In the first code range for C source line 23, the @code{for} loop
initialize and test, @code{N_SLINE} (68) records the line number:

@example
.stabn N_SLINE, NIL,
       @var{line},
       @var{address}

58 .stabn 68,0,23,LM2
59 LM2:
60      st %g0,[%fp-20]
61 L2:
62      sethi %hi(_s_g_repeat),%o0
63      ld [%fp-20],%o1
64      ld [%o0+%lo(_s_g_repeat)],%o0
65      cmp %o1,%o0
66      bge L3
67      nop

@exdent label for the @code{N_LBRAC} (start block) marking the start of @code{for} loop

68 LBB3:
69 .stabn 68,0,25,LM3
70 LM3:
71      sethi %hi(LC0),%o1
72      or %o1,%lo(LC0),%o0
73      call _printf,0
74      nop
75 .stabn 68,0,26,LM4
76 LM4:

@exdent label for the @code{N_RBRAC} (end block) stab marking the end of the @code{for} loop

77 LBE3:
@end example

@noindent
Now we come to the second code range for source line 23, the @code{for}
loop increment and return.  Once again, @code{N_SLINE} (68) records the
source line number:

@example
.stabn, N_SLINE, NIL,
        @var{line},
        @var{address}

78 .stabn 68,0,23,LM5
79 LM5:
80 L4:
81      ld [%fp-20],%o0
82      add %o0,1,%o1
83      st %o1,[%fp-20]
84      b,a L2
85 L3:
86 .stabn 68,0,27,LM6
87 LM6:

@exdent label for the @code{N_RBRAC} (end block) stab marking the end of the @code{for} loop

88 LBE2:
89 .stabn 68,0,27,LM7
90 LM7:
91 L1:
92      ret
93      restore
94 .stabs "main:F1",36,0,0,_main
95 .stabs "argc:p1",160,0,0,68
96 .stabs "argv:p20=*21=*2",160,0,0,72
97 .stabs "s_flap:V12",40,0,0,_s_flap.0
98 .stabs "times:1",128,0,0,-20
@end example

@noindent
Here is an illustration of stabs describing nested scopes.  The scope
nesting is reflected in the nested bracketing stabs (@code{N_LBRAC},
192, appears here).

@example
.stabn N_LBRAC,NIL,NIL,
       @var{block-start-address}

99  .stabn 192,0,0,LBB2      ## begin proc label
100 .stabs "inner:1",128,0,0,-24
101 .stabn 192,0,0,LBB3      ## begin for label
@end example

@noindent
@code{N_RBRAC} (224), ``right brace'' ends a lexical block (scope).

@example
.stabn N_RBRAC,NIL,NIL,
       @var{block-end-address}

102 .stabn 224,0,0,LBE3      ## end for label
103 .stabn 224,0,0,LBE2      ## end proc label
@end example

@node Variables
@chapter Variables

@menu
* Automatic variables:: locally scoped
* Global variables::
* Register variables::
* Initialized statics::
* Un-initialized statics::
* Parameters::
@end menu

@node Automatic variables
@section Locally scoped automatic variables

@table @strong
@item Directive:
@code{.stabs}
@item Type:
@code{N_LSYM} 
@item Symbol Descriptor:
none
@end table


In addition to describing types, the @code{N_LSYM} stab type also
describes locally scoped automatic variables.  Refer again to the body
of @code{main} in @file{example2.c}.  It allocates two automatic
variables: @samp{times} is scoped to the body of @code{main}, and
@samp{inner} is scoped to the body of the @code{for} loop.
@samp{s_flap} is locally scoped but not automatic, and will be discussed
later.

@example
20 @{
21      static float s_flap;
22      int times;
23      for (times=0; times < s_g_repeat; times++)@{
24        int inner;
25        printf ("Hello world\n");
26      @}
27 @};
@end example

The @code{N_LSYM} stab for an automatic variable is located just before the
@code{N_LBRAC} stab describing the open brace of the block to which it is
scoped. 

@example
@exdent @code{N_LSYM} (128): automatic variable, scoped locally to @code{main}

.stabs "@var{name}:
        @var{type-ref}",
       N_LSYM, NIL, NIL,
       @var{frame-pointer-offset}

98  .stabs "times:1",128,0,0,-20
99  .stabn 192,0,0,LBB2      ## begin `main' N_LBRAC

@exdent @code{N_LSYM} (128): automatic variable, scoped locally to the @code{for} loop

.stabs "@var{name}:
        @var{type-ref}",
        N_LSYM, NIL, NIL,
       @var{frame-pointer-offset}

100 .stabs "inner:1",128,0,0,-24
101 .stabn 192,0,0,LBB3      ## begin `for' loop N_LBRAC
@end example

Since the character in the string field following the colon is not a
letter, there is no symbol descriptor.  This means that the stab
describes a local variable, and that the number after the colon is a
type reference.  In this case it a a reference to the basic type @code{int}.
Notice also that the frame pointer offset is negative number for
automatic variables.


@node Global Variables
@section Global Variables

@table @strong
@item Directive:
@code{.stabs}
@item Type:
@code{N_GSYM}
@item Symbol Descriptor:
@code{G}
@end table

Global variables are represented by the @code{N_GSYM} stab type.  The symbol
descriptor, following the colon in the string field, is @samp{G}.  Following
the @samp{G} is a type reference or type definition.  In this example it is a
type reference to the basic C type, @code{char}.  The first source line in
@file{example2.c},
  
@example
1  char g_foo = 'c';
@end example

@noindent
yields the following stab.  The stab immediately precedes the code that
allocates storage for the variable it describes.

@example
@exdent @code{N_GSYM} (32): global symbol

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