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* stabs.texinfo (Stack Variables): Re-write.
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Jim Kingdon
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Tue Aug 17 15:57:27 1993 Jim Kingdon (kingdon@lioth.cygnus.com)
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* stabs.texinfo (Stack Variables): Re-write.
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Mon Aug 16 21:20:08 1993 Jim Kingdon (kingdon@lioth.cygnus.com)
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* stabs.texinfo (Stabs-in-elf): Talk about getting the start
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@ -218,21 +218,21 @@ descriptors}.
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The @samp{c} symbol descriptor is an exception in that it is not
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followed by type information. @xref{Constants}.
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Type information is either a @var{type_number}, or a
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Type information is either a @var{type_number}, or
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@samp{@var{type_number}=}. The @var{type_number} alone is a type
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reference, referring directly to a type that has already been defined.
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The @samp{@var{type_number}=} is a type definition, where the number
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represents a new type which is about to be defined. The type definition
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may refer to other types by number, and those type numbers may be
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followed by @samp{=} and nested definitions.
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The @samp{@var{type_number}=} form is a type definition, where the
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number represents a new type which is about to be defined. The type
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definition may refer to other types by number, and those type numbers
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may be followed by @samp{=} and nested definitions.
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In a type definition, if the character that follows the equals sign is
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non-numeric then it is a @var{type_descriptor}, and tells what kind of
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type is about to be defined. Any other values following the
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@var{type_descriptor} vary, depending on the @var{type_descriptor}. If
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a number follows the @samp{=} then the number is a @var{type_reference}.
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This is described more thoroughly in the section on types. @xref{Type
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For a full description of types, @ref{Types}. @xref{Type
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Descriptors,,Table D: Type Descriptors}, for a list of
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@var{type_descriptor} values.
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@ -840,7 +840,7 @@ nesting is reflected in the nested bracketing stabs (@code{N_LBRAC},
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@chapter Variables
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@menu
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* Automatic variables:: Variables allocated on the stack.
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* Stack Variables:: Variables allocated on the stack.
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* Global Variables:: Variables used by more than one source file.
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* Register variables:: Variables in registers.
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* Common Blocks:: Variables statically allocated together.
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@ -848,70 +848,54 @@ nesting is reflected in the nested bracketing stabs (@code{N_LBRAC},
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* Parameters:: Variables for arguments to functions.
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@end menu
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@node Automatic variables
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@section Locally scoped automatic variables
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@node Stack Variables
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@section Automatic Variables Allocated on the Stack
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@table @strong
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@item Directive:
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@code{.stabs}
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@item Type:
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@code{N_LSYM}
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@item Symbol Descriptor:
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none
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@end table
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If a variable is declared whose scope is local to a function and whose
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lifetime is only as long as that function executes (C calls such
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variables automatic), they can be allocated in a register
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(@pxref{Register variables}) or on the stack.
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In addition to describing types, the @code{N_LSYM} stab type also
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describes locally scoped automatic variables. Refer again to the body
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of @code{main} in @file{example2.c}. It allocates two automatic
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variables: @samp{times} is scoped to the body of @code{main}, and
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@samp{inner} is scoped to the body of the @code{for} loop.
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@samp{s_flap} is locally scoped but not automatic, and will be discussed
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later.
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For variables allocated on the stack, each variable has a stab with the
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symbol descriptor omitted. Since type information should being with a
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digit, @samp{-}, or @samp{(}, only digits, @samp{-}, and @samp{(} are
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precluded from being used for symbol descriptors by this fact. However,
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the Acorn RISC machine (ARM) is said to get this wrong: it puts out a
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mere type definition here, without the preceding
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@code{@var{typenumber}=}. This is a bad idea; there is no guarantee
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that type descriptors are distinct from symbol descriptors.
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These stabs have the @code{N_LSYM} stab type. The value of the stab is
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the offset of the variable within the local variables. On most machines
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this is an offset from the frame pointer and is negative.
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The stab for an automatic variable is located just before the
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@code{N_LBRAC} stab describing the open brace of the block to which it
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is scoped, except for some compilers which put the automatic variables
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after the @code{N_LBRAC} (see @code{VARIABLES_INSIDE_BLOCK} in GDB).
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For example, the following C code
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@example
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20 @{
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21 static float s_flap;
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22 int times;
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23 for (times=0; times < s_g_repeat; times++)@{
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24 int inner;
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25 printf ("Hello world\n");
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26 @}
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27 @};
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int
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main ()
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@{
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int x;
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@}
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@end example
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The @code{N_LSYM} stab for an automatic variable is located just before the
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@code{N_LBRAC} stab describing the open brace of the block to which it is
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scoped.
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produces the following stabs
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@example
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@exdent @code{N_LSYM} (128): automatic variable, scoped locally to @code{main}
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.stabs "@var{name}:
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@var{type information}",
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N_LSYM, NIL, NIL,
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@var{frame-pointer-offset}
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98 .stabs "times:1",128,0,0,-20
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99 .stabn 192,0,0,LBB2 ## begin `main' N_LBRAC
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@exdent @code{N_LSYM} (128): automatic variable, scoped locally to the @code{for} loop
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.stabs "@var{name}:
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@var{type information}",
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N_LSYM, NIL, NIL,
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@var{frame-pointer-offset}
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100 .stabs "inner:1",128,0,0,-24
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101 .stabn 192,0,0,LBB3 ## begin `for' loop N_LBRAC
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.stabs "main:F1",36,0,0,_main # N_FUN
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.stabs "x:1",128,0,0,-12 # N_LSYM
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.stabn 192,0,0,LBB2 # N_LBRAC
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.stabn 224,0,0,LBE2 # N_RBRAC
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@end example
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The symbol descriptor is omitted for automatic variables. Since type
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information should being with a digit, @samp{-}, or @samp{(}, only
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digits, @samp{-}, and @samp{(} are precluded from being used for symbol
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descriptors by this fact. However, the Acorn RISC machine (ARM) is said
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to get this wrong: it puts out a mere type definition here, without the
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preceding @code{@var{typenumber}=}. This is a bad idea; there is no
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guarantee that type descriptors are distinct from symbol descriptors.
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@xref{Procedures} for more information on the @samp{F} symbol desciptor,
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and @ref{Block Structure} for more information on the @code{N_LBRAC} and
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@code{N_RBRAC} symbols.
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@node Global Variables
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@section Global Variables
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@ -1861,9 +1845,11 @@ element of. So the definition of structure type 16 contains an type
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definition for an element which is a pointer to type 16.
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@node Typedefs
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@section Giving a type a name
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@section Giving a Type a Name
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To give a type a name, use the @samp{t} symbol descriptor. For example,
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To give a type a name, use the @samp{t} symbol descriptor. The type
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specified by the type information (@pxref{Stabs Format}) for the stab.
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For example,
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@example
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.stabs "s_typedef:t16",128,0,0,0
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@ -1883,7 +1869,7 @@ means---is it always the same as the name of the type, or is this type
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descriptor used with a nameless stab (@pxref{Stabs Format})? There
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optionally follows a comma followed by type information which defines
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the type of this type. If omitted, a semicolon is used in place of the
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comma and the type information, and, the type is much like a generic
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comma and the type information, and the type is much like a generic
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pointer type---it has a known size but little else about it is
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specified.
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@ -1937,8 +1923,7 @@ After that is a list of union element descriptions. Their format is
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name:type, bit offset into the union, and number of bytes for the
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element;.
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The stab for the union variable follows. Notice that the frame
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pointer offset for local variables is negative.
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The stab for the union variable follows.
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@display
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<128> N_LSYM - local variable (with no symbol descriptor)
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@ -3106,7 +3091,7 @@ Last stab for module (Solaris2).
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Path and name of source file, @xref{Source Files}.
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@item 0x80 N_LSYM
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Stack variable or type definition, @xref{N_LSYM}, @xref{Typedefs}.
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Stack variable (@pxref{Stack Variables}) or type (@pxref{Typedefs}).
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@item 0x82 N_BINCL
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Beginning of an include file (Sun only), @xref{Source Files}.
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@ -3181,7 +3166,7 @@ Gould non-base registers, @xref{Gould}.
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@item @var{digit}
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@itemx (
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@itemx -
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Local variable, @xref{Automatic variables}.
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Variable on the stack, @xref{Stack Variables}.
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@item a
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Parameter passed by reference in register, @xref{Parameters}.
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@ -3440,7 +3425,6 @@ Finally, any further information.
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* N_MOD2:: Modula2 information "for imc"
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* N_CATCH:: GNU C++ "catch" clause
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* N_SSYM:: Structure or union element
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* N_LSYM:: Automatic variable
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* N_ENTRY:: Alternate entry point
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* N_SCOPE:: Modula2 scope information (Sun only)
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* Gould:: non-base register symbols used on Gould systems
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@ -3589,38 +3573,6 @@ Value is offset in the structure.
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<<?looking at structs and unions in C I didn't see these>>
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@node N_LSYM
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@section 128 - 0x80 - N_LSYM
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Automatic var in the stack (also used for type descriptors.)
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@display
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.stabs "name" N_LSYM, NIL, NIL, value
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@end display
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@example
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@exdent @emph{For stack based local variables:}
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"name" -> name of the variable
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value -> offset from frame pointer (negative)
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@exdent @emph{For type descriptors:}
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"name" -> "name_of_the_type:#type"
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# -> t
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type -> type_ref (or) type_def
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type_ref -> type_number
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type_def -> type_number=type_desc etc.
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@end example
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Type may be either a type reference or a type definition. A type
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reference is a number that refers to a previously defined type. A
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type definition is the number that will refer to this type, followed
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by an equals sign, a type descriptor and the additional data that
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defines the type. See the Table D for type descriptors and the
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section on types for what data follows each type descriptor.
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@node N_ENTRY
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@section 164 - 0xa4 - N_ENTRY
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