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* Stabs: (stabs).                 The "stabs" debugging information format.

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   This document describes the stabs debugging symbol tables.

   Copyright 1992, 93, 94, 95, 97, 1998 Free Software Foundation, Inc.

Contributed by Cygnus Support.  Written by Julia Menapace, Jim Kingdon,

and David MacKenzie.

   Permission is granted to make and distribute verbatim copies of this

manual provided the copyright notice and this permission notice are

preserved on all copies.

   Permission is granted to copy or distribute modified versions of this

manual under the terms of the GPL (for which purpose this text may be

regarded as a program in the language TeX).

File: stabs.info,  Node: Conformant Arrays,  Prev: Reference Parameters,  Up: Parameters

Passing Conformant Array Parameters

-----------------------------------

   Conformant arrays are a feature of Modula-2, and perhaps other

languages, in which the size of an array parameter is not known to the

called function until run-time.  Such parameters have two stabs: a `x'

for the array itself, and a `C', which represents the size of the

array.  The value of the `x' stab is the offset in the argument list

where the address of the array is stored (it this right?  it is a

guess); the value of the `C' stab is the offset in the argument list

where the size of the array (in elements? in bytes?) is stored.

File: stabs.info,  Node: Types,  Next: Symbol Tables,  Prev: Variables,  Up: Top

Defining Types

**************

   The examples so far have described types as references to previously

defined types, or defined in terms of subranges of or pointers to

previously defined types.  This chapter describes the other type

descriptors that may follow the `=' in a type definition.

* Menu:

* Builtin Types::               Integers, floating point, void, etc.

* Miscellaneous Types::         Pointers, sets, files, etc.

* Cross-References::            Referring to a type not yet defined.

* Subranges::                   A type with a specific range.

* Arrays::                      An aggregate type of same-typed elements.

* Strings::                     Like an array but also has a length.

* Enumerations::                Like an integer but the values have names.

* Structures::                  An aggregate type of different-typed elements.

* Typedefs::                    Giving a type a name.

* Unions::                      Different types sharing storage.

* Function Types::

File: stabs.info,  Node: Builtin Types,  Next: Miscellaneous Types,  Up: Types

Builtin Types

=============

   Certain types are built in (`int', `short', `void', `float', etc.);

the debugger recognizes these types and knows how to handle them.

Thus, don't be surprised if some of the following ways of specifying

builtin types do not specify everything that a debugger would need to

know about the type--in some cases they merely specify enough

information to distinguish the type from other types.

   The traditional way to define builtin types is convolunted, so new

ways have been invented to describe them.  Sun's `acc' uses special

builtin type descriptors (`b' and `R'), and IBM uses negative type

numbers.  GDB accepts all three ways, as of version 4.8; dbx just

accepts the traditional builtin types and perhaps one of the other two

formats.  The following sections describe each of these formats.

* Menu:

* Traditional Builtin Types::   Put on your seatbelts and prepare for kludgery

* Builtin Type Descriptors::    Builtin types with special type descriptors

* Negative Type Numbers::       Builtin types using negative type numbers

File: stabs.info,  Node: Traditional Builtin Types,  Next: Builtin Type Descriptors,  Up: Builtin Types

Traditional Builtin Types

-------------------------

   This is the traditional, convoluted method for defining builtin

types.  There are several classes of such type definitions: integer,

floating point, and `void'.

* Menu:

* Traditional Integer Types::

* Traditional Other Types::

File: stabs.info,  Node: Traditional Integer Types,  Next: Traditional Other Types,  Up: Traditional Builtin Types

Traditional Integer Types

.........................

   Often types are defined as subranges of themselves.  If the bounding

values fit within an `int', then they are given normally.  For example:

     .stabs "int:t1=r1;-2147483648;2147483647;",128,0,0,0    # 128 is N_LSYM

     .stabs "char:t2=r2;0;127;",128,0,0,0

   Builtin types can also be described as subranges of `int':

     .stabs "unsigned short:t6=r1;0;65535;",128,0,0,0

   If the lower bound of a subrange is 0 and the upper bound is -1, the

type is an unsigned integral type whose bounds are too big to describe

in an `int'.  Traditionally this is only used for `unsigned int' and

`unsigned long':

     .stabs "unsigned int:t4=r1;0;-1;",128,0,0,0

   For larger types, GCC 2.4.5 puts out bounds in octal, with one or

more leading zeroes.  In this case a negative bound consists of a number

which is a 1 bit (for the sign bit) followed by a 0 bit for each bit in

the number (except the sign bit), and a positive bound is one which is a

1 bit for each bit in the number (except possibly the sign bit).  All

known versions of dbx and GDB version 4 accept this (at least in the

sense of not refusing to process the file), but GDB 3.5 refuses to read

the whole file containing such symbols.  So GCC 2.3.3 did not output the

proper size for these types.  As an example of octal bounds, the string

fields of the stabs for 64 bit integer types look like:

     long int:t3=r1;001000000000000000000000;000777777777777777777777;

     long unsigned int:t5=r1;000000000000000000000000;001777777777777777777777;

   If the lower bound of a subrange is 0 and the upper bound is

negative, the type is an unsigned integral type whose size in bytes is

the absolute value of the upper bound.  I believe this is a Convex

convention for `unsigned long long'.

   If the lower bound of a subrange is negative and the upper bound is

0, the type is a signed integral type whose size in bytes is the

absolute value of the lower bound.  I believe this is a Convex

convention for `long long'.  To distinguish this from a legitimate

subrange, the type should be a subrange of itself.  I'm not sure whether

this is the case for Convex.

File: stabs.info,  Node: Traditional Other Types,  Prev: Traditional Integer Types,  Up: Traditional Builtin Types

Traditional Other Types

.......................

   If the upper bound of a subrange is 0 and the lower bound is

positive, the type is a floating point type, and the lower bound of the

subrange indicates the number of bytes in the type:

     .stabs "float:t12=r1;4;0;",128,0,0,0

     .stabs "double:t13=r1;8;0;",128,0,0,0

   However, GCC writes `long double' the same way it writes `double',

so there is no way to distinguish.

     .stabs "long double:t14=r1;8;0;",128,0,0,0

   Complex types are defined the same way as floating-point types;

there is no way to distinguish a single-precision complex from a

double-precision floating-point type.

   The C `void' type is defined as itself:

     .stabs "void:t15=15",128,0,0,0

   I'm not sure how a boolean type is represented.

File: stabs.info,  Node: Builtin Type Descriptors,  Next: Negative Type Numbers,  Prev: Traditional Builtin Types,  Up: Builtin Types

Defining Builtin Types Using Builtin Type Descriptors

-----------------------------------------------------

   This is the method used by Sun's `acc' for defining builtin types.

These are the type descriptors to define builtin types:

`b SIGNED CHAR-FLAG WIDTH ; OFFSET ; NBITS ;'

     Define an integral type.  SIGNED is `u' for unsigned or `s' for

     signed.  CHAR-FLAG is `c' which indicates this is a character

     type, or is omitted.  I assume this is to distinguish an integral

     type from a character type of the same size, for example it might

     make sense to set it for the C type `wchar_t' so the debugger can

     print such variables differently (Solaris does not do this).  Sun

     sets it on the C types `signed char' and `unsigned char' which

     arguably is wrong.  WIDTH and OFFSET appear to be for small

     objects stored in larger ones, for example a `short' in an `int'

     register.  WIDTH is normally the number of bytes in the type.

     OFFSET seems to always be zero.  NBITS is the number of bits in

     the type.

     Note that type descriptor `b' used for builtin types conflicts with

     its use for Pascal space types (*note Miscellaneous Types::.);

     they can be distinguished because the character following the type

     descriptor will be a digit, `(', or `-' for a Pascal space type, or

     `u' or `s' for a builtin type.

`w'

     Documented by AIX to define a wide character type, but their

     compiler actually uses negative type numbers (*note Negative Type

     Numbers::.).

`R FP-TYPE ; BYTES ;'

     Define a floating point type.  FP-TYPE has one of the following

     values:

    `1 (NF_SINGLE)'

          IEEE 32-bit (single precision) floating point format.

    `2 (NF_DOUBLE)'

          IEEE 64-bit (double precision) floating point format.

    `3 (NF_COMPLEX)'

    `4 (NF_COMPLEX16)'

    `5 (NF_COMPLEX32)'

          These are for complex numbers.  A comment in the GDB source

          describes them as Fortran `complex', `double complex', and

          `complex*16', respectively, but what does that mean?  (i.e.,

          Single precision?  Double precison?).

    `6 (NF_LDOUBLE)'

          Long double.  This should probably only be used for Sun format

          `long double', and new codes should be used for other floating

          point formats (`NF_DOUBLE' can be used if a `long double' is

          really just an IEEE double, of course).

     BYTES is the number of bytes occupied by the type.  This allows a

     debugger to perform some operations with the type even if it

     doesn't understand FP-TYPE.

`g TYPE-INFORMATION ; NBITS'

     Documented by AIX to define a floating type, but their compiler

     actually uses negative type numbers (*note Negative Type

     Numbers::.).

`c TYPE-INFORMATION ; NBITS'

     Documented by AIX to define a complex type, but their compiler

     actually uses negative type numbers (*note Negative Type

     Numbers::.).

   The C `void' type is defined as a signed integral type 0 bits long:

     .stabs "void:t19=bs0;0;0",128,0,0,0

   The Solaris compiler seems to omit the trailing semicolon in this

case.  Getting sloppy in this way is not a swift move because if a type

is embedded in a more complex expression it is necessary to be able to

tell where it ends.

   I'm not sure how a boolean type is represented.

File: stabs.info,  Node: Negative Type Numbers,  Prev: Builtin Type Descriptors,  Up: Builtin Types

Negative Type Numbers

---------------------

   This is the method used in XCOFF for defining builtin types.  Since

the debugger knows about the builtin types anyway, the idea of negative

type numbers is simply to give a special type number which indicates

the builtin type.  There is no stab defining these types.

   There are several subtle issues with negative type numbers.

   One is the size of the type.  A builtin type (for example the C types

`int' or `long') might have different sizes depending on compiler

options, the target architecture, the ABI, etc.  This issue doesn't

come up for IBM tools since (so far) they just target the RS/6000; the

sizes indicated below for each size are what the IBM RS/6000 tools use.

To deal with differing sizes, either define separate negative type

numbers for each size (which works but requires changing the debugger,

and, unless you get both AIX dbx and GDB to accept the change,

introduces an incompatibility), or use a type attribute (*note String

Field::.) to define a new type with the appropriate size (which merely

requires a debugger which understands type attributes, like AIX dbx or

GDB).  For example,

     .stabs "boolean:t10=@s8;-16",128,0,0,0

   defines an 8-bit boolean type, and

     .stabs "boolean:t10=@s64;-16",128,0,0,0

   defines a 64-bit boolean type.

   A similar issue is the format of the type.  This comes up most often

for floating-point types, which could have various formats (particularly

extended doubles, which vary quite a bit even among IEEE systems).

Again, it is best to define a new negative type number for each

different format; changing the format based on the target system has

various problems.  One such problem is that the Alpha has both VAX and

IEEE floating types.  One can easily imagine one library using the VAX

types and another library in the same executable using the IEEE types.

Another example is that the interpretation of whether a boolean is true

or false can be based on the least significant bit, most significant

bit, whether it is zero, etc., and different compilers (or different

options to the same compiler) might provide different kinds of boolean.

   The last major issue is the names of the types.  The name of a given

type depends _only_ on the negative type number given; these do not

vary depending on the language, the target system, or anything else.

One can always define separate type numbers--in the following list you

will see for example separate `int' and `integer*4' types which are

identical except for the name.  But compatibility can be maintained by

not inventing new negative type numbers and instead just defining a new

type with a new name.  For example:

     .stabs "CARDINAL:t10=-8",128,0,0,0

   Here is the list of negative type numbers.  The phrase "integral

type" is used to mean twos-complement (I strongly suspect that all

machines which use stabs use twos-complement; most machines use

twos-complement these days).

`-1'

     `int', 32 bit signed integral type.

`-2'

     `char', 8 bit type holding a character.   Both GDB and dbx on AIX

     treat this as signed.  GCC uses this type whether `char' is signed

     or not, which seems like a bad idea.  The AIX compiler (`xlc')

     seems to avoid this type; it uses -5 instead for `char'.

`-3'

     `short', 16 bit signed integral type.

`-4'

     `long', 32 bit signed integral type.

`-5'

     `unsigned char', 8 bit unsigned integral type.

`-6'

     `signed char', 8 bit signed integral type.

`-7'

     `unsigned short', 16 bit unsigned integral type.

`-8'

     `unsigned int', 32 bit unsigned integral type.

`-9'

     `unsigned', 32 bit unsigned integral type.

`-10'

     `unsigned long', 32 bit unsigned integral type.

`-11'

     `void', type indicating the lack of a value.

`-12'

     `float', IEEE single precision.

`-13'

     `double', IEEE double precision.

`-14'

     `long double', IEEE double precision.  The compiler claims the size

     will increase in a future release, and for binary compatibility

     you have to avoid using `long double'.  I hope when they increase

     it they use a new negative type number.

`-15'

     `integer'.  32 bit signed integral type.

`-16'

     `boolean'.  32 bit type.  GDB and GCC assume that zero is false,

     one is true, and other values have unspecified meaning.  I hope

     this agrees with how the IBM tools use the type.

`-17'

     `short real'.  IEEE single precision.

`-18'

     `real'.  IEEE double precision.

`-19'

     `stringptr'.  *Note Strings::.

`-20'

     `character', 8 bit unsigned character type.

`-21'

     `logical*1', 8 bit type.  This Fortran type has a split

     personality in that it is used for boolean variables, but can also

     be used for unsigned integers.  0 is false, 1 is true, and other

     values are non-boolean.

`-22'

     `logical*2', 16 bit type.  This Fortran type has a split

     personality in that it is used for boolean variables, but can also

     be used for unsigned integers.  0 is false, 1 is true, and other

     values are non-boolean.

`-23'

     `logical*4', 32 bit type.  This Fortran type has a split

     personality in that it is used for boolean variables, but can also

     be used for unsigned integers.  0 is false, 1 is true, and other

     values are non-boolean.

`-24'

     `logical', 32 bit type.  This Fortran type has a split personality

     in that it is used for boolean variables, but can also be used for

     unsigned integers.  0 is false, 1 is true, and other values are

     non-boolean.

`-25'

     `complex'.  A complex type consisting of two IEEE single-precision

     floating point values.

`-26'

     `complex'.  A complex type consisting of two IEEE double-precision

     floating point values.

`-27'

     `integer*1', 8 bit signed integral type.

`-28'

     `integer*2', 16 bit signed integral type.

`-29'

     `integer*4', 32 bit signed integral type.

`-30'

     `wchar'.  Wide character, 16 bits wide, unsigned (what format?

     Unicode?).

`-31'

     `long long', 64 bit signed integral type.

`-32'

     `unsigned long long', 64 bit unsigned integral type.

`-33'

     `logical*8', 64 bit unsigned integral type.

`-34'

     `integer*8', 64 bit signed integral type.

File: stabs.info,  Node: Miscellaneous Types,  Next: Cross-References,  Prev: Builtin Types,  Up: Types

Miscellaneous Types

===================

`b TYPE-INFORMATION ; BYTES'

     Pascal space type.  This is documented by IBM; what does it mean?

     This use of the `b' type descriptor can be distinguished from its

     use for builtin integral types (*note Builtin Type Descriptors::.)

     because the character following the type descriptor is always a

     digit, `(', or `-'.

`B TYPE-INFORMATION'

     A volatile-qualified version of TYPE-INFORMATION.  This is a Sun

     extension.  References and stores to a variable with a

     volatile-qualified type must not be optimized or cached; they must

     occur as the user specifies them.

`d TYPE-INFORMATION'

     File of type TYPE-INFORMATION.  As far as I know this is only used

     by Pascal.

`k TYPE-INFORMATION'

     A const-qualified version of TYPE-INFORMATION.  This is a Sun

     extension.  A variable with a const-qualified type cannot be

     modified.

`M TYPE-INFORMATION ; LENGTH'

     Multiple instance type.  The type seems to composed of LENGTH

     repetitions of TYPE-INFORMATION, for example `character*3' is

     represented by `M-2;3', where `-2' is a reference to a character

     type (*note Negative Type Numbers::.).  I'm not sure how this

     differs from an array.  This appears to be a Fortran feature.

     LENGTH is a bound, like those in range types; see *Note

     Subranges::.

`S TYPE-INFORMATION'

     Pascal set type.  TYPE-INFORMATION must be a small type such as an

     enumeration or a subrange, and the type is a bitmask whose length

     is specified by the number of elements in TYPE-INFORMATION.

     In CHILL, if it is a bitstring instead of a set, also use the `S'

     type attribute (*note String Field::.).

`* TYPE-INFORMATION'

     Pointer to TYPE-INFORMATION.

File: stabs.info,  Node: Cross-References,  Next: Subranges,  Prev: Miscellaneous Types,  Up: Types

Cross-References to Other Types

===============================

   A type can be used before it is defined; one common way to deal with

that situation is just to use a type reference to a type which has not

yet been defined.

   Another way is with the `x' type descriptor, which is followed by

`s' for a structure tag, `u' for a union tag, or `e' for a enumerator

tag, followed by the name of the tag, followed by `:'.  If the name

contains `::' between a `<' and `>' pair (for C++ templates), such a

`::' does not end the name--only a single `:' ends the name; see *Note

Nested Symbols::.

   For example, the following C declarations:

     struct foo;

     struct foo *bar;

produce:

     .stabs "bar:G16=*17=xsfoo:",32,0,0,0

   Not all debuggers support the `x' type descriptor, so on some

machines GCC does not use it.  I believe that for the above example it

would just emit a reference to type 17 and never define it, but I

haven't verified that.

   Modula-2 imported types, at least on AIX, use the `i' type

descriptor, which is followed by the name of the module from which the

type is imported, followed by `:', followed by the name of the type.

There is then optionally a comma followed by type information for the

type.  This differs from merely naming the type (*note Typedefs::.) in

that it identifies the module; I don't understand whether the name of

the type given here is always just the same as the name we are giving

it, or whether this type descriptor is used with a nameless stab (*note

String Field::.), or what.  The symbol ends with `;'.

File: stabs.info,  Node: Subranges,  Next: Arrays,  Prev: Cross-References,  Up: Types

Subrange Types

==============

   The `r' type descriptor defines a type as a subrange of another

type.  It is followed by type information for the type of which it is a

subrange, a semicolon, an integral lower bound, a semicolon, an

integral upper bound, and a semicolon.  The AIX documentation does not

specify the trailing semicolon, in an effort to specify array indexes

more cleanly, but a subrange which is not an array index has always

included a trailing semicolon (*note Arrays::.).

   Instead of an integer, either bound can be one of the following:

`A OFFSET'

     The bound is passed by reference on the stack at offset OFFSET

     from the argument list.  *Note Parameters::, for more information

     on such offsets.

`T OFFSET'

     The bound is passed by value on the stack at offset OFFSET from

     the argument list.

`a REGISTER-NUMBER'

     The bound is pased by reference in register number REGISTER-NUMBER.

`t REGISTER-NUMBER'

     The bound is passed by value in register number REGISTER-NUMBER.

`J'

     There is no bound.

   Subranges are also used for builtin types; see *Note Traditional

Builtin Types::.

File: stabs.info,  Node: Arrays,  Next: Strings,  Prev: Subranges,  Up: Types

Array Types

===========

   Arrays use the `a' type descriptor.  Following the type descriptor

is the type of the index and the type of the array elements.  If the

index type is a range type, it ends in a semicolon; otherwise (for

example, if it is a type reference), there does not appear to be any

way to tell where the types are separated.  In an effort to clean up

this mess, IBM documents the two types as being separated by a

semicolon, and a range type as not ending in a semicolon (but this is

not right for range types which are not array indexes, *note

Subranges::.).  I think probably the best solution is to specify that a

semicolon ends a range type, and that the index type and element type

of an array are separated by a semicolon, but that if the index type is

a range type, the extra semicolon can be omitted.  GDB (at least

through version 4.9) doesn't support any kind of index type other than a

range anyway; I'm not sure about dbx.

   It is well established, and widely used, that the type of the index,

unlike most types found in the stabs, is merely a type definition, not

type information (*note String Field::.) (that is, it need not start

with `TYPE-NUMBER=' if it is defining a new type).  According to a

comment in GDB, this is also true of the type of the array elements; it

gives `ar1;1;10;ar1;1;10;4' as a legitimate way to express a two

dimensional array.  According to AIX documentation, the element type

must be type information.  GDB accepts either.

   The type of the index is often a range type, expressed as the type

descriptor `r' and some parameters.  It defines the size of the array.

In the example below, the range `r1;0;2;' defines an index type which

is a subrange of type 1 (integer), with a lower bound of 0 and an upper

bound of 2.  This defines the valid range of subscripts of a

three-element C array.

   For example, the definition:

     char char_vec[3] = {'a','b','c'};

produces the output:

     .stabs "char_vec:G19=ar1;0;2;2",32,0,0,0

          .global _char_vec

          .align 4

     _char_vec:

          .byte 97

          .byte 98

          .byte 99

   If an array is "packed", the elements are spaced more closely than

normal, saving memory at the expense of speed.  For example, an array

of 3-byte objects might, if unpacked, have each element aligned on a

4-byte boundary, but if packed, have no padding.  One way to specify

that something is packed is with type attributes (*note String

Field::.).  In the case of arrays, another is to use the `P' type

descriptor instead of `a'.  Other than specifying a packed array, `P'

is identical to `a'.

   An open array is represented by the `A' type descriptor followed by

type information specifying the type of the array elements.

   An N-dimensional dynamic array is represented by

     D DIMENSIONS ; TYPE-INFORMATION

   DIMENSIONS is the number of dimensions; TYPE-INFORMATION specifies

the type of the array elements.

   A subarray of an N-dimensional array is represented by

     E DIMENSIONS ; TYPE-INFORMATION

   DIMENSIONS is the number of dimensions; TYPE-INFORMATION specifies

the type of the array elements.

File: stabs.info,  Node: Strings,  Next: Enumerations,  Prev: Arrays,  Up: Types

Strings

=======

   Some languages, like C or the original Pascal, do not have string

types, they just have related things like arrays of characters.  But

most Pascals and various other languages have string types, which are

indicated as follows:

`n TYPE-INFORMATION ; BYTES'

     BYTES is the maximum length.  I'm not sure what TYPE-INFORMATION

     is; I suspect that it means that this is a string of

     TYPE-INFORMATION (thus allowing a string of integers, a string of

     wide characters, etc., as well as a string of characters).  Not

     sure what the format of this type is.  This is an AIX feature.

`z TYPE-INFORMATION ; BYTES'

     Just like `n' except that this is a gstring, not an ordinary

     string.  I don't know the difference.

`N'

     Pascal Stringptr.  What is this?  This is an AIX feature.

   Languages, such as CHILL which have a string type which is basically

just an array of characters use the `S' type attribute (*note String

Field::.).

File: stabs.info,  Node: Enumerations,  Next: Structures,  Prev: Strings,  Up: Types

Enumerations

============

   Enumerations are defined with the `e' type descriptor.

   The source line below declares an enumeration type at file scope.

The type definition 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.  Therefore it does

not describe a block local symbol, but a file local one.

   The source line:

     enum e_places {first,second=3,last};

generates the following stab:

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

   The symbol descriptor (`T') says that the stab describes a

structure, enumeration, or union 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 `;'.  The fact that VALUE is specified as an integer can cause

problems if the value is large.  GCC 2.5.2 tries to output it in octal

in that case with a leading zero, which is probably a good thing,

although GDB 4.11 supports octal only in cases where decimal is

perfectly good.  Negative decimal values are supported by both GDB and

dbx.

   There is no standard way to specify the size of an enumeration type;

it is determined by the architecture (normally all enumerations types

are 32 bits).  Type attributes can be used to specify an enumeration

type of another size for debuggers which support them; see *Note String

Field::.

   Enumeration types are unusual in that they define symbols for the

enumeration values (`first', `second', and `third' in the above

example), and even though these symbols are visible in the file as a

whole (rather than being in a more local namespace like structure

member names), they are defined in the type definition for the

enumeration type rather than each having their own symbol.  In order to

be fast, GDB will only get symbols from such types (in its initial scan

of the stabs) if the type is the first thing defined after a `T' or `t'

symbol descriptor (the above example fulfills this requirement).  If

the type does not have a name, the compiler should emit it in a

nameless stab (*note String Field::.); GCC does this.

File: stabs.info,  Node: Structures,  Next: Typedefs,  Prev: Enumerations,  Up: Types

Structures

==========

   The encoding of structures in stabs can be shown with an example.

   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.  Seperate stabs are 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 the bss section.

     struct s_tag {

       int   s_int;

       float s_float;

       char  s_char_vec[8];

       struct s_tag* s_next;

     } g_an_s;

     typedef struct s_tag s_typedef;

   The structure tag has an `N_LSYM' stab type because, like the

enumeration, the symbol has file scope.  Like the enumeration, the

symbol descriptor is `T', for enumeration, structure, or tag type.  The

type descriptor `s' following the `16=' of the type definition narrows

the symbol type to structure.

   Following the `s' type descriptor is the number of bytes the

structure 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, NUMBER OF BITS IN THE ELEMENT.

     # 128 is N_LSYM

     .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

   In this example, the first two 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 a type

definition for an element which is a pointer to type 16.

   If a field is a static member (this is a C++ feature in which a

single variable appears to be a field of every structure of a given

type) it still starts out with the field name, a colon, and the type,

but then instead of a comma, bit position, comma, and bit size, there

is a colon followed by the name of the variable which each such field

refers to.

   If the structure has methods (a C++ feature), they follow the

non-method fields; see *Note Cplusplus::.

File: stabs.info,  Node: Typedefs,  Next: Unions,  Prev: Structures,  Up: Types

Giving a Type a Name

====================

   To give a type a name, use the `t' symbol descriptor.  The type is

specified by the type information (*note String Field::.) for the stab.

For example,

     .stabs "s_typedef:t16",128,0,0,0     # 128 is N_LSYM

   specifies that `s_typedef' refers to type number 16.  Such stabs

have symbol type `N_LSYM' (or `C_DECL' for XCOFF).  (The Sun

documentation mentions using `N_GSYM' in some cases).

   If you are specifying the tag name for a structure, union, or

enumeration, use the `T' symbol descriptor instead.  I believe C is the

only language with this feature.

   If the type is an opaque type (I believe this is a Modula-2 feature),

AIX provides a type descriptor to specify it.  The type descriptor is

`o' and is followed by a name.  I don't know what the name means--is it

always the same as the name of the type, or is this type descriptor

used with a nameless stab (*note String Field::.)?  There optionally

follows a comma followed by type information which defines the type of

this type.  If omitted, a semicolon is used in place of the comma and

the type information, and the type is much like a generic pointer

type--it has a known size but little else about it is specified.

File: stabs.info,  Node: Unions,  Next: Function Types,  Prev: Typedefs,  Up: Types

Unions

======

     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 infomation 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 (*note Stack

Variables::.).

File: stabs.info,  Node: Function Types,  Prev: Unions,  Up: Types

Function Types

==============

   Various types can be defined for function variables.  These types are

not used in defining functions (*note Procedures::.); they are used for

things like pointers to functions.

   The simple, traditional, type is type descriptor `f' is followed by

type information for the return type of the function, followed by a

semicolon.

   This does not deal with functions for which the number and types of

the parameters are part of the type, as in Modula-2 or ANSI C.  AIX

provides extensions to specify these, using the `f', `F', `p', and `R'

type descriptors.

   First comes the type descriptor.  If it is `f' or `F', this type

involves a function rather than a procedure, and the type information

for the return type of the function follows, followed by a comma.  Then

comes the number of parameters to the function and a semicolon.  Then,

for each parameter, there is the name of the parameter followed by a

colon (this is only present for type descriptors `R' and `F' which

represent Pascal function or procedure parameters), type information

for the parameter, a comma, 0 if passed by reference or 1 if passed by

value, and a semicolon.  The type definition ends with a semicolon.

   For example, this variable definition:

     int (*g_pf)();

generates the following code:

     .stabs "g_pf:G24=*25=f1",32,0,0,0

         .common _g_pf,4,"bss"

   The variable defines a new type, 24, which is a pointer to another

new type, 25, which is a function returning `int'.

File: stabs.info,  Node: Symbol Tables,  Next: Cplusplus,  Prev: Types,  Up: Top

Symbol Information in Symbol Tables

***********************************

   This chapter describes the format of symbol table entries and how

stab assembler directives map to them.  It also describes the

transformations that the assembler and linker make on data from stabs.

* Menu:

* Symbol Table Format::

* Transformations On Symbol Tables::

File: stabs.info,  Node: Symbol Table Format,  Next: Transformations On Symbol Tables,  Up: Symbol Tables

Symbol Table Format

===================

   Each time the assembler encounters a stab directive, it puts each

field of the stab into a corresponding field in a symbol table entry of

its output file.  If the stab contains a string field, the symbol table

entry for that stab points to a string table entry containing the

string data from the stab.  Assembler labels become relocatable

addresses.  Symbol table entries in a.out have the format: