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.
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 convoluted, 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.
This is the traditional, convoluted method for defining builtin types.
There are several classes of such type definitions: integer, floating
point, and void.
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.
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.
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 ;
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 (see section 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
R fp-type ; bytes ;
1 (NF_SINGLE)
2 (NF_DOUBLE)
3 (NF_COMPLEX)
4 (NF_COMPLEX16)
5 (NF_COMPLEX32)
complex, double complex, and
complex*16, respectively, but what does that mean? (i.e., Single
precision? Double precision?).
6 (NF_LDOUBLE)
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).
g type-information ; nbits
c type-information ; nbits
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.
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
(see section The 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. See section 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.
b type-information ; bytes
B type-information
d type-information
k type-information
M type-information ; length
character*3 is
represented by `M-2;3', where `-2' is a reference to a
character type (see section 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 section Subrange Types.
S type-information
* type-information
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 section Defining a Symbol Within Another Type.
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 (see section Giving a Type a Name) 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 (see section The String Field), or what. The symbol ends with `;'.
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 (see section Array Types).
Instead of an integer, either bound can be one of the following:
A offset
T offset
a register-number
t register-number
J
Subranges are also used for builtin types; see section Traditional Builtin 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, see section Subrange Types). 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 (see section The 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 (see section The 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.
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
z type-information ; bytes
N
Languages, such as CHILL which have a string type which is basically just an array of characters use the `S' type attribute (see section The String Field).
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 section The 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 (see section The String Field); GCC does this.
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. Separate stabs are generated for the
structure tag, the structure typedef, and the structure instance. The
stabs for the tag and the typedef are emitted 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 stand-alone 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 section GNU C++ Stabs.
To give a type a name, use the `t' symbol descriptor. The type is specified by the type information (see section The 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 (see section The 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.
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 section Automatic Variables Allocated on the Stack).
Various types can be defined for function variables. These types are not used in defining functions (see section 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.
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