This is Info file stabs.info, produced by Makeinfo version 1.68 from the input file ./stabs.texinfo. START-INFO-DIR-ENTRY * Stabs: (stabs). The "stabs" debugging information format. END-INFO-DIR-ENTRY 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: struct internal_nlist { unsigned long n_strx; /* index into string table of name */ unsigned char n_type; /* type of symbol */ unsigned char n_other; /* misc info (usually empty) */ unsigned short n_desc; /* description field */ bfd_vma n_value; /* value of symbol */ }; If the stab has a string, the `n_strx' field holds the offset in bytes of the string within the string table. The string is terminated by a NUL character. If the stab lacks a string (for example, it was produced by a `.stabn' or `.stabd' directive), the `n_strx' field is zero. Symbol table entries with `n_type' field values greater than 0x1f originated as stabs generated by the compiler (with one random exception). The other entries were placed in the symbol table of the executable by the assembler or the linker.  File: stabs.info, Node: Transformations On Symbol Tables, Prev: Symbol Table Format, Up: Symbol Tables Transformations on Symbol Tables ================================ The linker concatenates object files and does fixups of externally defined symbols. You can see the transformations made on stab data by the assembler and linker by examining the symbol table after each pass of the build. To do this, use `nm -ap', which dumps the symbol table, including debugging information, unsorted. For stab entries the columns are: VALUE, OTHER, DESC, TYPE, STRING. For assembler and linker symbols, the columns are: VALUE, TYPE, STRING. The low 5 bits of the stab type tell the linker how to relocate the value of the stab. Thus for stab types like `N_RSYM' and `N_LSYM', where the value is an offset or a register number, the low 5 bits are `N_ABS', which tells the linker not to relocate the value. Where the value of a stab contains an assembly language label, it is transformed by each build step. The assembler turns it into a relocatable address and the linker turns it into an absolute address. * Menu: * Transformations On Static Variables:: * Transformations On Global Variables:: * Stab Section Transformations:: For some object file formats, things are a bit different.  File: stabs.info, Node: Transformations On Static Variables, Next: Transformations On Global Variables, Up: Transformations On Symbol Tables Transformations on Static Variables ----------------------------------- This source line defines a static variable at file scope: static int s_g_repeat The following stab describes the symbol: .stabs "s_g_repeat:S1",38,0,0,_s_g_repeat The assembler transforms the stab into this symbol table entry in the `.o' file. The location is expressed as a data segment offset. 00000084 - 00 0000 STSYM s_g_repeat:S1 In the symbol table entry from the executable, the linker has made the relocatable address absolute. 0000e00c - 00 0000 STSYM s_g_repeat:S1  File: stabs.info, Node: Transformations On Global Variables, Next: Stab Section Transformations, Prev: Transformations On Static Variables, Up: Transformations On Symbol Tables Transformations on Global Variables ----------------------------------- Stabs for global variables do not contain location information. In this case, the debugger finds location information in the assembler or linker symbol table entry describing the variable. The source line: char g_foo = 'c'; generates the stab: .stabs "g_foo:G2",32,0,0,0 The variable is represented by two symbol table entries in the object file (see below). The first one originated as a stab. The second one is an external symbol. The upper case `D' signifies that the `n_type' field of the symbol table contains 7, `N_DATA' with local linkage. The stab's value is zero since the value is not used for `N_GSYM' stabs. The value of the linker symbol is the relocatable address corresponding to the variable. 00000000 - 00 0000 GSYM g_foo:G2 00000080 D _g_foo These entries as transformed by the linker. The linker symbol table entry now holds an absolute address: 00000000 - 00 0000 GSYM g_foo:G2 ... 0000e008 D _g_foo  File: stabs.info, Node: Stab Section Transformations, Prev: Transformations On Global Variables, Up: Transformations On Symbol Tables Transformations of Stabs in separate sections --------------------------------------------- For object file formats using stabs in separate sections (*note Stab Sections::.), use `objdump --stabs' instead of `nm' to show the stabs in an object or executable file. `objdump' is a GNU utility; Sun does not provide any equivalent. The following example is for a stab whose value is an address is relative to the compilation unit (*note ELF Linker Relocation::.). For example, if the source line static int ld = 5; appears within a function, then the assembly language output from the compiler contains: .Ddata.data: ... .stabs "ld:V(0,3)",0x26,0,4,.L18-Ddata.data # 0x26 is N_STSYM ... .L18: .align 4 .word 0x5 Because the value is formed by subtracting one symbol from another, the value is absolute, not relocatable, and so the object file contains Symnum n_type n_othr n_desc n_value n_strx String 31 STSYM 0 4 00000004 680 ld:V(0,3) without any relocations, and the executable file also contains Symnum n_type n_othr n_desc n_value n_strx String 31 STSYM 0 4 00000004 680 ld:V(0,3)  File: stabs.info, Node: Cplusplus, Next: Stab Types, Prev: Symbol Tables, Up: Top GNU C++ Stabs ************* * Menu: * Class Names:: C++ class names are both tags and typedefs. * Nested Symbols:: C++ symbol names can be within other types. * Basic Cplusplus Types:: * Simple Classes:: * Class Instance:: * Methods:: Method definition * Method Type Descriptor:: The `#' type descriptor * Member Type Descriptor:: The `@' type descriptor * Protections:: * Method Modifiers:: * Virtual Methods:: * Inheritence:: * Virtual Base Classes:: * Static Members::  File: stabs.info, Node: Class Names, Next: Nested Symbols, Up: Cplusplus C++ Class Names =============== In C++, a class name which is declared with `class', `struct', or `union', is not only a tag, as in C, but also a type name. Thus there should be stabs with both `t' and `T' symbol descriptors (*note Typedefs::.). To save space, there is a special abbreviation for this case. If the `T' symbol descriptor is followed by `t', then the stab defines both a type name and a tag. For example, the C++ code struct foo {int x;}; can be represented as either .stabs "foo:T19=s4x:1,0,32;;",128,0,0,0 # 128 is N_LSYM .stabs "foo:t19",128,0,0,0 or .stabs "foo:Tt19=s4x:1,0,32;;",128,0,0,0  File: stabs.info, Node: Nested Symbols, Next: Basic Cplusplus Types, Prev: Class Names, Up: Cplusplus Defining a Symbol Within Another Type ===================================== In C++, a symbol (such as a type name) can be defined within another type. In stabs, this is sometimes represented by making the name of a symbol which contains `::'. Such a pair of colons does not end the name of the symbol, the way a single colon would (*note String Field::.). I'm not sure how consistently used or well thought out this mechanism is. So that a pair of colons in this position always has this meaning, `:' cannot be used as a symbol descriptor. For example, if the string for a stab is `foo::bar::baz:t5=*6', then `foo::bar::baz' is the name of the symbol, `t' is the symbol descriptor, and `5=*6' is the type information.  File: stabs.info, Node: Basic Cplusplus Types, Next: Simple Classes, Prev: Nested Symbols, Up: Cplusplus Basic Types For C++ =================== << the examples that follow are based on a01.C >> C++ adds two more builtin types to the set defined for C. These are the unknown type and the vtable record type. The unknown type, type 16, is defined in terms of itself like the void type. The vtable record type, type 17, is defined as a structure type and then as a structure tag. The structure has four fields: delta, index, pfn, and delta2. pfn is the function pointer. << In boilerplate $vtbl_ptr_type, what are the fields delta, index, and delta2 used for? >> This basic type is present in all C++ programs even if there are no virtual methods defined. .stabs "struct_name:sym_desc(type)type_def(17)=type_desc(struct)struct_bytes(8) elem_name(delta):type_ref(short int),bit_offset(0),field_bits(16); elem_name(index):type_ref(short int),bit_offset(16),field_bits(16); elem_name(pfn):type_def(18)=type_desc(ptr to)type_ref(void), bit_offset(32),field_bits(32); elem_name(delta2):type_def(short int);bit_offset(32),field_bits(16);;" N_LSYM, NIL, NIL .stabs "$vtbl_ptr_type:t17=s8 delta:6,0,16;index:6,16,16;pfn:18=*15,32,32;delta2:6,32,16;;" ,128,0,0,0 .stabs "name:sym_dec(struct tag)type_ref($vtbl_ptr_type)",N_LSYM,NIL,NIL,NIL .stabs "$vtbl_ptr_type:T17",128,0,0,0  File: stabs.info, Node: Simple Classes, Next: Class Instance, Prev: Basic Cplusplus Types, Up: Cplusplus Simple Class Definition ======================= The stabs describing C++ language features are an extension of the stabs describing C. Stabs representing C++ class types elaborate extensively on the stab format used to describe structure types in C. Stabs representing class type variables look just like stabs representing C language variables. Consider the following very simple class definition. class baseA { public: int Adat; int Ameth(int in, char other); }; The class `baseA' is represented by two stabs. The first stab describes the class as a structure type. The second stab describes a structure tag of the class type. Both stabs are of stab type `N_LSYM'. Since the stab is not located between an `N_FUN' and an `N_LBRAC' stab this indicates that the class is defined at file scope. If it were, then the `N_LSYM' would signify a local variable. A stab describing a C++ class type is similar in format to a stab describing a C struct, with each class member shown as a field in the structure. The part of the struct format describing fields is expanded to include extra information relevent to C++ class members. In addition, if the class has multiple base classes or virtual functions the struct format outside of the field parts is also augmented. In this simple example the field part of the C++ class stab representing member data looks just like the field part of a C struct stab. The section on protections describes how its format is sometimes extended for member data. The field part of a C++ class stab representing a member function differs substantially from the field part of a C struct stab. It still begins with `name:' but then goes on to define a new type number for the member function, describe its return type, its argument types, its protection level, any qualifiers applied to the method definition, and whether the method is virtual or not. If the method is virtual then the method description goes on to give the vtable index of the method, and the type number of the first base class defining the method. When the field name is a method name it is followed by two colons rather than one. This is followed by a new type definition for the method. This is a number followed by an equal sign and the type of the method. Normally this will be a type declared using the `#' type descriptor; see *Note Method Type Descriptor::; static member functions are declared using the `f' type descriptor instead; see *Note Function Types::. The format of an overloaded operator method name differs from that of other methods. It is `op$::OPERATOR-NAME.' where OPERATOR-NAME is the operator name such as `+' or `+='. The name ends with a period, and any characters except the period can occur in the OPERATOR-NAME string. The next part of the method description represents the arguments to the method, preceeded by a colon and ending with a semi-colon. The types of the arguments are expressed in the same way argument types are expressed in C++ name mangling. In this example an `int' and a `char' map to `ic'. This is followed by a number, a letter, and an asterisk or period, followed by another semicolon. The number indicates the protections that apply to the member function. Here the 2 means public. The letter encodes any qualifier applied to the method definition. In this case, `A' means that it is a normal function definition. The dot shows that the method is not virtual. The sections that follow elaborate further on these fields and describe the additional information present for virtual methods. .stabs "class_name:sym_desc(type)type_def(20)=type_desc(struct)struct_bytes(4) field_name(Adat):type(int),bit_offset(0),field_bits(32); method_name(Ameth)::type_def(21)=type_desc(method)return_type(int); :arg_types(int char); protection(public)qualifier(normal)virtual(no);;" N_LSYM,NIL,NIL,NIL .stabs "baseA:t20=s4Adat:1,0,32;Ameth::21=##1;:ic;2A.;;",128,0,0,0 .stabs "class_name:sym_desc(struct tag)",N_LSYM,NIL,NIL,NIL .stabs "baseA:T20",128,0,0,0