All of the following stabs normally use the N_FUN symbol type.
However, Sun's acc compiler on SunOS4 uses N_GSYM and
N_STSYM, which means that the value of the stab for the function
is useless and the debugger must get the address of the function from
the non-stab symbols instead. On systems where non-stab symbols have
leading underscores, the stabs will lack underscores and the debugger
needs to know about the leading underscore to match up the stab and the
non-stab symbol. BSD Fortran is said to use N_FNAME with the
same restriction; the value of the symbol is not useful (I'm not sure it
really does use this, because GDB doesn't handle this and no one has
complained).
A function is represented by an ‘F’ symbol descriptor for a global
(extern) function, and ‘f’ for a static (local) function. For
a.out, the value of the symbol is the address of the start of the
function; it is already relocated. For stabs in ELF, the SunPRO
compiler version 2.0.1 and GCC put out an address which gets relocated
by the linker. In a future release SunPRO is planning to put out zero,
in which case the address can be found from the ELF (non-stab) symbol.
Because looking things up in the ELF symbols would probably be slow, I'm
not sure how to find which symbol of that name is the right one, and
this doesn't provide any way to deal with nested functions, it would
probably be better to make the value of the stab an address relative to
the start of the file, or just absolute. See ELF Linker Relocation for more information on linker relocation of stabs in ELF
files. For XCOFF, the stab uses the C_FUN storage class and the
value of the stab is meaningless; the address of the function can be
found from the csect symbol (XTY_LD/XMC_PR).
The type information of the stab represents the return type of the
function; thus ‘foo:f5’ means that foo is a function returning type
5. There is no need to try to get the line number of the start of the
function from the stab for the function; it is in the next
N_SLINE symbol.
Some compilers (such as Sun's Solaris compiler) support an extension for
specifying the types of the arguments. I suspect this extension is not
used for old (non-prototyped) function definitions in C. If the
extension is in use, the type information of the stab for the function
is followed by type information for each argument, with each argument
preceded by ‘;’. An argument type of 0 means that additional
arguments are being passed, whose types and number may vary (‘...’
in ANSI C). GDB has tolerated this extension (parsed the syntax, if not
necessarily used the information) since at least version 4.8; I don't
know whether all versions of dbx tolerate it. The argument types given
here are not redundant with the symbols for the formal parameters
(see Parameters); they are the types of the arguments as they are
passed, before any conversions might take place. For example, if a C
function which is declared without a prototype takes a float
argument, the value is passed as a double but then converted to a
float. Debuggers need to use the types given in the arguments
when printing values, but when calling the function they need to use the
types given in the symbol defining the function.
If the return type and types of arguments of a function which is defined
in another source file are specified (i.e., a function prototype in ANSI
C), traditionally compilers emit no stab; the only way for the debugger
to find the information is if the source file where the function is
defined was also compiled with debugging symbols. As an extension the
Solaris compiler uses symbol descriptor ‘P’ followed by the return
type of the function, followed by the arguments, each preceded by
‘;’, as in a stab with symbol descriptor ‘f’ or ‘F’.
This use of symbol descriptor ‘P’ can be distinguished from its use
for register parameters (see Register Parameters) by the fact that it has
symbol type N_FUN.
The AIX documentation also defines symbol descriptor ‘J’ as an internal function. I assume this means a function nested within another function. It also says symbol descriptor ‘m’ is a module in Modula-2 or extended Pascal.
Procedures (functions which do not return values) are represented as
functions returning the void type in C. I don't see why this couldn't
be used for all languages (inventing a void type for this purpose if
necessary), but the AIX documentation defines ‘I’, ‘P’, and
‘Q’ for internal, global, and static procedures, respectively.
These symbol descriptors are unusual in that they are not followed by
type information.
The following example shows a stab for a function main which
returns type number 1. The _main specified for the value
is a reference to an assembler label which is used to fill in the start
address of the function.
.stabs "main:F1",36,0,0,_main # 36 is N_FUN
The stab representing a procedure is located immediately following the code of the procedure. This stab is in turn directly followed by a group of other stabs describing elements of the procedure. These other stabs describe the procedure's parameters, its block local variables, and its block structure.
If functions can appear in different sections, then the debugger may not
be able to find the end of a function. Recent versions of GCC will mark
the end of a function with an N_FUN symbol with an empty string
for the name. The value is the address of the end of the current
function. Without such a symbol, there is no indication of the address
of the end of a function, and you must assume that it ended at the
starting address of the next function or at the end of the text section
for the program.