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Procedure Name:symbols
Symbolic procedure names, which represent logical code addresses of the
type class LCPTR, may be used as operands in expressions. Thus, to
determine the virtual address of the procedure FOPEN, the WL command may
be used as follows:
$nmdebug > wl FOPEN
SYS $a.345498
$nmdebug >
In the above example, since no System Debug variable named FOPEN was
found, the expression evaluator searched for the symbol in the currently
loaded program file and libraries, finding it in NL.PUB.SYS.
Procedure name symbols stand for slightly different locations depending
on the mode of System Debug. In CM, they stand for the starting address
of the code bodies that they name. In NM, they stand for the entry
address. Since compilers may emit constants before executable
instructions in System Object Modules, breakpoints should always be set
at entry addresses. To find the entry address of a CM procedure, the
procedure symbol name should be prefixed by the question mark (?), as
explained below.
When searching program files and libraries for procedure symbols, System
Debug behaves differently depending on its mode. In NM, procedure names
are case sensitive, and the program file and libraries are searched in
the following order:
NM search order: first... PROG, GRP, PUB, USERs, SYS ...last
In CM, procedure names are case insensitive, and the following search
order is used:
CM search order: first... PROG, GRP, PUB, LGRP, LPUB, SYS ...last
Each of the above search orders, which visit all currently loaded files,
is known as a full search path. Note that this order is the same as that
used by the CM and NM loaders in satisfying external references in
program files and libraries, as specified in the LIB= and LIBLIST=
parameters of the RUN command.
Variations of certain commands, such as BREAK, DISPLAY, MODIFY,
TRANSLATE, FREEZE, and UNFREEZE, restrict the search path for procedure
name symbols in their parameters to a single loaded code file. In
addition, certain coercion functions (PROG, GRP, PUB, LGRP, LPUB, SYS)
also restrict the search path for procedure name symbols in their
parameters to a single loaded code file. This allows references to
procedure symbols in a particular library, that would otherwise be
inaccessible if they were redefined in preceding libraries on the full
search path.
Two symbol tables are present in NM executable libraries and program
files. The first symbol table is called the Loader Symbol Table (LST)
and is utilized by the native mode loader. It contains only exported
level 1 procedure names, which are hashed to support fast symbol name
lookups.
The second symbol table is called the System Object Module (SOM) symbol
table. This symbol table contains all compiler-generated symbols
(procedure, data, internal labels, try/recover, and so on), which are
maintained in no particular order. Any lookup attempt must be made
sequentially through the symbols.
If the SOM symbols are being searched and an ambiguous name is entered,
the first symbol that matches the name found during the sequential search
of the symbol table is used.
The symbol table used by the expression evaluator for symbol lookups is
based on the environment variable LOOKUP_ID. The variable may take on any
of the following values. (The default setting is LSTPROC.)
UNIVERSAL Search exported procedures in the SOM symbols.
LOCAL Search nonexported procedures in the SOM symbols.
NESTED Search nested procedures in the SOM symbols.
PROCEDURES Search local or exported procedures in the SOM symbols.
ALLPROC Search local/exported/nested procedures in the SOM symbols.
EXPORTSTUB Search export stubs in the SOM symbols.
DATAANY Search exported or local data SOM symbols.
DATAUNIV Search exported data SOM symbols.
DATALOCAL Search local data SOM symbols.
LSTPROC Search exported level 1 procedures in the LST.
LSTEXPORTSTUB Search export stubs in the LST.
ANY Search for any type of symbol in the SOM symbols.
NOTE Using the SOM symbol table is noticeably slower than using the LST.
!procedure_name
Just as System Debug variable names, which are composed of only the
letters "A" through "F" may conflict with hex constants, so may procedure
name symbols. Preceding such name symbols with an exclamation point
makes the expression scanner see the name as a symbol instead of a hex
constant. However, System Debug variable names take precedence over
procedure name symbols, so the variable name ADD makes a procedure of
that name invisible in expressions. In this case, the functions CMADDR
and NMADDR can be used to locate the procedure names.
?procedure_name
Sometimes the address that a procedure name symbol represents is not
appropriate for a particular use. By preceding a procedure name symbol
with a question mark, a different address is returned, depending on the
mode of System Debug.
In CM, ?procedure_name returns the entry point address for the named
procedure instead of its start address. This is the address of interest
when setting CM breakpoints. In NM, the question mark prefix returns the
export stub address of the procedure. This is the entry location used by
callers from external modules. Please refer to the Procedure Calling
Conventions Reference Manual (09740-90015) for a detailed discussion of
export stubs and native mode procedure organization.