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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.
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NOTE: Using the SOM symbol table is noticeably slower than using the LST.
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!procedure_name
Just as System Debug variable names 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 for a detailed
discussion of export stubs and native mode procedure organization.
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