SBL(6) SBL(6)
NAME
sbl - symbol table file
DESCRIPTION
A Limbo symbol table file provides type information for the
module in an associated Dis executable file, for use by
debuggers and similar applications. They are written by the
Limbo compiler when given the -g option. The files conven-
tionally have a .sbl suffix; they need not be stored in the
same directory as the corresponding Dis file.
The file consists of a header followed by five tables:
header file-table pc-table type-table fn-table data-
table
Each table starts with a line containing a decimal count of
the items in that table. The count can be zero. The follow-
ing sections describe the format of table items in each
table. In the description, the following terminals are
used.
A string is a sequence of letters, digits, and the charac-
ters _, ., -, and >. Letters are the Unicode characters a
through z and A through Z, and all Unicode characters with
encoded values greater than A0 hexadecimal.
An int is an optional minus sign followed by a sequence of
digits, 0 to 9.
In the following description, the presence of a space is
represented by • and a newline by \n. There are no other
spaces between syntactic elements in the file format. Other
special characters represent themselves.
Header
The header consists of two items.
header:
magic\n
module\n
magic:
limbo .sbl 2.0
module:
string
There have been two previous versions of symbol table for-
mat, distinguished by the number in magic. Version 1. was
the original; version 1.1 added more references back to the
source; and version 2.0 replaced the original adt table by a
Page 1 Plan 9 (printed 12/21/24)
SBL(6) SBL(6)
type table and added support for Limbo's pick construction.
Only version 2.0 is described here; the others are obsolete.
Module is the name of the module implemented in the Dis
file. Symbol file references to identifiers declared by the
implementation module are unqualified. A name referenced
that is imported from any other module is preceded by m->
where m is that module's identifier.
File table
The file table is a list of file names referenced by the
rest of the tables. Each file name is terminated by a new-
line character; within the name, any character other than a
newline is valid.
PC table
The PC table relates every instruction in the Dis file to
the source from which it was compiled. The table is indexed
by Dis PC to obtain a reference to the corresponding source.
Each item consists of a source description and a statement
reference:
pc-item:
src stmt \n
src:
pos,pos•
pos:
file:opt line.opt char
file, line, char, stmt:
int
A source description src selects source text as a range of
characters within lines of a source file. File is an index
into the file table (origin 0); line and char are positions
within that source file, with line numbers starting at 1 and
character positions at 0. If file or line is omitted, it is
assumed to have the previous value, or 0 if there is no pre-
vious value.
Stmt is the `statement number' of the instruction. Despite
its name, the scope of a statement number is smaller than a
Limbo statement: it identifies a region marked by the com-
piler for treatment as a unit when debugging. For instance,
in a for statement, the initial expression, increment, test,
and body all have unique statement numbers. All instruc-
tions compiled from the same region in a Limbo program have
the same statement number. The number is that of the inner-
most region that contains the instruction.
Type table
The type table describes the Limbo adts used in the Dis
file, both locally declared and imported. Each entry
Page 2 Plan 9 (printed 12/21/24)
SBL(6) SBL(6)
describes a type:
type:
@ type-index \n
a adt-item
p adt-item tag-table
t size . id-table
m module \n src
A type
C type
L type
R type
n
N
B
b
i
f
s
type-index:
int
Each leading character specifies a different Limbo type:
@ existing type referenced by type table index (not
self-referential)
A array of type
a adt without pick
B big
b byte
C chan of type
f real
i int
L list of type
m module
N nil's type
n no type (eg, function with no return type)
p adt with pick
R ref type
s string
t tuple
A type-index is an offset in the type table, representing
the corresponding type. Size is the size in bytes of a
value of the given type. The size of basic types is known
and does not appear explicitly: big and real are 64 bits;
all others including strings and reference types are 4
bytes. (Strings are represented internally by a pointer.)
Each id-table is preceded by a count of the number of
entries, followed by an id-item for each entry in the table:
Page 3 Plan 9 (printed 12/21/24)
SBL(6) SBL(6)
id-table:
count \n id-items
id-item:
id-offset : id-name : src type \n
id-offset:
int
id-name:
string
count:
int
Id-offset is the number of bytes from the start of the
enclosing object to the value of the object identified by
id-name.
An adt without a pick clause is described using an adt-item:
adt-item:
adt-name • src size \n id-table
adt-name:
string
size:
int
The id-table has an entry for every data element of adt-
name.
If an adt has a pick clause, the adt's type table entry uses
the p type character. The invariant part of the adt is
described by an adt-item (with zero size) and the variant
clause is described by a tag-table of the following form:
tag-table:
count \n tag-items
tag-item:
name : src size \n id-table
| name : src \n
The tag-table describes the variants of a Limbo adt that
includes a pick clause. The name is the pick tag for the
associated alternative. If the size and id-table are miss-
ing, the given variant has the same description as the pre-
vious one.
A tuple's id-table contains id-names of the form tn, where n
is the 0-origin index of the item in the tuple.
Src is as previously defined above in the PC table section.
Fn table
The function table describes each function implemented by
the Dis file.
Page 4 Plan 9 (printed 12/21/24)
SBL(6) SBL(6)
fn-item:
fn-pc : fn-name \n args locals return
fn-pc:
int
fn-name:
string
args, locals:
id-table
return:
type
Fn-pc is the starting pc for the Dis instructions for fn-
name. If the function is a member of an adt, the member name
is qualified by the adt name, so that fn-name has the form
`adt-name.member-name'. Within the id-tables for args and
locals, the id-offset fields give offsets relative to the
function's activation frame. Furthermore, no table entries
are made for args that are declared nil (unused).
Data table
The data table describes the global variables in the Dis
file. It is an id-table, with one entry for each global
variable.
SEE ALSO
limbo(1), wm-deb(1), debug(2), dis(6)
Page 5 Plan 9 (printed 12/21/24)