Print debugging statement: Difference between revisions
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Revision as of 18:12, 9 May 2020
From Wikipedia:
Print debugging (or tracing) is the act of watching (live or recorded) trace statements, or print statements, that indicate the flow of execution of a process. This is sometimes called printf debugging, due to the use of the printf function in C.
- Task
- Show the print debugging statements in the language.
- Demonstrate their ability to track provenance by displaying information about source code (e.g., code fragment, line and column number).
C
C doesn't have a built-in print debugging statement. However, it can be defined by users as a macro.
<lang C>#include <stdio.h>
- define DEBUG_INT(x) printf( #x " at line %d\nresult: %d\n\n", __LINE__, x)
int add(int x, int y) {
int result = x + y; DEBUG_INT(x); DEBUG_INT(y); DEBUG_INT(result); DEBUG_INT(result+1); return result;
}
int main() {
add(2, 7); return 0;
}</lang>
- Output:
x at line 7 result: 2 y at line 8 result: 7 result at line 9 result: 9 result+1 at line 10 result: 10
C++
<lang cpp>#include <iostream>
// requires support for variadic macros in this form
- define DEBUG(msg,...) fprintf(stderr, "[DEBUG %s@%d] " msg "\n", __FILE__, __LINE__, __VA_ARGS__)
// may be replace with and include of <source_location> (c++20) if it becomes part of the standard
int main() {
DEBUG("Hello world");
DEBUG("Some %d Things", 42);
return 0;
}</lang>
COBOL
Works with GnuCOBOL.
Explicit debug usually just uses DISPLAY, and usually directed at SYSERR. Compiler Directing Facility debug is supported with >>D directive statements. GnuCOBOL also supports full on line step tracing, controlled by both compile time switch, -ftraceall, and a truthy setting in a `COB_SET_TRACE` environment variable (so program step logging can be switched on and off without a recompile (for tracking down pesky production run mysteries)).
<lang cobol>gcobol*>
*> steptrace.cob
*> Tectonics: cobc -xj -fdebugging-line -ftraceall steptrace.cob
*> export COB_SET_TRACE=Y
*>
identification division.
program-id. steptrace.
data division.
working-storage section.
procedure division.
steptrace-main.
display "explicit line" upon syserr
>>Ddisplay "debug line" upon syserr
display "from " FUNCTION MODULE-ID " in " FUNCTION MODULE-SOURCE
goback.
end program steptrace.</lang>
That is fixed form COBOL, columns 1 through 6 ignored by the preprocessor (historical format, from the days of punch card input) with column 7 being a special indicator column, star for comments, dash for continuations, and D is supported by GnuCOBOL for Debug lines. The >>D form is a newer addition to the Compiler Directing Facility, and can float anywhere on a free format compile line.
Along with MODULE-ID and MODULE-SOURCE, there are intrinsics for MODULE-CALLER-ID, FUNCTION MODULE-DATE, FUNCTION MODULE-FORMATTED-DATE, FUNCTION MODULE-PATH, FUNCTION MODULE-TIME.
- Output:
prompt$ cobc -xj -fdebugging-line -ftraceall steptrace.cob explicit line debug line from steptrace in steptrace.cob prompt$ cobc -xj -ftraceall steptrace.cob explicit line from steptrace in steptrace.cob prompt$ export COB_SET_TRACE=Y prompt$ ./steptrace Source: 'steptrace.cob' Program-Id: steptrace Entry: steptrace Line: 22 Program-Id: steptrace Section: (None) Line: 22 Program-Id: steptrace Paragraph: steptrace-main Line: 22 Program-Id: steptrace Statement: DISPLAY Line: 24 explicit line Program-Id: steptrace Statement: DISPLAY Line: 28 from steptrace in steptrace.cob Program-Id: steptrace Statement: GOBACK Line: 30 Program-Id: steptrace Exit: steptrace
D
Using templates and default arguments provides options for specifying a debug function. The file and line could be included as either template arguments or function arguments. <lang d>import std.stdio;
void debugln(string file = __FILE__, size_t line = __LINE__, S...)(S args) {
write('[', file, '@', line, "] ", args, '\n');
}
void debugWrite(S...)(S args, string file = __FILE__, size_t line = __LINE__) {
write('[', file, '@', line, "] ", args, '\n');
}
void main() {
debugln();
debugln("Hello world!");
debugln("Hello", ' ', "world", '!');
debugWrite();
debugWrite("Goodbye world!");
debugWrite("Goodbye", ' ', "world", '!');
} </lang>
- Output:
[debug.d@12] [debug.d@13] Hello world! [debug.d@14] Hello world! [debug.d@16] [debug.d@17] Goodbye world! [debug.d@18] Goodbye world!
Go
Go doesn't have a built-in print debugging statement as such. Nor does it have macros.
However, as the following example shows, it is easy enough to mimic a C-like approach by writing a short 'debug' function which can show the value of an expression and its type at the appropriate line number in the program's source code.
Note that a label for the expression (whether it's a simple variable or not) must be passed to the 'debug' function as there is no way to deduce it otherwise. <lang go>package main
import (
"fmt" "runtime"
)
type point struct {
x, y float64
}
func add(x, y int) int {
result := x + y
debug("x", x)
debug("y", y)
debug("result", result)
debug("result+1", result+1)
return result
}
func debug(s string, x interface{}) {
_, _, lineNo, _ := runtime.Caller(1)
fmt.Printf("%q at line %d type '%T'\nvalue: %#v\n\n", s, lineNo, x, x)
}
func main() {
add(2, 7)
b := true
debug("b", b)
s := "Hello"
debug("s", s)
p := point{2, 3}
debug("p", p)
q := &p
debug("q", q)
}</lang>
- Output:
"x" at line 14 type 'int'
value: 2
"y" at line 15 type 'int'
value: 7
"result" at line 16 type 'int'
value: 9
"result+1" at line 17 type 'int'
value: 10
"b" at line 29 type 'bool'
value: true
"s" at line 31 type 'string'
value: "Hello"
"p" at line 33 type 'main.point'
value: main.point{x:2, y:3}
"q" at line 35 type '*main.point'
value: &main.point{x:2, y:3}
Julia
Julia has native print logging type functions, including a core Logging module. In addition, there are several additional downloadable modules, such as Memento, with extended logging functionality. Julia's builtin logging has defined macros for logging levels: @error, @warn, @info, and @debug. By default, debug level statements in code are not printed unless the logging level is set to allow debug statements to print, which can be enabled by setting the JULIA_DEBUG environment variable. <lang julia>function test()
@info "starting test()"
a = [1, 2]
for i in 1:4
if i > 3
@debug "debugging $a at line $(@__LINE__) of file $(@__FILE__)"
else
a .*= 2
end
end
@warn "exiting test()"
println()
end
test()
ENV["JULIA_DEBUG"] = "all"
test()
</lang>
- Output:
[ Info: starting test() ┌ Warning: exiting test() └ @ Main /usr/programming/test2.jl:13 [ Info: starting test() ┌ Debug: debugging [8, 16] at line 8 of file /usr/programming/test2.jl └ @ Main /usr/programming/test2.jl:8 ┌ Warning: exiting test() └ @ Main /usr/programming/test2.jl:13
Mercury
Mercury has trace goals which can be used within pure code, can summon !IO for use within the goal, can be made conditional off of compile-time or runtime flags, and which are pretty free with what they can do.
Together with data functors like $module, $pred, $line, trace goals can be used for debugging print statements as in the following example. Together with the require module's utilities, trace goals can be used for assert() statements or pre/post assertions.
<lang Mercury>:- module add.
- - interface.
- - import_module io.
- - pred main(io::di, io::uo) is det.
- - implementation.
- - import_module int, io, string, list.
- - func add(int, int) = int.
A `add` B = C :-
C = A + B,
trace [io(!IO), compile_time(grade(debug))] (
io.format("%s - %s(%d): %d `add` %d = %d\n",
[s($grade), s($pred), i($line), i(A), i(B), i(C)], !IO)
).
main(!IO) :-
2 `add` 7 = N, io.print_line(N, !IO).</lang>
- Output:
(with a non-debug grade)
9
- Output:
(with a debug grade)
asm_fast.par.gc.debug.stseg - function `add.add'/2(13): 2 `add` 7 = 9 9
Perl
Carp is a core module, always available.
<lang perl>use Carp;
$str = 'Resistance'; carp "'$str' is futile."; print "\n";
doodle($str); print "\n";
fiddle(7);
sub doodle { my ($str) = @_; carp "'$str' is still futile." }
sub fiddle { faddle(2*shift) } sub faddle { fuddle(3*shift) } sub fuddle { ( carp("'$_[0]', interesting number.") ); }</lang>
- Output:
'Resistance' is futile. at printf_debug.pl line 11.
'Resistance' is still futile. at printf_debug.pl line 17.
main::doodle("Resistance") called at printf_debug.pl line 13
'42', interesting number. at printf_debug.pl line 21.
main::fuddle(42) called at printf_debug.pl line 20
main::faddle(14) called at printf_debug.pl line 19
main::fiddle(7) called at printf_debug.pl line 15
Phix
The ? statement is used as a quick shorthand to dump variable contents and expression results.
For provenance, the following was added to builtins/reflections.e (for now/not an autoinclude, 0.8.1+), and throw()
was also tweaked to convert supplied addresses, which it previously did not. It proved to be quite an easy enhancement
to the language, albeit as yet undocumented.
<lang Phix>function _debug_info()
-- use throw to convert a return address and routine number
-- from the call stack into a proper line number, etc.
-- (private, not called direct/from outside this file)
integer rtn
atom ret_addr
#ilASM{
[32]
mov edx,[ebp+20] -- prev_ebp
mov eax,[edx+28] -- return address
mov edx,[edx+20] -- prev_ebp
lea edi,[ret_addr]
call :%pStoreMint
mov eax,[edx+8] -- calling routine no
mov [rtn],eax
[64]
mov rdx,[rbp+40] -- prev_ebp
mov rax,[rdx+56] -- return address
mov rdx,[rdx+40] -- prev_ebp
lea rdi,[ret_addr]
call :%pStoreMint
mov rax,[rdx+16] -- calling routine no
mov [rtn],rax
[]
}
try
throw({1,ret_addr-1,-1,rtn,-1,-1,-1})
catch e
return e
end try
end function
-- NOTE: following five routines must all use the exact same nesting level.
global function debug_info()
return _debug_info()
end function
global function debug_line()
return _debug_info()[E_LINE]
end function
global function debug_rtn()
return _debug_info()[E_NAME]
end function
global function debug_file()
return _debug_info()[E_FILE]
end function
global function debug_path()
return _debug_info()[E_PATH]
end function</lang> This can be used as follows (0.8.1+) <lang Phix>include builtins/reflections.e
?debug_info() ?debug_line() procedure test()
?debug_info()
?debug_line()
?debug_file()
printf(1,"This is line %d in file %s\n",{debug_line(),debug_file()})
end procedure test()</lang>
- Output:
{1,7544919,3,21,"-1","test.exw","C:\\Program Files (x86)\\Phix\\"}
4
{1,7545341,6,1034,"test","test.exw","C:\\Program Files (x86)\\Phix\\"}
7
"test.exw"
This is line 9 in file test.exw
See the throw() documentation for more details, especially regarding the debug_info() results.
There is no routine name for the first debug_info() call, so you get "-1" in [E_NAME].
The line numbers 3, 4, 6, 7, and 9 are returned for five of the seven calls.
Pyret
Pyret has the spy expression. The expression can print the value of an identifier, using the identifier itself as a label if it's not already given. It could also print the value of an arbitrary expression, but it needs an explicit label in this case.
<lang pyret>fun add(x, y):
result = x + y spy "in add": x, y, result, result-plus-one: result + 1 end result
end
add(2, 7)</lang>
- Output:
Spying "in add" (at file:///spies.arr:3:2-8:5) x: 2 y: 7 result: 9 result-plus-one: 10 9
Racket
Racket doesn't have a built-in print debugging statement. However, it can be defined by users as a macro.
<lang racket>#lang racket
(require syntax/parse/define)
(define (debug:core line col code val #:label [label #f])
;; if label exists, use it instead of the code fragment (printf "~a at line ~a column ~a\n" (or label code) line col) (printf "result: ~a\n\n" val) ;; return the value itself, so that we can wrap macro around an expression ;; without restructuring any code val)
(define-simple-macro (debug <x> option ...)
#:with line (datum->syntax this-syntax (syntax-line #'<x>)) #:with col (datum->syntax this-syntax (syntax-column #'<x>)) (debug:core line col (quote <x>) <x> option ...))
(define (add x y)
(define result (+ x y)) (debug x) (debug y) (debug (if #t (+ x y) (error 'impossible))) (debug (add1 result) #:label "result plus one") (debug result))
(add 2 7)</lang>
- Output:
x at line 20 column 9 result: 2 y at line 21 column 9 result: 7 (if #t (+ x y) (error 'impossible)) at line 22 column 9 result: 9 result plus one at line 23 column 9 result: 10 result at line 24 column 9 result: 9 9
Raku
(formerly Perl 6)
There isn't anything built-in to do this in Rakudo/Raku, though it's pretty easy to cobble something together piggybacking off of the exception system. It would probably be better to instantiate a specific "DEBUG" exception to avoid interfering with other user instantiated ad-hoc exceptions, but for a quick-and-dirty demo, this should suffice.
This example will report any payload contents passed to the exception. If you want specific information, it will need to be passed in, though some of it may be determinable through introspection. Reports the file name and line number where the "debug" call originated and unwinds the call stack to trace through the subroutine calls leading up to it. Will follow the call chain into included files and modules, though calls to the CORE setting and dispatcher are filtered out here to reduce noise.
Comments with the files line numbers are added here to make it easier to match up the debug output with the file. Typically you would be editing the file in an editor that provides line numbering so that wouldn't be necessary/helpful.
<lang perl6>my &pdb = ¨
CATCH {
when X::AdHoc {
my @frames = .backtrace[*];
say .payload;
for @frames {
# Filter out exception handing and dispatcher frames
next if .file.contains: 'SETTING' or .subname.chars < 1;
printf "%sfrom file: %s,%s line: %s\n",
(' ' x $++), .file,
(my $s = .subname) eq '<unit>' ?? !! " sub: $s,", .line;
}
say ;
.resume;
}
default {}
}
- Testing / demonstration
- helper subs #line 22
sub alpha ($a) { #line 23
pdb ('a =>', $a + 3); #line 24
pdb 'string'; #line 25
beta(7); #line 26
} #line 27 sub beta ($b) { pdb $b } #line 28 sub gamma ($c) { beta $c } #line 29 sub delta ($d) { gamma $d } #line 30
#line 31
my $a = 10; #line 32 pdb (.VAR.name, $_) with $a; #line 33 alpha($a); #line 34 delta("Δ"); #line 35 .&beta for ^3; #line 36</lang>
- Output:
($a 10) from file: debug.p6, line: 33 (a => 13) from file: debug.p6, sub: alpha, line: 24 from file: debug.p6, line: 34 string from file: debug.p6, sub: alpha, line: 25 from file: debug.p6, line: 34 7 from file: debug.p6, sub: beta, line: 28 from file: debug.p6, sub: alpha, line: 26 from file: debug.p6, line: 34 Δ from file: debug.p6, sub: beta, line: 28 from file: debug.p6, sub: gamma, line: 29 from file: debug.p6, sub: delta, line: 30 from file: debug.p6, line: 35 0 from file: debug.p6, sub: beta, line: 28 from file: debug.p6, line: 36 1 from file: debug.p6, sub: beta, line: 28 from file: debug.p6, line: 36 2 from file: debug.p6, sub: beta, line: 28 from file: debug.p6, line: 36
REXX
Since REXX is an interpretive language, it is easy to add judicious use of the say which is an easy
way to visually examine the values of any variable throughout the program's execution.
When that might not prove feasible (maybe because of copious output before the problem occurs),
using the trace instruction might be a better choice.
Some of the options for the trace instruction are to display:
- what commands have a non-zero return code
- result of clauses
- what commands are being executed
- the (name of) labels being executed
- command errors
- command failures
- commands executed that have a negative return code
- an interactive mode that pauses and lets the programmer display values of variables
One of the options that shows the detailed information is the i option which
is the most informative and
shows intermediate results within a REXX statement as it's being evaluated.
The first number (for the trace output) is the line number for the REXX program.
(Blank lines are not traced.)
The following output is from the Regina REXX interpreter. <lang rexx>/*REXX program to demonstrate debugging (TRACE) information while executing a program*/ /*────────────────────────────────────────────── (below) the I is for information. */ trace i parse arg maxDiv . if maxDiv== | maxDiv=="," then maxDiv= 1000 /*obtain optional argument from the CL.*/ say 'maximum random divisor is:' maxDiv /*display the max divisor being used. */ total= 0
do j=1 to 100
total= total + j/random(maxDiv)
end /*j*/
say 'total=' total /*stick a fork in it, we're all done. */</lang>
- output when using the input of: 9
4 *-* parse arg maxDiv .
>>> "9"
>.> ""
5 *-* if maxDiv=='' | maxDiv=="," then maxDiv= 1000 /*obtain optional argument from the CL.*/
>V> "9"
>L> ","
>O> "0"
>V> "9"
>L> ""
>O> "0"
>U> "0"
6 *-* say 'maximum random divisor is:' maxDiv /*display the max divisor being used. */
>L> "maximum random divisor is:"
>V> "9"
>O> "maximum random divisor is: 9"
maximum random divisor is: 9
7 *-* total= 0
>L> "0"
9 *-* do j=1 to 100
>L> "1"
>L> "100"
>V> "1"
10 *-* total= total + j/random(maxDiv)
>V> "0"
>V> "1"
>V> "9"
>F> "3"
>O> "0.333333333"
>O> "0.333333333"
11 *-* end /*j*/
9 *-* do j=1 to 100
>V> "1"
>V> "2"
10 *-* total= total + j/random(maxDiv)
>V> "0.333333333"
>V> "2"
>V> "9"
>F> "0"
10 +++ total= total + j/random(maxDiv)
Error 42 running "c:\debuggin.rex", line 10: Arithmetic overflow/underflow
Error 42.3: Arithmetic overflow; divisor must not be zero
Programming note: this error occurred because when the random BIF is invoked
with one argument (as is here), the
range of random numbers generated are integers from zero to the value of the argument (inclusive).
zkl
Print debugging is similar to C. The _debug_ keyword conditionally compiles code (ie the debug code isn't compiled unless debugging is turned on). <lang zkl>fcn ds(line=__LINE__){
println("This is line %d of file %s compiled on %s"
.fmt(line,__FILE__,__DATE__));
}(); _debug_{
ds(__LINE__); println("Debug level is ",__DEBUG__);
vm.stackTrace().println();
}</lang>
- Output:
Run with debugging turned off: $ zkl rs This is line 39 of file rs.zkl compiled on 2019-08-28 Run with debugging turned on: Due to some brain deadness, we need to set the debug level (-d), compile the file (-c) then run it and quit out of the REPL: $ zkl -dc rs --run --quit Compiled Class(rs) (0.0 seconds, ??? lines/sec) This is line 44 of file rs.zkl compiled on 2019-08-28 This is line 49 of file rs.zkl compiled on 2019-08-28 Debug level is 1 Stack trace for VM#1 (): rs.__constructor@stackTrace addr:25 args(0) reg(0) startup.__constructor@__constructor addr:1767 args(0) reg(22) R