variadic macros and and empty replacement for __VA_ARGS__

[Urs Thuermann]

I want to write a macro that produces debug output and has a variable
number of arguments, so that I can use like this:

int i;
char *s;

DBG("simple message\n");
DBG("message with an int argument, value is %d\n", i);
DBG("two arguments: int %d, strings %s\n", i, s);

The output should be a fixed prefix, say "DEBUG ", followed by the
function name the macros is used in, and then by the string passed to
the macro, including further arguments.

The best solution I have found so far, is

#define DBG(fmt, ...) printf("DEBUG %s:" fmt, __func__, __VA_ARGS__)

This works as intended as long as there is at least one argument
following the format strings, but it expands to
printf("DEBUG %s:" fmt, __func__,) when there is no argument and that
causes a syntax error.

The draft of C99 I have here, disallows to give no arguments for the
ellipsis ... as stated in 6.10.3:

[#4] If the identifier-list in the macro definition does not end
with an ellipsis, the number of arguments, including those
arguments consisting of no preprocessing tokens, in an invocation
of a function-like macro shall agree with the number of parameters
in the macro definition. Otherwise, there shall be more arguments
in the invocation than there are parameters in the macro
definition (excluding the ...). There shall exist a )
preprocessing token that terminates the invocation.
Is there a better way to write the DBG() macro to avoid this?

urs

[John Devereux]

Urs Thuermann <ur*@isnogud.es cape.de> writes:

I want to write a macro that produces debug output and has a variable
number of arguments, so that I can use like this:

int i;
char *s;

DBG("simple message\n");
DBG("message with an int argument, value is %d\n", i);
DBG("two arguments: int %d, strings %s\n", i, s);

The output should be a fixed prefix, say "DEBUG ", followed by the
function name the macros is used in, and then by the string passed to
the macro, including further arguments.

The best solution I have found so far, is

#define DBG(fmt, ...) printf("DEBUG %s:" fmt, __func__, __VA_ARGS__)
<SNIP> Is there a better way to write the DBG() macro to avoid this?

There was a related thread a couple of weeks ago:

<http://groups.google.c o.uk/group/comp.lang.c/browse_frm/thread/ac0bda529a91d75 4/949239c624c9fe7 1?lnk=st&q=logg ing+group%3Acom p.lang.c&rnum=3 &hl=en#949239c6 24c9fe71>

This basically used a *function* instead of a macro. The logging
function accepts variadic arguments, and these can then be modified
and passed on to a varargs form of printf (e.g. vfprintf).

--

John Devereux

[Richard G. Riley]

On 2006-03-27, John Devereux <jd******@THISd evereux.me.uk> wrote:

Urs Thuermann <ur*@isnogud.es cape.de> writes:

I want to write a macro that produces debug output and has a variable
number of arguments, so that I can use like this:

int i;
char *s;

DBG("simple message\n");
DBG("message with an int argument, value is %d\n", i);
DBG("two arguments: int %d, strings %s\n", i, s);

The output should be a fixed prefix, say "DEBUG ", followed by the
function name the macros is used in, and then by the string passed to
the macro, including further arguments.

The best solution I have found so far, is

#define DBG(fmt, ...) printf("DEBUG %s:" fmt, __func__, __VA_ARGS__)

<SNIP>

Is there a better way to write the DBG() macro to avoid this?

There was a related thread a couple of weeks ago:

<http://groups.google.c o.uk/group/comp.lang.c/browse_frm/thread/ac0bda529a91d75 4/949239c624c9fe7 1?lnk=st&q=logg ing+group%3Acom p.lang.c&rnum=3 &hl=en#949239c6 24c9fe71>

This basically used a *function* instead of a macro. The logging
function accepts variadic arguments, and these can then be modified
and passed on to a varargs form of printf (e.g. vfprintf).

One little thing you might consider at an early stage : have an
importance or category enum as the first/second parameter : very
useful for filtering/enabling/disabling runtime logging for different
subsets in a non trivial system. You might even use the function name
and have settings to enable/disable logging for the particular function.

[raxip]

> This works as intended as long as there is at least one argument

following the format strings, but it expands to
printf("DEBUG %s:" fmt, __func__,) when there is no argument and that
causes a syntax error.

#define macroFunc(a,b,. ..) ( Func(a, b, ##__VA_ARGS__) )

[pemo]

John Devereux wrote:

Urs Thuermann <ur*@isnogud.es cape.de> writes:

I want to write a macro that produces debug output and has a variable
number of arguments, so that I can use like this:

int i;
char *s;

DBG("simple message\n");
DBG("message with an int argument, value is %d\n", i);
DBG("two arguments: int %d, strings %s\n", i, s);

The output should be a fixed prefix, say "DEBUG ", followed by the
function name the macros is used in, and then by the string passed to
the macro, including further arguments.

The best solution I have found so far, is

#define DBG(fmt, ...) printf("DEBUG %s:" fmt, __func__, __VA_ARGS__)

<SNIP>

Is there a better way to write the DBG() macro to avoid this?

There was a related thread a couple of weeks ago:

<http://groups.google.c o.uk/group/comp.lang.c/browse_frm/thread/ac0bda529a91d75 4/949239c624c9fe7 1?lnk=st&q=logg ing+group%3Acom p.lang.c&rnum=3 &hl=en#949239c6 24c9fe71>

This basically used a *function* instead of a macro. The logging
function accepts variadic arguments, and these can then be modified
and passed on to a varargs form of printf (e.g. vfprintf).

See this one too:

http://groups.google.co.uk/group/com...d17634c1ed7065

--
==============
Not a pedant
==============

[Michael Mair]

raxip schrieb:

This works as intended as long as there is at least one argument
following the format strings, but it expands to
printf("DEB UG %s:" fmt, __func__,) when there is no argument and that
causes a syntax error.

#define macroFunc(a,b,. ..) ( Func(a, b, ##__VA_ARGS__) )

This is "gcc-C".
It is not valid C99, though.

-Michael
--
E-Mail: Mine is an /at/ gmx /dot/ de address.

[raxip]

Assumptions are wonderous creatures.

[Default User]

raxip wrote:

Assumptions are wonderous creatures.

That's nice. What does it have to do with anything? See my .sig below
for valuable information.

Brian

--
Please quote enough of the previous message for context. To do so from
Google, click "show options" and use the Reply shown in the expanded
header.

[raxip]

> That's nice. What does it have to do with anything? See my .sig below

for valuable information.

Is this better?

[Default User]

raxip wrote:

That's nice. What does it have to do with anything? See my .sig
below for valuable information.

Is this better?

Somewhat. Where are the attributions (the who said what part)? I know
that Google puts them in if you use the correct Reply.

Brian