#define macro for debug printing in C?

I use something like this:

#ifdef DEBUG
 #define D if(1) 
#else
 #define D if(0) 
#endif

Than I just use D as a prefix:

D printf("x=%0.3f\n",x);

Compiler sees the debug code, there is no comma problem and it works everywhere. Also it works when printf is not enough, say when you must dump an array or calculate some diagnosing value that is redundant to the program itself.

EDIT: Ok, it might generate a problem when there is else somewhere near that can be intercepted by this injected if. This is a version that goes over it:

#ifdef DEBUG
 #define D 
#else
 #define D for(;0;)
#endif

Here's the version I use:

#ifdef NDEBUG
#define Dprintf(FORMAT, ...) ((void)0)
#define Dputs(MSG) ((void)0)
#else
#define Dprintf(FORMAT, ...) \
    fprintf(stderr, "%s() in %s, line %i: " FORMAT "\n", \
        __func__, __FILE__, __LINE__, __VA_ARGS__)
#define Dputs(MSG) Dprintf("%s", MSG)
#endif

For a portable (ISO C90) implementation, you could use double parentheses, like this;

#include <stdio.h>
#include <stdarg.h>

#ifndef NDEBUG
#  define debug_print(msg) stderr_printf msg
#else
#  define debug_print(msg) (void)0
#endif

void
stderr_printf(const char *fmt, ...)
{
  va_list ap;
  va_start(ap, fmt);
  vfprintf(stderr, fmt, ap);
  va_end(ap);
}

int
main(int argc, char *argv[])
{
  debug_print(("argv[0] is %s, argc is %d\n", argv[0], argc));
  return 0;
}

or (hackish, wouldn't recommend it)

#include <stdio.h>

#define _ ,
#ifndef NDEBUG
#  define debug_print(msg) fprintf(stderr, msg)
#else
#  define debug_print(msg) (void)0
#endif

int
main(int argc, char *argv[])
{
  debug_print("argv[0] is %s, argc is %d"_ argv[0] _ argc);
  return 0;
}

I would do something like

#ifdef DEBUG
#define debug_print(fmt, ...) fprintf(stderr, fmt, __VA_ARGS__)
#else
#define debug_print(fmt, ...) do {} while (0)
#endif

I think this is cleaner.

According to http://gcc.gnu.org/onlinedocs/cpp/Variadic-Macros.html, there should be a ## before __VA_ARGS__.

Otherwise, a macro #define dbg_print(format, ...) printf(format, __VA_ARGS__) will not compile the following example: dbg_print("hello world");.

#define debug_print(FMT, ARGS...) do { \
    if (DEBUG) \
        fprintf(stderr, "%s:%d " FMT "\n", __FUNCTION__, __LINE__, ## ARGS); \
    } while (0)

This is what I use:

#if DBG
#include <stdio.h>
#define DBGPRINT printf
#else
#define DBGPRINT(...) /**/  
#endif

It has the nice benefit to handle printf properly, even without additional arguments. In case DBG ==0, even the dumbest compiler gets nothing to chew upon, so no code is generated.

So, when using gcc, I like:

#define DBGI(expr) ({int g2rE3=expr; fprintf(stderr, "%s:%d:%s(): ""%s->%i\n", __FILE__,  __LINE__, __func__, #expr, g2rE3); g2rE3;})

Because it can be inserted into code.

Suppose you're trying to debug

printf("%i\n", (1*2*3*4*5*6));

720

Then you can change it to:

printf("%i\n", DBGI(1*2*3*4*5*6));

hello.c:86:main(): 1*2*3*4*5*6->720
720

And you can get an analysis of what expression was evaluated to what.

It's protected against the double-evaluation problem, but the absence of gensyms does leave it open to name-collisions.

However it does nest:

DBGI(printf("%i\n", DBGI(1*2*3*4*5*6)));

hello.c:86:main(): 1*2*3*4*5*6->720
720
hello.c:86:main(): printf("%i\n", DBGI(1*2*3*4*5*6))->4

So I think that as long as you avoid using g2rE3 as a variable name, you'll be OK.

Certainly I've found it (and allied versions for strings, and versions for debug levels etc) invaluable.

I've been stewing on how to do this for years, and finally come up with a solution. However, I didn't know that there were other solutions here already. First, at difference with Leffler's answer, I don't see his argument that debug prints should always be compiled. I'd rather not have tons of unneeded code executing in my project, when not needed, in cases where I need to test and they might not be getting optimized out.

Not compiling every time might sound worse than it is in actual practice. You do wind up with debug prints that don't compile sometimes, but it's not so hard to compile and test them before finalizing a project. With this system, if you are using three levels of debugs, just put it on debug message level three, fix your compile errors and check for any others before you finalize yer code. (Since of course, debug statements compiling are no guarantee that they are still working as intended.)

My solution provides for levels of debug detail also; and if you set it to the highest level, they all compile. If you've been using a high debug detail level recently, they all were able to compile at that time. Final updates should be pretty easy. I've never needed more than three levels, but Jonathan says he's used nine. This method (like Leffler's) can be extended to any number of levels. The usage of my method may be simpler; requiring just two statements when used in your code. I am, however, coding the CLOSE macro too - although it doesn't do anything. It might if I were sending to a file.

Against the cost the extra step of testing them to see that they will compile before delivery, is that

1. You must trust them to get optimized out, which admittedly SHOULD happen if you have a sufficient optimization level.
2. Furthermore, they probably won't if you make a release compile with optimization turned off for testing purposes (which is admittedly rare); and they almost certainly won't at all during debug - thereby executing dozens or hundreds of "if (DEBUG)" statements at runtime; thus slowing execution (which is my principle objection) and less importantly, increasing your executable or dll size; and hence execution and compile times. Jonathan, however, informs me his method can be made to also not compile statements at all.

Branches are actually relatively pretty costly in modern pre-fetching processors. Maybe not a big deal if your app is not a time-critical one; but if performance is an issue, then, yes, a big enough deal that I'd prefer to opt for somewhat faster-executing debug code (and possibly faster release, in rare cases, as noted).

So, what I wanted is a debug print macro that does not compile if it is not to be printed, but does if it is. I also wanted levels of debugging, so that, e.g. if I wanted performance-crucial parts of the code not to print at some times, but to print at others, I could set a debug level, and have extra debug prints kick in. I came across a way to implement debug levels that determined if the print was even compiled or not. I achieved it this way:

DebugLog.h:

// FILE: DebugLog.h
// REMARKS: This is a generic pair of files useful for debugging.  It provides three levels of 
// debug logging, currently; in addition to disabling it.  Level 3 is the most information.
// Levels 2 and 1 have progressively more.  Thus, you can write: 
//     DEBUGLOG_LOG(1, "a number=%d", 7);
// and it will be seen if DEBUG is anything other than undefined or zero.  If you write
//     DEBUGLOG_LOG(3, "another number=%d", 15);
// it will only be seen if DEBUG is 3.  When not being displayed, these routines compile
// to NOTHING.  I reject the argument that debug code needs to always be compiled so as to 
// keep it current.  I would rather have a leaner and faster app, and just not be lazy, and 
// maintain debugs as needed.  I don't know if this works with the C preprocessor or not, 
// but the rest of the code is fully C compliant also if it is.

#define DEBUG 1

#ifdef DEBUG
#define DEBUGLOG_INIT(filename) debuglog_init(filename)
#else
#define debuglog_init(...)
#endif

#ifdef DEBUG
#define DEBUGLOG_CLOSE debuglog_close
#else
#define debuglog_close(...)
#endif

#define DEBUGLOG_LOG(level, fmt, ...) DEBUGLOG_LOG ## level (fmt, ##__VA_ARGS__)

#if DEBUG == 0
#define DEBUGLOG_LOG0(...)
#endif

#if DEBUG >= 1
#define DEBUGLOG_LOG1(fmt, ...) debuglog_log (fmt, ##__VA_ARGS__)
#else
#define DEBUGLOG_LOG1(...)
#endif

#if DEBUG >= 2
#define DEBUGLOG_LOG2(fmt, ...) debuglog_log (fmt, ##__VA_ARGS__)
#else
#define DEBUGLOG_LOG2(...)
#endif

#if DEBUG == 3
#define DEBUGLOG_LOG3(fmt, ...) debuglog_log (fmt, ##__VA_ARGS__)
#else
#define DEBUGLOG_LOG3(...)
#endif

void debuglog_init(char *filename);
void debuglog_close(void);
void debuglog_log(char* format, ...);

DebugLog.cpp:

// FILE: DebugLog.h
// REMARKS: This is a generic pair of files useful for debugging.  It provides three levels of 
// debug logging, currently; in addition to disabling it.  See DebugLog.h's remarks for more 
// info.

#include <stdio.h>
#include <stdarg.h>

#include "DebugLog.h"

FILE *hndl;
char *savedFilename;

void debuglog_init(char *filename)
{
    savedFilename = filename;
    hndl = fopen(savedFilename, "wt");
    fclose(hndl);
}

void debuglog_close(void)
{
    //fclose(hndl);
}

void debuglog_log(char* format, ...)
{
    hndl = fopen(savedFilename,"at");
    va_list argptr;
    va_start(argptr, format);
    vfprintf(hndl, format, argptr);
    va_end(argptr);
    fputc('\n',hndl);
    fclose(hndl);
}

Using the macros

To use it, just do:

DEBUGLOG_INIT("afile.log");

To write to the log file, just do:

DEBUGLOG_LOG(1, "the value is: %d", anint);

To close it, you do:

DEBUGLOG_CLOSE();

although currently this isn't even necessary, technically speaking, as it does nothing. I'm still using the CLOSE right now, however, in case I change my mind about how it works, and want to leave the file open between logging statements.

Then, when you want to turn on debug printing, just edit the first #define in the header file to say, e.g.

#define DEBUG 1

To have logging statements compile to nothing, do

#define DEBUG 0

If you need info from a frequently executed piece of code (i.e. a high level of detail), you may want to write:

 DEBUGLOG_LOG(3, "the value is: %d", anint);

If you define DEBUG to be 3, logging levels 1, 2 & 3 compile. If you set it to 2, you get logging levels 1 & 2. If you set it to 1, you only get logging level 1 statements.

As to the do-while loop, since this evaluates to either a single function or nothing, instead of an if statement, the loop is not needed. OK, castigate me for using C instead of C++ IO (and Qt's QString::arg() is a safer way of formatting variables when in Qt, too — it's pretty slick, but takes more code and the formatting documentation isn't as organized as it might be - but still I've found cases where its preferable), but you can put whatever code in the .cpp file you want. It also might be a class, but then you would need to instantiate it and keep up with it, or do a new() and store it. This way, you just drop the #include, init and optionally close statements into your source, and you are ready to begin using it. It would make a fine class, however, if you are so inclined.

I'd previously seen a lot of solutions, but none suited my criteria as well as this one.

1. It can be extended to do as many levels as you like.
2. It compiles to nothing if not printing.
3. It centralizes IO in one easy-to-edit place.
4. It's flexible, using printf formatting.
5. Again, it does not slow down debug runs, whereas always-compiling debug prints are always executed in debug mode. If you are doing computer science, and not easier to write information processing, you may find yourself running a CPU-consuming simulator, to see e.g. where the debugger stops it with an index out of range for a vector. These run extra-slowly in debug mode already. The mandatory execution of hundreds of debug prints will necessarily slow such runs down even further. For me, such runs are not uncommon.

Not terribly significant, but in addition:

6. It requires no hack to print without arguments (e.g. DEBUGLOG_LOG(3, "got here!");); thus allowing you to use, e.g. Qt's safer .arg() formatting. It works on MSVC, and thus, probably gcc. It uses ## in the #defines, which is non-standard, as Leffler points out, but is widely supported. (You can recode it not to use ## if necessary, but you will have to use a hack such as he provides.)

Warning: If you forget to provide the logging level argument, MSVC unhelpfully claims the identifier is not defined.

You might want to use a preprocessor symbol name other than DEBUG, as some source also defines that symbol (eg. progs using ./configure commands to prepare for building). It seemed natural to me when I developed it. I developed it in an application where the DLL is being used by something else, and it's more convent to send log prints to a file; but changing it to vprintf() would work fine, too.

I hope this saves many of you grief about figuring out the best way to do debug logging; or shows you one you might prefer. I've half-heartedly been trying to figure this one out for decades. Works in MSVC 2012 & 2015, and thus probably on gcc; as well as probably working on many others, but I haven't tested it on them.

I mean to make a streaming version of this one day, too.

Note: Thanks go to Leffler, who has cordially helped me format my message better for StackOverflow.

My favourite of the below is var_dump, which when called as:

var_dump("%d", count);

produces output like:

patch.c:150:main(): count = 0

Credit to @"Jonathan Leffler". All are C89-happy:

Code

#define DEBUG 1
#include <stdarg.h>
#include <stdio.h>
void debug_vprintf(const char *fmt, ...)
{
    va_list args;
    va_start(args, fmt);
    vfprintf(stderr, fmt, args);
    va_end(args);
}

/* Call as: (DOUBLE PARENTHESES ARE MANDATORY) */
/* var_debug(("outfd = %d, somefailed = %d\n", outfd, somefailed)); */
#define var_debug(x) do { if (DEBUG) { debug_vprintf ("%s:%d:%s(): ", \
    __FILE__,  __LINE__, __func__); debug_vprintf x; }} while (0)

/* var_dump("%s" variable_name); */
#define var_dump(fmt, var) do { if (DEBUG) { debug_vprintf ("%s:%d:%s(): ", \
    __FILE__,  __LINE__, __func__); debug_vprintf ("%s = " fmt, #var, var); }} while (0)

#define DEBUG_HERE do { if (DEBUG) { debug_vprintf ("%s:%d:%s(): HERE\n", \
    __FILE__,  __LINE__, __func__); }} while (0)

#define PRINT_LOG(str_format, ...) { \
    time_t curtime=time (NULL); \
    struct tm *ltm = localtime (&curtime); \
    printf("[%d-%02d-%02d %02d:%02d:%02d] " str_format, \
        ltm->tm_year + 1900, ltm->tm_mon + 1, ltm->tm_mday, \
        ltm->tm_hour, ltm->tm_min, ltm->tm_sec, ##__VA_ARGS__); \
}
    
PRINT_LOG("[%d] Serving client, str=%s, number=%d\n", getpid(), "my str", 10);

I believe this variation of the theme gives debug categories without the need to have a separate macro name per category.

I used this variation in an Arduino project where program space is limited to 32K and dynamic memory is limited to 2K. The addition of debug statements and trace debug strings quickly uses up space. So it is essential to be able to limit the debug trace that is included at compile time to the minimum necessary each time the code is built.

debug.h

#ifndef DEBUG_H
#define DEBUG_H

#define PRINT(DEBUG_CATEGORY, VALUE)  do { if (DEBUG_CATEGORY & DEBUG_MASK) Serial.print(VALUE);} while (0);

#endif

calling .cpp file

#define DEBUG_MASK 0x06
#include "Debug.h"

...
PRINT(4, "Time out error,\t");
...

If you don't care that the output goes to stdout, you can use this:

int doDebug = DEBUG;  // Where DEBUG may be supplied in compiler command
#define trace if (doDebug) printf

trace("whatever %d, %i\n", arg1, arg2);

Generic debugging macros to conditionally compile and run any code in C or C++ based on debug levels

Here's one way that uses the preprocessor and works with any code in C or C++:

debug.h:

#pragma once

// 1. Your debug levels to choose from:

#define DEBUG_LEVEL_OFF     100 // Print/run nothing. 

#define DEBUG_LEVEL_ERROR   50  // Print/run `DEBUG_ERROR()` statements & higher.
#define DEBUG_LEVEL_WARNING 40  // Print/run `DEBUG_WARNING()` statements & higher.
#define DEBUG_LEVEL_LOG     30  // Print/run `DEBUG_LOG()` statements & higher.
#define DEBUG_LEVEL_DEBUG   20  // Print/run `DEBUG_DEBUG()` statements & higher.
#define DEBUG_LEVEL_INFO    10  // Print/run `DEBUG_INFO()` statements & higher.

#define DEBUG_LEVEL_ALL     0   // Print/run everything. 


// 2. Your debug macros to call: 

// Call `code` if `macroDebugLevel` >= `yourDebugLevel`.
//
// Example 1: to conditionally run a **single line of code**:
// ```c
// #define MY_DEBUG_LEVEL  DEBUG_LEVEL_INFO
// DEBUG_INFO(MY_DEBUG_LEVEL, printf("vals: %i; %f\n", my_int, my_float));
// ```
//
// Example 2: to conditionally run **multiple lines of code**:
// ```c
// #define MY_DEBUG_LEVEL  DEBUG_LEVEL_INFO
// DEBUG_INFO(MY_DEBUG_LEVEL,
//     int my_int = 7;
//     float my_float = 3.14;
//     printf("vals: %i; %f\n", my_int, my_float);
//     my_int = 8;
//     my_float = 2.718;
//     printf("vals: %i; %f\n", my_int, my_float);
// );
// ```
#define DEBUG_ERROR(yourDebugLevel, code) \
    _DEBUG_RUN(DEBUG_LEVEL_ERROR, (yourDebugLevel), code)
#define DEBUG_WARNING(yourDebugLevel, code) \
    _DEBUG_RUN(DEBUG_LEVEL_WARNING, (yourDebugLevel), code)
#define DEBUG_LOG(yourDebugLevel, code) \
    _DEBUG_RUN(DEBUG_LEVEL_LOG, (yourDebugLevel), code)
#define DEBUG_DEBUG(yourDebugLevel, code) \
    _DEBUG_RUN(DEBUG_LEVEL_DEBUG, (yourDebugLevel), code)
#define DEBUG_INFO(yourDebugLevel, code) \
    _DEBUG_RUN(DEBUG_LEVEL_INFO, (yourDebugLevel), code)

// Internal macro to run `code` if `macroDebugLevel` >= `yourDebugLevel`.
// - Since the `if` statement is determined by the compiler at compile time, 
//   the entire `code` block is optimized out for all compiler optimization 
//   levels other than `-O0` (no optimization). This removes unnecessary 
//   code and runtime processing from the final binary.
#define _DEBUG_RUN(macroDebugLevel, yourDebugLevel, code) \
    do \
    { \
        if ((macroDebugLevel) >= (yourDebugLevel)) \
        { \
            code; \
        } \
    } while (0)

Usage:

#include "debug.h"

// With this define, the debug code below WILL run since I call `DEBUG_INFO()`,
// whose `macroDebugLevel` (`DEBUG_LEVEL_INFO`) **is** >= `MY_DEBUG_LEVEL` 
// (also `DEBUG_LEVEL_INFO`).
#define MY_DEBUG_LEVEL  DEBUG_LEVEL_INFO

// With this define, the debug code below will NOT run since I call 
// `DEBUG_INFO()` but the level is set higher, to `DEBUG_LEVEL_WARNING`.
// #define MY_DEBUG_LEVEL  DEBUG_LEVEL_WARNING

// Example 1: single line of code to print
// - Put this inside some function where you want to write some debug info
//   if `MY_DEBUG_LEVEL` is low enough, or otherwise stated: the `DEBUG_*()` 
//   macro is high enough.
DEBUG_INFO(MY_DEBUG_LEVEL, printf("vals: %i; %f\n", my_int, my_float));

// Example 2: multiple lines of code
// - Put this inside some function where you want to run some debug code
//   if `MY_DEBUG_LEVEL` is low enough, or otherwise stated: the `DEBUG_*()`
//   macro is high enough.
DEBUG_INFO(MY_DEBUG_LEVEL,
    int my_int = 7;
    float my_float = 3.14;
    printf("vals: %i; %f\n", my_int, my_float);
    my_int = 8;
    my_float = 2.718;
    printf("vals: %i; %f\n", my_int, my_float);
);

In the above example, the DEBUG_INFO macro will only run the passed-in code statement or statements if MY_DEBUG_LEVEL is less than or equal to DEBUG_LEVEL_INFO, or, otherwise stated, the code will run if the macro debug level is >= your debug level. To force these debug lines to run, lower MY_DEBUG_LEVEL to DEBUG_LEVEL_INFO or lower, or increase the DEBUG_INFO() macro to a sufficiently high level.

Example: if MY_DEBUG_LEVEL is set to DEBUG_LEVEL_WARNING, then the debug code will only run if you use the DEBUG_WARNING() or DEBUG_ERROR() macros to call your code to run.

The nice thing about the above technique is that it does not rely on any particular debug print mechanism or function, such as printf(). Rather, the debug code above can be universally applied to any code that you want to run conditionally based on a debug level.

Additionally, if you have compile-time optimizations turned on, you'll end up with something like this after preprocessing:

do
{
    if (2 <= 1)
    {
        printf("vals: %i; %f\n", my_int, my_float);
    }
} while (0);

...and if the if statement is false, such as if (2 <= 1) in this case, the compiler will see that it's always false, at compile time, and optimize it out so that you don't have any runtime overhead nor program memory storage for debug code that isn't supposed to run.

Note that the purpose of the do while loop is to ensure that the macro can be used in any context and is always counted as a single statement, even if you were to later add an else statement or something after your if statement.

_{Note: this answer was moved from here, where I've now deleted it.}

See also

1. My answer: Debug Print Macro in C? - my more specific printf() variadic macro approach