How to convert a variable argument function into a macro?

Question

I have a variable argument function which prints error messages in my application, whose code is given below:

void error(char *format,...)
{   va_list args;
    printf("Error: ");
    va_start(args, format);
    vfprintf(stderr, format, args);
    va_end(args);
    printf("\n");
    abort(); 
}

This function is used in error conditions as follows:

error("invalid image width %d and image height %d in GIF file %s",wid,hei,name);

The error() function is called from different places with different arguments (variable argument function).

The function approach works fine.

Now, if I have to convert this function into a macro, how do I do it? I tried doing it as:

#define error(format)  {va_list args;\
    printf("Error: ");\
    va_start(args, format);\
    vfprintf(stderr, format, args);\
    va_end(args);\
    printf("\n"); abort()}

But this does not print the arguments correctly.

What is wrong in the macro definition above?

What is the fix?

score 2 · Accepted Answer · edited Feb 01 '17 at 22:54

2

If your compiler supports ISO style variadic macros, you can define a macro as such:

#define error(...) \    
    fprintf(stderr, "Error: ");   \
    fprintf(stderr, __VA_ARGS__); \
    fprintf(stderr, "\n"); \
    abort();

Alternatively, if you are using GCC, there's also the GNU style variadic macro used as such:

#define error(format, args...) \    
    fprintf(stderr, "Error: ");   \
    fprintf(stderr, format , ## args); \
    fprintf(stderr, "\n"); \
    abort();

For more information, see http://gcc.gnu.org/onlinedocs/cpp/Variadic-Macros.html

Update

If your compiler does not have variadic macro support, a (poor?) alternative would be to stick to the va_list in function approach. If you wish the definition to reside in a header file, then perhaps a static inline function?

static inline void error(const char *fmt, ...) {
#define PRINT_ERROR
    va_list args;
    fprintf(stderr, "Error: "); 
    va_start(args, fmt);
    vfprintf(stderr, fmt, args);
    va_end(args);
    fprintf(stderr, "\n");
    abort();
#endif
}

edited Feb 01 '17 at 22:54

Nisse Engström

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answered Aug 25 '09 at 12:14

Shawn Chin

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@shawnchin: Thanks. I tried it in MSVC6.0, MSVS-2005 compilers, it doesnot seem to support variable argument macros. I guess GCC supports, but do u know of any windows based compiler supporting this? – goldenmean Aug 25 '09 at 12:32
@goldenmean: This site says that ISO style Variadic macros was introduced in MSVS-2005 Visual C++ (http://msdn.microsoft.com/en-us/library/ms177415(VS.80).aspx) – Shawn Chin Aug 25 '09 at 12:47
@shawnchin: Are there any peculiar MSVisual Studio project options which one needs to set to enable the compiler to compile this variadic macros. I tried setting few options(e.g. Compile code as C++) Is there any macro to force the compiler to accept these variadic macro extensions(Are they C99 extensions)? – goldenmean Aug 25 '09 at 13:19
While i was going through other SO posts, i can across this link: http://stackoverflow.com/questions/139479/how-universally-is-c99-supported/139506#139506/ which says MSVS2005 might not have support for C99 features(Varuiadic macros being one of C99 feature). – goldenmean Aug 25 '09 at 13:31
I'm afraid I don't have an MSVS installation to test it out. However, google search results and this SO post (http://stackoverflow.com/questions/65037/is-there-a-way-to-write-macros-with-a-variable-argument-list-in-visual-c/65103#65103) claim that variadic macros are indeed supported from VC++ 2005 onwards. – Shawn Chin Aug 25 '09 at 13:40

score 1 · Answer 2 · answered Aug 25 '09 at 12:11

1

Here is an article with some examples on variable arguments used in a macro. It looks like it should do what you're looking for. You can use __VA_ARGS__ in your macro.

Which compiler are you using?

answered Aug 25 '09 at 12:11

Kieveli

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It looks like it would work all right. but WHY replace a perfectly good function call with a macro? It's not like in-lining the code is going to make any difference to performance or anything in this case. It's just taking some decent code and turning it into crap code – Glen Aug 25 '09 at 12:24

score 0 · Answer 3 · answered Aug 25 '09 at 12:09

0

Macros do not (yet?) support variadic arguments and anyway, using va_list wouldn't work here, it only works on variadic function arguments.

Why do you want to replace the function (which works fine, according to you) with a macro anyway?

answered Aug 25 '09 at 12:09

Konrad Rudolph

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laalto · Answer 4 · 2009-08-25T12:23:45.877

0

Many compilers support GNU style variadic macros, like this:

#define error(format,args...) do { \
  fprintf(stderr, "error: " format "\n", ##args); \
  abort(); \
} while(0)

However, if you aim for portability, do not use variadic macros.

edited Aug 25 '09 at 12:23

answered Aug 25 '09 at 12:13

laalto

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@Laalto: Thanks. I tried it in MSVC6.0, MSVS-2005 compilers, it doesnot seem to support variable argument macros. I guess GCC supports, but do u know of any windows based compiler supporting this? – goldenmean Aug 25 '09 at 12:32
I've used this construct on Metrowerks CodeWarrior and various flavors of GCC on Windows. – laalto Aug 25 '09 at 12:56

score 0 · Answer 5 · answered Aug 25 '09 at 12:15

There's a common extension that does what you want, simply write:

#define error(args...) (fputs("Error: ",stdout),printf(args),puts(""))

C99 users can also say:

#define error(...) (fputs("Error: ",stdout),printf(__VA_ARGS__),puts(""))

but there are some problems with using __VA_ARGS__. Fortunately there's a GCC extension to deal with it, but then you're back to using a compiler-specific extension, and the args... mode is more widely available.

How to convert a variable argument function into a macro?

5 Answers5