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printf can not be a macro defined in terms of fprintf ?

Posted on 2014-03-10
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Please explain following sentence from 'C' language book:

"Note, though, that unlike getchar and fgetc, printf can not be a macro defined in terms of fprintf, because macros can not have a variable number of arguments."

Please provide example of how there will be variable number of arguments if printf is a macro defined in terms of fprintf ?
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Question by:naseeam
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Expert Comment

by:SStory
ID: 39918397
printf can have variable number of args.

printf("Good morning, %s, did you know that %d + %d = %d?",name,5,5,10);
printf("Good morning, %s",name);

Here printf has 4 args (5 including the string) in the first, and 1 or 2 in the second. It is saying you can't do this in the macro because it won't allow variable args..I think.
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by:naseeam
ID: 39918478
Please provide example of printf being a macro defined in terms of fprintf.  Then, explain why it can't be done.
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Accepted Solution

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jkr earned 1300 total points
ID: 39919055
Well, you can create 'getchar()' as a macro from 'fgetc()' like

#define getchar fgetc(stdin)

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The same or something similar is isn't possible because 'printf()' takes a variable number of 0 up to n arguments, which cannot be taken care of with a macro, since the latter always takes a defined number of arguments (with the notable exception of gcc, but that's a non-standard compiler extendion).
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Assisted Solution

by:SStory
SStory earned 700 total points
ID: 39919199
#define dontwork fprintf(stdin,txt)

It won't work because fprintf can take tons of args based on the format string.  For every %d or %s etc in the string there must be a ,arg1,arg2...,argn

I don not know how much better anyone can explain this.  The above macro expects a single text string with no variables.  How many would you make?  You can not know.  Normally functions like that have an argv array of items or something and maybe the count argc...
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Author Closing Comment

by:naseeam
ID: 39919273
Great explanation with great examples.
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