Implementing min function in C by using typeof compiler extension gives error

Question

When i attempted to implement min function in C as C lacks this function the compiler gives an error. I've used type of for implementing it just like this

MIN and MAX in C

My code

For quick reference:

>#define min(a,b) \
  ({ typeof (a) _a = (a); \
  typeof (b) _b = (b); \        //Min defined
  _a < _b ? _a : _b; })
  .
  .
  .
  c= min(c+floor(1.0/f), bi.biWidth) ;
  r= min(r+floor(1.0/f),abs(bi.biHeight)) ;

Error generated:

 >make resize
 clang -fsanitize=integer -fsanitize=undefined -ggdb3 -O0 -std=c11 -Wall       
 -Werror -Wextra -Wno-sign-compare -Wshadow    resize.c  -lcrypt -lcs50      
 -lm  -o resize

resize.c:191:18: error: implicit declaration of function 'typeof' is    
invalid in C99 [-Werror,-Wimplicit-function-declaration]
          c= min(c+floor(1.0/f), bi.biWidth) ;
             ^
resize.c:11:6: note: expanded from macro 'min'
({ typeof (a) _a = (a); \
 ^
 resize.c:191:18: error: expected ';' after expression
 resize.c:11:17: note: expanded from macro 'min'
 ({ typeof (a) _a = (a); \
            ^

 resize.c:191:18: error: use of undeclared identifier '_a'

 resize.c:11:17: note: expanded from macro 'min'
 ({ typeof (a) _a = (a); \
            ^

 resize.c:191:18: error: expected ';' after expression

 resize.c:12:18: note: expanded from macro 'min'
  typeof (b) _b = (b); \

                  ^

 resize.c:191:18: error: use of undeclared identifier '_b'

 resize.c:12:18: note: expanded from macro 'min'
  typeof (b) _b = (b); \
             ^

 resize.c:191:18: error: use of undeclared identifier '_a'

 resize.c:13:5: note: expanded from macro 'min'
 _a < _b ? _a : _b; })
 ^

 resize.c:191:18: error: use of undeclared identifier '_b'

 resize.c:13:10: note: expanded from macro 'min'
 _a < _b ? _a : _b; })
     ^

 resize.c:191:18: error: use of undeclared identifier '_b'

 resize.c:13:20: note: expanded from macro 'min'
 _a < _b ? _a : _b; })
               ^

 resize.c:193:13: error: expected ';' after expression
     r= min(r+floor(1.0/f),abs(bi.biHeight)) ;
        ^

 resize.c:11:17: note: expanded from macro 'min'
  ({ typeof (a) _a = (a); \
            ^

 resize.c:193:13: error: use of undeclared identifier '_a'

 resize.c:11:17: note: expanded from macro 'min'
 ({ typeof (a) _a = (a); \
            ^

 resize.c:193:13: error: expected ';' after expression

 resize.c:12:18: note: expanded from macro 'min'
  typeof (b) _b = (b); \
             ^

 resize.c:193:13: error: use of undeclared identifier '_b'

 resize.c:12:18: note: expanded from macro 'min'
  typeof (b) _b = (b); \
             ^

 resize.c:193:13: error: use of undeclared identifier '_a'

 resize.c:13:5: note: expanded from macro 'min'
 _a < _b ? _a : _b; })
 ^

 resize.c:193:13: error: use of undeclared identifier '_b'

 resize.c:13:10: note: expanded from macro 'min'
 _a < _b ? _a : _b; })
     ^

 resize.c:193:13: error: use of undeclared identifier '_b'

 resize.c:13:20: note: expanded from macro 'min'
 _a < _b ? _a : _b; })
               ^

 15 errors generated.

How do you expect us to say what's wrong with your code if you don't show us your code? Please update your question with a Minimal, Complete, Verifiable Example. — dbush
– dbush, Commented Jun 7, 2017 at 18:54
You should check if your compiler supports typeof. If not you have to think about an alternative. — Andre Kampling
– Andre Kampling, Commented Jun 7, 2017 at 20:10

Andre Kampling · Accepted Answer · 2017-06-15 12:44:24Z

1

It should be:

#define min(a,b) \
   ({ __typeof__ (a) _a = (a); \
      __typeof__ (b) _b = (b); // Min defined \
      _a < _b ? _a : _b; })

Your comment should not be after the \ but before it. Also you write typeof without the underscores. It is recommended to use them, the explanation is given here at SO.

typeof(), __typeof__() and __typeof() are compiler-specific extensions to the C language, because standard C does not include such an operator.

Macro without typeof:

If your compiler does not support the typeof "operator", you could define the min macro without typeof:

#define min(a,b) (((a) < (b)) ? (a) : (b))

But with that you will have side effects when using postfix or prefix in/decrement operators on variables, because a double evaluation will occur.

Inline function:

The other possibility to make it safe is an inline function. It will be typesafe and no double evaluation will happen:

inline int min(int a, int b)
{
   if (a < b)
   {
      return a;
   }
   return b;
}

You loose type genericity, but there is no other way. You could also think of using double since integers will be implicitly casted. But remember the loose of precision.

edited Jun 15, 2017 at 12:44

answered Jun 7, 2017 at 19:02

Andre Kampling

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7 Comments

Prabhat Sharma Over a year ago

What's the difference between both? I've used this documentation code of typeof gcc doc

Prabhat Sharma Over a year ago

Moreover it didn't solved the problem! The errors are almost same except instead of typeof there is typeof

Prabhat Sharma Over a year ago

Means it's all compiler specific and we should not use it. Right? But how then we define min or max function in C?

Andre Kampling Over a year ago

You could implement it without of the use of typeof: #define min(a,b) (((a) < (b)) ? (a) : (b)).

Prabhat Sharma Over a year ago

But this will cause problem in some cases like if we write (++a, ++b) etc. See this stackoverflow.com/questions/3437404/min-and-max-in-c But I admit this will not cause any problem in my case!

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