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](http://stackoverflow.com/help/mcve). — dbush, Jun 07 '17 at 18:54
@PrabhatSharma add the code snippet *verbatim in the question* please, thanks! — , Jun 07 '17 at 18:59
I've added the link and marked the use of min through comments! — Prabhat Sharma, Jun 07 '17 at 18:59
@PrabhatSharma voting to close here: code must be included in the question, not in an external link. — , Jun 07 '17 at 19:10
Sorry, but I've done both. Since the actual code was quite long I didn't include the whole! — Prabhat Sharma, Jun 07 '17 at 19:16
`typeof (b) _b = (b); \ //Min defined` --> typeof (b) _b = (b); //Min defined\ — BLUEPIXY, Jun 07 '17 at 19:38
You should check if your compiler supports typeof. If not you have to think about an alternative. — Andre Kampling, Jun 07 '17 at 20:10
Note: `typeof` is a [compiler extension](https://stackoverflow.com/q/12081502/2410359) not in C11, C99,... — chux - Reinstate Monica, Jun 07 '17 at 21:46

Andre Kampling · Accepted Answer · 2017-06-15T12:44:24.580

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 '17 at 12:44

answered Jun 07 '17 at 19:02

Andre Kampling

5,476
2
20
47

What's the difference between both? I've used this documentation code of typeof [gcc doc](https://gcc.gnu.org/onlinedocs/gcc/Typeof.html) – Prabhat Sharma Jun 07 '17 at 19:05
Moreover it didn't solved the problem! The errors are almost same except instead of typeof there is _typeof_ – Prabhat Sharma Jun 07 '17 at 19:08
Means it's all compiler specific and we should not use it. Right? But how then we define min or max function in C? – Prabhat Sharma Jun 07 '17 at 19:12
You could implement it without of the use of **typeof**: `#define min(a,b) (((a) < (b)) ? (a) : (b))`. – Andre Kampling Jun 07 '17 at 19:15
1

But this will cause problem in some cases like if we write (++a, ++b) etc. See this https://stackoverflow.com/questions/3437404/min-and-max-in-c But I admit this will not cause any problem in my case! – Prabhat Sharma Jun 07 '17 at 19:21
You could use an inline function as I edited my answer. – Andre Kampling Jun 07 '17 at 19:47
Thanks! @Andre Kampling – Prabhat Sharma Jun 07 '17 at 19:57

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

1 Answers1