MIN and MAX in C

Question

Where are MIN and MAX defined in C, if at all? What is the best way to implement these, as generically and type safe as possible (compiler extensions/builtins for mainstream compilers preferred).

Answer 1

There's a std::min and std::max in C++, but AFAIK, there's no equivalent in the C standard library. You can define them yourself with macros like

#define MAX(x, y) (((x) > (y)) ? (x) : (y))
#define MIN(x, y) (((x) < (y)) ? (x) : (y))

But this causes problems if you write something like MAX(++a, ++b).

Answer 2

Here's some gcc doc about min/max: http://gcc.gnu.org/onlinedocs/gcc-3.4.6/gcc/Min-and-Max.html

Answer 3

I don't think that they are standardised macros. There are standardised functions for floating point already, fmax and fmin (and fmaxf for floats, and fmaxl for long doubles).

You can implement them as macros as long as you are aware of the issues of side-effects/double-evaluation.

#define MAX(a,b) ((a) > (b) ? a : b)
#define MIN(a,b) ((a) < (b) ? a : b)

In most cases, you can leave it to the compiler to determine what you're trying to do and optimise it as best it can. While this causes problems when used like MAX(i++, j++), I doubt there is ever much need in checking the maximum of incremented values in one go. Increment first, then check.

Answer 4

Where are MIN and MAX defined in C, if at all?

They aren't.

What is the best way to implement these, as generically and type safe as possible (compiler extensions/builtins for mainstream compilers preferred).

As functions. I wouldn't use macros like #define MIN(X,Y) ((X) < (Y) ? : (X) : (Y)) , especially if you plan to deploy your code. Either write your own or use something like http://www.gnu.org/s/libc/manual/html_node/Misc-FP-Arithmetic.html#Misc-FP-Arithmetic or fix the macro using http://gcc.gnu.org/onlinedocs/gcc-3.4.6/gcc/Typeof.html#Typeof (you get typesafety bonus too):

 #define max(a,b) \
   ({ __typeof__ (a) _a = (a); \
       __typeof__ (b) _b = (b); \
     _a > _b ? _a : _b; })

Everyone says "oh I know about double evaluation, it's no problem" and a few months down the road, you'll be debugging the silliest problems for hours on end.

Edit: note the use of __typeof__ instead of typeof:

If you are writing a header file that must work when included in ISO C programs, write __typeof__ instead of typeof.

Answer 5

It's also provided in the GNU libc (Linux) and FreeBSD versions of sys/param.h, and has the definition provided by dreamlax.

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On Debian:

$ uname -sr
Linux 2.6.11

$ cat /etc/debian_version
5.0.2

$ egrep 'MIN\(|MAX\(' /usr/include/sys/param.h
#define MIN(a,b) (((a)<(b))?(a):(b))
#define MAX(a,b) (((a)>(b))?(a):(b))

$ head -n 2 /usr/include/sys/param.h | grep GNU
This file is part of the GNU C Library.

On FreeBSD:

$ uname -sr
FreeBSD 5.5-STABLE

$ egrep 'MIN\(|MAX\(' /usr/include/sys/param.h
#define MIN(a,b) (((a)<(b))?(a):(b))
#define MAX(a,b) (((a)>(b))?(a):(b))

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The source repositories are here:

• GNU C Library
• FreeBSD