Any C library which defines a macro named max in its standard headers is broken beyond imagination. Fortunately, an easy workaround if you need to support such platforms is to #undef max (and any other problematic macros it defines) after including the system headers and before any of your own headers/code.
Note that everyone else is saying to wrap your definition in #ifndef max ... #endif. This is not a good idea. Defining max in a system header is an indication that the implementor was incompetent, and it's possible that certain versions of the environment have incorrect macros (for example, ones which do not properly protect arguments with parentheses, but I've even seen a max macro that was incorrectly performing min instead of max at least once in my life!). Just use #undef and be safe.
As for why it's so broken for stdlib.h to define max, the C standard is very specific about what names are reserved for the application and what names are reserved for standard functions and/or internal use by the implementation. There are very good reasons for this. Defining macro names in system headers that could clash with variable/function names used in the application program is dangerous. In the best case it leads to compile-time errors with an obvious cause, but in other cases it can cause very strange behavior that's hard to debug. In any case it makes it very difficult to write portable code because you never know what names will already be taken by the library.
Any C library which defines a macro named max in its standard headers is broken beyond imagination. Fortunately, an easy workaround if you need to support such platforms is to #undef max (and any other problematic macros it defines) after including the system headers and before any of your own headers/code.
Note that everyone else is saying to wrap your definition in #ifndef max ... #endif. This is not a good idea. Defining max in a system header is an indication that the implementor was incompetent, and it's possible that certain versions of the environment have incorrect macros (for example, ones which do not properly protect arguments with parentheses, but I've even seen a max macro that was incorrectly performing min instead of max at least once in my life!). Just use #undef and be safe.
As for why it's so broken for stdlib.h to define max, the C standard is very specific about what names are reserved for the application and what names are reserved for standard functions and/or internal use by the implementation. There are very good reasons for this. Defining macro names in system headers that could clash with variable/function names used in the application program is dangerous. In the best case it leads to compile-time errors with an obvious cause, but in other cases it can cause very strange behavior that's hard to debug. In any case it makes it very difficult to write portable code because you never know what names will already be taken by the library.
So answering your main question:
Is max(a,b) defined in stdlib.h or not?
No it isn't, it's defined in windef.h around line 187:
#ifndef NOMINMAX #ifndef max #define max(a,b) (((a) > (b)) ? (a) : (b)) #endif #ifndef min #define min(a,b) (((a) < (b)) ? (a) : (b)) #endif #endif /* NOMINMAX */
Sorry for the newbie question, I'm looking for a way to find the maximum between two values in C and I stumbled upon this, which make it sound like max() is a macro defined in stdlib.h, however when I look up the file I can't find it, and when I try to compile the example I get an 'implicit declaration' warning then an 'undefined reference' at execution.
That stackoverflow discussion seems to go that way and suggests that one uses fmax from the math library, or write the max() themselves.
I don't know if I'm just bad at reading documentation or if there's actually something special about that I should know about. Either way if someone can help me figuring this out, i'd be grateful.
(PS : it's for a school project where I won't have any issues with double evaluation so a dirty solution will do)
MIN and MAX in C - Stack Overflow
min/max in stdlib.h?!
Is max() predefined or not ?
How much of the stdlib is useless when working on embedded systems?
Where are
MINandMAXdefined 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, use something like standard fmax or fmin, or fix the macro using GCC's typeof (you get typesafety bonus too) in a GCC statement expression:
#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.
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 oftypeof.
It's also provided in the GNU libc (Linux) and FreeBSD versions of sys/param.h, and has the definition provided by dreamlax.
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))
The source repositories are here:
- GNU C Library
- FreeBSD