Thread: question - linux - gcc - c standards

Originally Posted by AMAMH

I want my code to be compatible at least under all Linux distros, but I would also like if it compiles under other operating systems too.

The POSIX Standard is upheld by all linux and unix-based systems:

The Open Group Base Specifications Issue 7

if you look under alphabetical index, this is just the manual pages for all standard C functions as implemented under POSIX. They are pretty good man pages too.

Generally, tho, just use the GNU C Reference
The GNU C Library
since your C library is built by GNU, and they do include explicit notes there about "gnu extensions" that may effect portability.

Portability issues are specific issues. You will become aware of them as they become involved with your programs, don't worry. For now, you do not need to add any compiler flags except this one:

gcc -Wall

That will help to keep your code proper and as portable as possible, eg it will warn you when you use a gnu specific extension like this:

warning: implicit declaration of function ‘strcasestr’

because GNU's strcasestr is GNU specific.

Originally Posted by AMAMH

thanks MK27 for ur reply.
u really helped me, especially for pointing that I should use -Wall to disable gnu extensions, but isn't gcc available for all operating systems?

This doesn't actually disable them; it is just to let you know. If you are using the GNU C library with your compiler, you can use strcasestr. What that message ("warning: implicit declaration of function") usually means is that you don't have a prototype defined for your function -- which makes it slightly confusing, so try to remember this!

To make a long story short, if you run across that warning because of a library function (not your own*) and you want to get rid of it and still compile with -Wall, just copy the prototype from the manual into your program code at the top:

Code:

char * strcasestr (const char *haystack, const char *needle);

Now you can compile with -Wall and there will be no warnings. I don't know how or why gcc uses this method. It seems to me:

warning: strcasestr is a GNU specific extension to the C standard

would have been fine. In any case, "forcing" you to copy the prototype in does serve as a strong reminder of the fact that that function may have limited portability.

There are actually not many functions like that, strcasestr is the only one I could think of off the top of my head.

And yes, gcc is available for available for more OS's than I've ever heard of. I don't think it has to use the GNU C library to work; certainly MS users use it and they are not using the GNU library. It is good you are thinking pro-actively, but don't sweat about this stuff too much right now.

* if it's because of your own function you forgot to include a prototype: that is the original meaning of "warning: implicit declaration..."

Originally Posted by AMAMH

should I just ignore the c99 standard and use the default gnu89 or c89?

If you want to maximise portability, sticking with C89 would be your best bet.

Originally Posted by AMAMH

is c99 implemented enough in gcc that I can use it?

What specific language features of C99 do you have in mind?

Originally Posted by AMAMH

Actually c99 is planned to be the next default in gcc, so I can't just ignore it, besides, I HATE living in the past.

I believe that any valid C89 program should be a valid C99 program, the use of obsolescent language features and language extensions aside.

Originally Posted by sandeep_patra

should not we use
extern char * strcasestr(const char *, const char *);

rather than using
char * strcasestr (const char *haystack, const char *needle);

Arguably yes, but I do not think that declaring function declarations as extern really makes any difference.