How can I check presence of flag at compile time?

I am using the -LITTLE flag for choosing little endian calculation and

-BIG for big endian calculation in my project whil开发者_运维问答e compiling.

#ifdef LITTLE   
   {    
    // i'm using i for operating one loop 
   }
   #endif

 /* If the system is big-endian, store bytes in array as forward order */
#ifdef  BIG 
   {
   // using i for loop
   }
   #endif

like

gcc -LITTLE my_c_file.c

I want to check if user hasn't given any of flag at compile time then compilation does not takes place and give an error.

How can I do that?

I think you mean gcc -DLITTLE.

You can use something like:

#if !defined(LITTLE) && !defined(BIG)
#error either LITTLE or BIG must be defined
#endif

At a guess, you might also want:

#if defined(LITTLE) && defined(BIG)
#error only one of LITTLE or BIG must be defined
#endif

Of course, it's better if you can write code that doesn't care about the endianness of the maching that you are running on and avoid the whole mess.

Try

#if !defined(LITTLE) && !defined(BIG)
#error "Either LITTLE or BIG has to be defined"
#endif

You can detect the byte sex of the platform automatically. See boost/detail/endian.hpp.