When writing your C program, you can include files in two ways. The first way is to surround the file you want to include with the angled brackets < and >. This method of inclusion tells the preprocessor to look for the file in the predefined default location. This predefined default location is often an INCLUDE environment variable that denotes the path to your include files. For instance, given the INCLUDE variable
INCLUDE=C:\COMPILER\INCLUDE;S:\SOURCE\HEADERS;
using the #include version of file inclusion, the compiler first checks the C:\COMPILER\INCLUDE directory for the specified file. If the file is not found there, the compiler then checks the S:\SOURCE\HEADERS directory. If the file is still not found, the preprocessor checks the current directory.
The second way to include files is to surround the file you want to include with double quotation marks. This method of inclusion tells the preprocessor to look for the file in the current directory first, then look for it in the predefined locations you have set up. Using the #include "file" version of file inclusion and applying it to the preceding example, the preprocessor first checks the current directory for the specified file. If the file is not found in the current directory, the C:\COMPILER\INCLUDE directory is searched. If the file is still not found, the preprocessor checks the S:\SOURCE\HEADERS directory.
The #include method of file inclusion is often used to include standard headers such as stdio.h orstdlib.h. This is because these headers are rarely (if ever) modified, and they should always be read from your compiler's standard include file directory.
The #include "file" method of file inclusion is often used to include nonstandard header files that you have created for use in your program. This is because these headers are often modified in the current directory, and you will want the preprocessor to use your newly modified version of the header rather than the older, unmodified version.

Yes. This can be done by using the #if, #else, and #endif preprocessor directives. For example, certain compilers use different names for header files. One such case is between Borland C++, which uses the header file alloc.h, and Microsoft C++, which uses the header file malloc.h. Both of these headers serve the same purpose, and each contains roughly the same definitions. If, however, you are writing a program that is to support Borland C++ and Microsoft C++, you must define which header to include at compile time. The following example shows how this can be done:
#ifdef __BORLANDC__
#include
#else
#include
#endif
When you compile your program with Borland C++, the __BORLANDC__ symbolic name is automatically defined by the compiler. You can use this predefined symbolic name to determine whether your program is being compiled with Borland C++. If it is, you must include the alloc.h file rather than the malloc.h file.

Yes. Include files can be nested any number of times. As long as you use precautionary measures, you can avoid including the same file twice. In the past, nesting header files was seen as bad programming practice, because it complicates the dependency tracking function of the MAKE program and thus slows down compilation. Many of today's popular compilers make up for this difficulty by implementing a concept called precompiled headers, in which all headers and associated dependencies are stored in a precompiled state.
Many programmers like to create a custom header file that has #include statements for every header needed for each module. This is perfectly acceptable and can help avoid potential problems relating to #include files, such as accidentally omitting an #include file in a module.

Even though there is no limit to the number of levels of nested include files you can have, your compiler might run out of stack space while trying to include an inordinately high number of files. This number varies according to your hardware configuration and possibly your compiler.
In practice, although nesting include files is perfectly legal, you should avoid getting nest-crazy and purposely implementing a large number of include levels. You should create an include level only where it makes sense, such as creating one include file that has an #include statement for each header required by the module you are working with.

The concatenation operator (##) is used to concatenate (combine) two separate strings into one single string. The concatenation operator is often used in C macros, as the following program demonstrates:
#include
#define SORT(x) sort_function ## x
void main(void);
void main(void)
{
char* array;
int elements, element_size;
...
SORT(3)(array, elements, element_size);
...
}
In the preceding example, the SORT macro uses the concatenation operator to combine the strings sort_function and whatever is passed in the parameter x. This means that the line
SORT(3)(array, elements, element_size);
is run through the preprocessor and is translated into the following line:
sort_function3(array, elements, element_size);
As you can see, the concatenation operator can come in handy when you do not know what function to call until runtime. Using the concatenation operator, you can dynamically construct the name of the function you want to call, as was done with the SORT macro.