Preprocessors in c


1. What is a macro, and how do you use it?
A macro is a preprocessor directive that provides a mechanism for token replacement in your source code. Macros are created by using the #define statement. Here is an example of a macro:
#define VERSION_STAMP "1.02"
The macro being defined in this example is commonly referred to as a symbol. The symbol VERSION_STAMP is simply a physical representation of the string "1.02". When the preprocessor is invoked, every occurrence of the VERSION_STAMP symbol is replaced with the literal string "1.02". Here is another example of a macro:
#define CUBE(x) ((x) * (x) * (x))
The macro being defined here is named CUBE, and it takes one argument, x. The rest of the code on the line represents the body of the CUBE macro. Thus, the simplistic macro CUBE(x) will represent the more complex expression ((x) * (x) * (x)). When the preprocessor is invoked, every instance of the macro CUBE(x) in your program is replaced with the code ((x) * (x) * (x)).
Macros can save you many keystrokes when you are coding your program. They can also make your program much more readable and reliable, because you enter a macro in one place and use it in potentially several places. There is no overhead associated with macros, because the code that the macro represents is expanded in-place, and no jump in your program is invoked. Additionally, the arguments are not type-sensitive, so you don't have to worry about what data type you are passing to the macro.
Note that there must be no white space between your macro name and the parentheses containing the argument definition. Also, you should enclose the body of the macro in parentheses to avoid possible ambiguity regarding the translation of the macro. For instance, the following example shows the CUBE macro defined incorrectly:
#define CUBE (x) x * x * x
You also should be careful with what is passed to a macro. For instance, a very common mistake is to pass an incremented variable to a macro, as in the following example:
#include <stdio.h>
#define CUBE(x) (x*x*x)
void main(void);
void main(void)
{
     int x, y;
     x = 5;
     y = CUBE(++x);
     printf("y is %d\n", y);
}
What will y be equal to? You might be surprised to find out that y is not equal to 125 (the cubed value of 5) and not equal to 336 (6 * 7 * 8), but rather is 512. This is because the variable x is incremented while being passed as a parameter to the macro. Thus, the expanded CUBE macro in the preceding example actually appears as follows:
y = ((++x) * (++x) * (++x));
Each time x is referenced, it is incremented, so you wind up with a very different result from what you had intended. Because x is referenced three times and you are using a prefix increment operator, x is actually 8 when the code is expanded. Thus, you wind up with the cubed value of 8 rather than 5. This common mistake is one you should take note of because tracking down such bugs in your software can be a very frustrating experience. I personally have seen this mistake made by people with many years of C programming under their belts. I recommend that you type the example program and see for yourself how surprising the resulting value (512) is.
Macros can also utilize special operators such as the stringizing operator (#) and the concatenation operator (##). The stringizing operator can be used to convert macro parameters to quoted strings, as in the following example:
#define DEBUG_VALUE(v) printf(#v " is equal to %d.\n", v)
In your program, you can check the value of a variable by invoking the DEBUG_VALUE macro: ... int x = 20; DEBUG_VALUE(x); ... The preceding code prints "x is equal to 20." on-screen. This example shows that the stringizing operator used with macros can be a very handy debugging tool.
The concatenation operator (##) is used to concatenate (combine) two separate strings into one single string.
2. What will the preprocessor do for a program?
The C preprocessor is used to modify your program according to the preprocessor directives in your source code. A preprocessor directive is a statement (such as #define) that gives the preprocessor specific instructions on how to modify your source code. The preprocessor is invoked as the first part of your compiler program's compilation step. It is usually hidden from the programmer because it is run automatically by the compiler.
The preprocessor reads in all of your include files and the source code you are compiling and creates a preprocessed version of your source code. This preprocessed version has all of its macros and constant symbols replaced by their corresponding code and value assignments. If your source code contains any conditional preprocessor directives (such as #if), the preprocessor evaluates the condition and modifies your source code accordingly.
Here is an example of a program that uses the preprocessor extensively:
#include <stdio.h>
#define TRUE         1
#define FALSE        (!TRUE)
#define GREATER(a,b) ((a) > (b) ? (TRUE) : (FALSE))
#define PIG_LATIN    FALSE
void main(void);
void main(void)
{
     int x, y;
#if PIG_LATIN
     printf("Easeplay enternay ethay aluevay orfay xnay: ");
     scanf("%d", &x);
     printf("Easeplay enternay ethay aluevay orfay ynay: ");
     scanf("%d", &y);
#else
     printf("Please enter the value for x: ");
     scanf("%d", &x);
     printf("Please enter the value for y: ");
     scanf("%d", &y);
#endif
     if (GREATER(x,y) == TRUE)
     {
#if PIG_LATIN
          printf("xnay islay eatergray anthay ynay!\n");
#else
          printf("x is greater than y!\n");
#endif
     }
     else
     {
     #if PIG_LATIN
          printf("xnay islay otnay eatergray anthay ynay!\n");
#else
          printf("x is not greater than y!\n");
#endif
     }
}
This program uses preprocessor directives to define symbolic constants (such as TRUE, FALSE, and PIG_LATIN), a macro (such as GREATER(a,b)), and conditional compilation (by using the #if statement). When the preprocessor is invoked on this source code, it reads in the stdio.h file and interprets its preprocessor directives, then it replaces all symbolic constants and macros in your program with the corresponding values and code. Next, it evaluates whether PIG_LATIN is set to TRUE and includes either the pig latin text or the plain English text.
If PIG_LATIN is set to FALSE, as in the preceding example, a preprocessed version of the source code would look like this:
/* Here is where all the include files
   would be expanded. */
void main(void)
{
     int x, y;
     printf("Please enter the value for x: ");
     scanf("%d", &x);
     printf("Please enter the value for y: ");
     scanf("%d", &y);
     if (((x) > (y) ? (1) : (!1)) == 1)
     {
          printf("x is greater than y!\n");
     }
     else
     {
          printf("x is not greater than y!\n");
     }
}
This preprocessed version of the source code can then be passed on to the compiler. If you want to see a preprocessed version of a program, most compilers have a command-line option or a standalone preprocessor program to invoke only the preprocessor and save the preprocessed version of your source code to a file. This capability can sometimes be handy in debugging strange errors with macros and other preprocessor directives, because it shows your source code after it has been run through the preprocessor.
3. How can you avoid including a header more than once?
One easy technique to avoid multiple inclusions of the same header is to use the #ifndef and #define preprocessor directives. When you create a header for your program, you can #define a symbolic name that is unique to that header. You can use the conditional preprocessor directive named #ifndef to check whether that symbolic name has already been assigned. If it is assigned, you should not include the header, because it has already been preprocessed. If it is not defined, you should define it to avoid any further inclusions of the header. The following header illustrates this technique:
#ifndef _FILENAME_H
#define _FILENAME_H
#define VER_NUM      "1.00.00"
#define REL_DATE     "08/01/94"
#if __WINDOWS__
#define OS_VER       "WINDOWS"
#else
#define OS_VER       "DOS"
#endif
#endif
When the preprocessor encounters this header, it first checks to see whether _FILENAME_H has been defined. If it hasn't been defined, the header has not been included yet, and the _FILENAME_H symbolic name is defined. Then, the rest of the header is parsed until the last #endif is encountered, signaling the end of the conditional #ifndef _FILENAME_H statement. Substitute the actual name of the header file for "FILENAME" in the preceding example to make it applicable for your programs.
4. Can a file other than a .h file be included with #include?
The preprocessor will include whatever file you specify in your #include statement. Therefore, if you have the line
#include <macros.inc>
in your program, the file macros.inc will be included in your precompiled program. It is, however, unusual programming practice to put any file that does not have a .h or .hpp extension in an #include statement. You should always put a .h extension on any of your C files you are going to include. This method makes it easier for you and others to identify which files are being used for preprocessing purposes.
For instance, someone modifying or debugging your program might not know to look at the macros.inc file for macro definitions. That person might try in vain by searching all files with .h extensions and come up empty. If your file had been named macros.h, the search would have included the macros.h file, and the searcher would have been able to see what macros you defined in it.
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