Function question and Answers
1. When should I declare a function?
1. When should I declare a function?
Functions that are used only in the current source file should be declared as static, and the function's declaration should appear in the current source file along with the definition of the function. Functions used outside of the current source file should have their declarations put in a header file, which can be included in whatever source file is going to use that function. For instance, if a function named stat_func() is used only in the source file stat.c, it should be declared as shown here:
/* stat.c */
#include <stdio.h>
static int stat_func(int, int); /* static declaration of stat_func() */
void main(void)
{
rc = stat_func(1, 2);
}
/* definition (body) of stat_func() */
static int stat_func(int arg1, int arg2)
{
return rc;
}
In this example, the function named stat_func() is never used outside of the source file stat.c. There is therefore no reason for the prototype (or declaration) of the function to be visible outside of the stat.c source file. Thus, to avoid any confusion with other functions that might have the same name, the declaration ofstat_func() should be put in the same source file as the declaration of stat_func().
In the following example, the function glob_func() is declared and used in the source file global.c and is used in the source file extern.c. Because glob_func() is used outside of the source file in which it's declared, the declaration of glob_func() should be put in a header file (in this example, named proto.h) to be included in both the global.c and the extern.c source files. This is how it's done:
/* proto.h */
int glob_func(int, int); /* declaration of the glob_func() function */
/* global.c */
#include <stdio.h>
#include "proto.h"
/* include this proto.h file for the declaration of glob_func() */
void main(void);
void main(void)
{
rc = glob_func(1, 2);
}
/* definition (body) of the glob_func() function */
int glob_func(int arg1, int arg2)
{
return rc;
}
/* extern.c */
#include <stdio.h>
#include "proto.h"
/* include this proto.h file for the declaration of glob_func() */
void ext_func(void);
void ext_func(void)
{
/* call glob_func(), which is defined in the global.c source file */
rc = glob_func(10, 20);
}
In the preceding example, the declaration of glob_func() is put in the header file named proto.h becauseglob_func() is used in both the global.c and the extern.c source files. Now, whenever glob_func() is going to be used, you simply need to include the proto.h header file, and you will automatically have the function's declaration. This will help your compiler when it is checking parameters and return values from global functions you are using in your programs. Notice that your function declarations should always appear before the first function declaration in your source file.
In general, if you think your function might be of some use outside of the current source file, you should put its declaration in a header file so that other modules can access it. Otherwise, if you are sure your function will never be used outside of the current source file, you should declare the function as static and include the declaration only in the current source file.
2. Why should I prototype a function?
A function prototype tells the compiler what kind of arguments a function is looking to receive and what kind of return value a function is going to give back. This approach helps the compiler ensure that calls to a function are made correctly and that no erroneous type conversions are taking place. For instance, consider the following prototype:
int some_func(int, char*, long);
Looking at this prototype, the compiler can check all references (including the definition of some_func()) to ensure that three parameters are used (an integer, a character pointer, and then a long integer) and that a return value of type integer is received. If the compiler finds differences between the prototype and calls to the function or the definition of the function, an error or a warning can be generated to avoid errors in your source code. For instance, the following examples would be flagged as incorrect, given the preceding
prototype of some_func():
x = some_func(1); /* not enough arguments passed */
x = some_func("HELLO!", 1, "DUDE!"); /* wrong type of arguments used */
x = some_func(1, str, 2879, "T"); /* too many arguments passed */
/* In the following example, the return value expected
from some_func() is not an integer: */
long* lValue;
lValue = some_func(1, str, 2879); /* some_func() returns an int,
not a long* */
Using prototypes, the compiler can also ensure that the function definition, or body, is correct and correlates with the prototype. For instance, the following definition of some_func() is not the same as its prototype, and it therefore would be flagged by the compiler:
int some_func(char* string, long lValue, int iValue) /* wrong order ofparameters */
{
}
The bottom line on prototypes is that you should always include them in your source code because they provide a good error-checking mechanism to ensure that your functions are being used correctly. Besides, many of today's popular compilers give you warnings when compiling if they can't find a prototype for a function that is being referenced.
3. How many parameters should a function have?
There is no set number or "guideline" limit to the number of parameters your functions can have. However, it is considered bad programming style for your functions to contain an inordinately high (eight or more) number of parameters. The number of parameters a function has also directly affects the speed at which it is called—the more parameters, the slower the function call. Therefore, if possible, you should minimize the number of parameters you use in a function. If you are using more than four parameters, you might want to rethink your function design and calling conventions.
One technique that can be helpful if you find yourself with a large number of function parameters is to put your function parameters in a structure. Consider the following program, which contains a function namedprint_report() that uses 10 parameters. Instead of making an enormous function declaration and proto- type, the print_report() function uses a structure to get its parameters:
#include <stdio.h>
typedef struct
{
int orientation;
char rpt_name[25];
char rpt_path[40];
int destination;
char output_file[25];
int starting_page;
int ending_page;
char db_name[25];
char db_path[40];
int draft_quality;
} RPT_PARMS;
void main(void);
int print_report(RPT_PARMS*);
void main(void)
{
RPT_PARMS rpt_parm; /* define the report parameter
structure variable */
/* set up the report parameter structure variable to pass to the print_report() function */
rpt_parm.orientation = ORIENT_LANDSCAPE;
rpt_parm.rpt_name = "QSALES.RPT";
rpt_parm.rpt_path = "C:\REPORTS";
rpt_parm.destination = DEST_FILE;
rpt_parm.output_file = "QSALES.TXT";
rpt_parm.starting_page = 1;
rpt_parm.ending_page = RPT_END;
rpt_parm.db_name = "SALES.DB";
rpt_parm.db_path = "C:\DATA";
rpt_parm.draft_quality = TRUE;
/* Call the print_report() function, passing it a pointer to theparameters instead of passing it a long list of 10 separateparameters. */
ret_code = print_report(&rpt_parm);
}
int print_report(RPT_PARMS* p)
{
int rc;
/* access the report parameters passed to the print_report()function */
orient_printer(p->orientation);
set_printer_quality((p->draft_quality == TRUE) ? DRAFT : NORMAL);
return rc;
}
The preceding example avoided a large, messy function prototype and definition by setting up a predefined structure of type RPT_PARMS to hold the 10 parameters that were needed by the print_report() function. The only possible disadvantage to this approach is that by removing the parameters from the function definition, you are bypassing the compiler's capability to type-check each of the parameters for validity during the compile stage.
Generally, you should keep your functions small and focused, with as few parameters as possible to help with execution speed. If you find yourself writing lengthy functions with many parameters, maybe you should rethink your function design or consider using the structure-passing technique presented here. Additionally, keeping your functions small and focused will help when you are trying to isolate and fix bugs in your programs.
4. What is a static function?
A static function is a function whose scope is limited to the current source file. Scope refers to the visibility of a function or variable. If the function or variable is visible outside of the current source file, it is said to haveglobal, or external, scope. If the function or variable is not visible outside of the current source file, it is said to have local, or static, scope.
A static function therefore can be seen and used only by other functions within the current source file. When you have a function that you know will not be used outside of the current source file or if you have a function that you do not want being used outside of the current source file, you should declare it as static. Declaring local functions as static is considered good programming practice. You should use static functions often to avoid possible conflicts with external functions that might have the same name.
For instance, consider the following example program, which contains two functions. The first function,open_customer_table(), is a global function that can be called by any module. The second function,open_customer_indexes(), is a local function that will never be called by another module. This is because you can't have the customer's index files open without first having the customer table open. Here is the code:
#include <stdio.h>
int open_customer_table(void); /* global function, callable from any module */
static int open_customer_indexes(void); /* local function, used only in this module */
int open_customer_table(void)
{
int ret_code;
/* open the customer table */
if (ret_code == OK)
{
ret_code = open_customer_indexes();
}
return ret_code;
}
static int open_customer_indexes(void)
{
int ret_code;
/* open the index files used for this table */
return ret_code;
}
Generally, if the function you are writing will not be used outside of the current source file, you should declare it as static.
5. Should a function contain a return statement if it does not return a value?
In C, void functions (those that do not return a value to the calling function) are not required to include a return statement. Therefore, it is not necessary to include a return statement in your functions declared as being void.
In some cases, your function might trigger some critical error, and an immediate exit from the function might be necessary. In this case, it is perfectly acceptable to use a return statement to bypass the rest of the function's code. However, keep in mind that it is not considered good programming practice to litter your functions with return statements-generally, you should keep your function's exit point as focused and clean as possible.
6. How can you pass an array to a function by value?
An array can be passed to a function by value by declaring in the called function the array name with square brackets ([ and ]) attached to the end. When calling the function, simply pass the address of the array (that is, the array's name) to the called function. For instance, the following program passes the array x[] to the function named byval_func() by value:
#include <stdio.h>
void byval_func(int[]); /* the byval_func() function is passed an
integer array by value */
void main(void);
void main(void)
{
int x[10];
int y;
/* Set up the integer array. */
for (y=0; y<10; y++)
x[y] = y;
/* Call byval_func(), passing the x array by value. */
byval_func(x);
}
/* The byval_function receives an integer array by value. */
void byval_func(int i[])
{
int y;
/* Print the contents of the integer array. */
for (y=0; y<10; y++)
printf("%d\n", i[y]);
}
In this example program, an integer array named x is defined and initialized with 10 values. The functionbyval_func() is declared as follows:
int byval_func(int[]);
The int[] parameter tells the compiler that the byval_func() function will take one argument—an array of integers. When the byval_func() function is called, you pass the address of the array to byval_func():byval_func(x);
Because the array is being passed by value, an exact copy of the array is made and placed on the stack. The called function then receives this copy of the array and can print it. Because the array passed to byval_func()is a copy of the original array, modifying the array within the byval_func() function has no effect on the original array.
Passing arrays of any kind to functions can be very costly in several ways. First, this approach is very inefficient because an entire copy of the array must be made and placed on the stack. This takes up valuable program time, and your program execution time is degraded. Second, because a copy of the array is made, more memory (stack) space is required. Third, copying the array requires more code generated by the compiler, so your program is larger.
Instead of passing arrays to functions by value, you should consider passing arrays to functions by reference: this means including a pointer to the original array. When you use this method, no copy of the array is made. Your programs are therefore smaller and more efficient, and they take up less stack space. To pass an array by reference, you simply declare in the called function prototype a pointer to the data type you are holding in the array.
Consider the following program, which passes the same array (x) to a function:
#include <stdio.h>
void const_func(const int*);
void main(void);
void main(void)
{
int x[10];
int y;
/* Set up the integer array. */
for (y=0; y<10; y++)
x[y] = y;
/* Call const_func(), passing the x array by reference. */
const_func(x);
}
/* The const_function receives an integer array by reference.
Notice that the pointer is declared as const, which renders
it unmodifiable by the const_func() function. */
void const_func(const int* i)
{
int y;
/* Print the contents of the integer array. */
for (y=0; y<10; y++)
printf("%d\n", *(i+y));
}
In the preceding example program, an integer array named x is defined and initialized with 10 values. Thefunction const_func() is declared as follows:
int const_func(const int*);
The const int* parameter tells the compiler that the const_func() function will take one argument—a constant pointer to an integer. When the const_func() function is called, you pass the address of the array toconst_func():
const_func(x);
Because the array is being passed by reference, no copy of the array is made and placed on the stack. The called function receives simply a constant pointer to an integer. The called function must be coded to be smart enough to know that what it is really receiving is a constant pointer to an array of integers. The const modifier is used to prevent the const_func() from accidentally modifying any elements of the original array.
The only possible drawback to this alternative method of passing arrays is that the called function must be coded correctly to access the array—it is not readily apparent by the const_func() function prototype or definition that it is being passed a reference to an array of integers. You will find, however, that this method is much quicker and more efficient, and it is recommended when speed is of utmost importance.
7. Is it possible to execute code even after the program exits the main() function?
The standard C library provides a function named atexit() that can be used to perform "cleanup" operations when your program terminates. You can set up a set of functions you want to perform automatically when your program exits by passing function pointers to the atexit() function. Here's an example of a program that uses the atexit() function:
#include <stdio.h>
#include <stdlib.h>
void close_files(void);
void print_registration_message(void);
int main(int, char**);
int main(int argc, char** argv)
{
atexit(print_registration_message);
atexit(close_files);
while (rec_count < max_records)
{
process_one_record();
}
exit(0);
}
This example program uses the atexit() function to signify that the close_files() function and theprint_registration_message() function need to be called automatically when the program exits. When themain() function ends, these two functions will be called to close the files and print the registration message. There are two things that should be noted regarding the atexit() function. First, the functions you specify to execute at program termination must be declared as void functions that take no parameters. Second, the functions you designate with the atexit() function are stacked in the order in which they are called withatexit(), and therefore they are executed in a last-in, first-out (LIFO) method. Keep this information in mind when using the atexit() function. In the preceding example, the atexit() function is stacked as shown here:
atexit(print_registration_message);
atexit(close_files);
Because the LIFO method is used, the close_files() function will be called first, and then theprint_registration_message() function will be called.
The atexit() function can come in handy when you want to ensure that certain functions (such as closing your program's data files) are performed before your program terminates.
8. What does a function declared as PASCAL do differently?
C function declared as PASCAL uses a different calling convention than a "regular" C function. Normally, C function parameters are passed right to left; with the PASCAL calling convention, the parameters are passed left to right.
Consider the following function, which is declared normally in a C program:
int regular_func(int, char*, long);
Using the standard C calling convention, the parameters are pushed on the stack from right to left. This means that when the regular_func() function is called in C, the stack will contain the following parameters:
long char*
int
The function calling regular_func() is responsible for restoring the stack when regular_func() returns.
When the PASCAL calling convention is being used, the parameters are pushed on the stack from left to right.
Consider the following function, which is declared as using the PASCAL calling convention:
int PASCAL pascal_func(int, char*, long);
When the function pascal_func() is called in C, the stack will contain the following parameters:
int char*long
The function being called is responsible for restoring the stack pointer. Why does this matter? Is there any benefit to using PASCAL functions?
Functions that use the PASCAL calling convention are more efficient than regular C functions—the function calls tend to be slightly faster. Microsoft Windows is an example of an operating environment that uses the PASCAL calling convention. The Windows SDK (Software Development Kit) contains hundreds of functions declared as PASCAL.
When Windows was first designed and written in the late 1980s, using the PASCAL modifier tended to make a noticeable difference in program execution speed. In today's world of fast machinery, the PASCAL modifier is much less of a catalyst when it comes to the speed of your programs. In fact, Microsoft has abandoned the PASCAL calling convention style for the Windows NT operating system.
In your world of programming, if milliseconds make a big difference in your programs, you might want to use the PASCAL modifier when declaring your functions. Most of the time, however, the difference in speed is hardly noticeable, and you would do just fine to use C's regular calling convention.
Is using exit() the same as using return?
No. The exit() function is used to exit your program and return control to the operating system. The return statement is used to return from a function and return control to the calling function. If you issue a return from the main() function, you are essentially returning control to the calling function, which is the operating system. In this case, the return statement and exit() function are similar. Here is an example of a program that uses the exit() function and return statement:
#include <stdio.h>
#include <stdlib.h>
int main(int, char**);
int do_processing(void);
int do_something_daring();
int main(int argc, char** argv)
{
int ret_code;
if (argc < 3)
{
printf("Wrong number of arguments used!\n");
/* return 1 to the operating system */
exit(1);
}
ret_code = do_processing();
/* return 0 to the operating system */
exit(0);
}
int do_processing(void)
{
int rc;
rc = do_something_daring();
if (rc == ERROR)
{
printf("Something fishy is going on around here..."\n);
/* return rc to the operating system */
exit(rc);
}
/* return 0 to the calling function */
return 0;
}
In the main() function, the program is exited if the argument count (argc) is less than 3. The statementexit(1); tells the program to exit and return the number 1 to the operating system. The operating system can then decide what to do based on the return value of the program. For instance, many DOS batch files check the environment variable named ERRORLEVEL for the return value of executable programs.
Preprocessor
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 theVERSION_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 #definepreprocessor 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.
5. What is the benefit of using #define to declare a constant?
Using the #define method of declaring a constant enables you to declare a constant in one place and use it throughout your program. This helps make your programs more maintainable, because you need to maintain only the #define statement and not several instances of individual constants throughout your program. For instance, if your program used the value of pi (approximately 3.14159) several times, you might want to declare a constant for pi as follows:
#define PI 3.14159
This way, if you wanted to expand the precision of pi for more accuracy, you could change it in one place rather than several places. Usually, it is best to put #define statements in an include file so that several modules can use the same constant value.
Using the #define method of declaring a constant is probably the most familiar way of declaring constants to traditional C programmers. Besides being the most common method of declaring constants, it also takes up the least memory. Constants defined in this manner are simply placed directly into your source code, with no variable space allocated in memory. Unfortunately, this is one reason why most debuggers cannot inspect constants created using the #define method.
Constants defined with the #define method can also be overridden using the #undef preprocessor directive. This means that if a symbol such as NULL is not defined the way you would like to see it defined, you can remove the previous definition of NULL and instantiate your own custom definition.
6. What is the benefit of using enum to declare a constant?
Using the enum keyword to define a constant can have several benefits. First, constants declared with enum are automatically generated by the compiler, thereby relieving the programmer of manually assigning unique values to each constant. Also, constants declared with enum tend to be more readable to the programmer, because there is usually an enumerated type identifier associated with the constant's definition.
Additionally, enumerated constants can usually be inspected during a debugging session. This can be an enormous benefit, especially when the alternative is having to manually look up the constant's value in a header file. Unfortunately, using the enum method of declaring constants takes up slightly more memory space than using the #define method of declaring constants, because a memory location must be set up to store the constant.
Here is an example of an enumerated constant used for tracking errors in your program: enum Error_Code
{
OUT_OF_MEMORY,
NSUFFICIENT_DISK_SPACE,
LOGIC_ERROR,
FILE_NOT_FOUND
};
7. What is the benefit of using an enum rather than a #define constant?
The use of an enumeration constant (enum) has many advantages over using the traditional symbolic constant style of #define. These advantages include a lower maintenance requirement, improved program readability, and better debugging capability. The first advantage is that enumerated constants are generated automati- cally by the compiler. Conversely, symbolic constants must be manually assigned values by the programmer. For instance, if you had an enumerated constant type for error codes that could occur in your program, your enumdefinition could look something like this:
enum Error_Code
{
OUT_OF_MEMORY,
INSUFFICIENT_DISK_SPACE,
LOGIC_ERROR,
FILE_NOT_FOUND
};
In the preceding example, OUT_OF_MEMORY is automatically assigned the value of 0 (zero) by the compiler because it appears first in the definition. The compiler then continues to automatically assign numbers to the enumerated constants, making INSUFFICIENT_DISK_SPACE equal to 1, LOGIC_ERROR equal to 2, and so on.
If you were to approach the same example by using symbolic constants, your code would look something like this:
#define OUT_OF_MEMORY 0
#define INSUFFICIENT_DISK_SPACE 1
#define LOGIC_ERROR 2
#define FILE_NOT_FOUND 3
Each of the two methods arrives at the same result: four constants assigned numeric values to represent error codes. Consider the maintenance required, however, if you were to add two constants to represent the error codes DRIVE_NOT_READY and CORRUPT_FILE. Using the enumeration constant method, you simply would put these two constants anywhere in the enum definition. The compiler would generate two unique values for these constants. Using the symbolic constant method, you would have to manually assign two new numbers to these constants. Additionally, you would want to ensure that the numbers you assign to these constants are unique. Because you don't have to worry about the actual values, defining your constants using the enumerated method is easier than using the symbolic constant method. The enumerated method also helps prevent accidentally reusing the same number for different constants.
Another advantage of using the enumeration constant method is that your programs are more readable and thus can be understood better by others who might have to update your program later. For instance, consider the following piece of code:
void copy_file(char* source_file_name, char* dest_file_name)
{
Error_Code err;
if (drive_ready() != TRUE)
err = DRIVE_NOT_READY;
}
Looking at this example, you can derive from the definition of the variable err that err should be assigned only numbers of the enumerated type Error_Code. Hence, if another programmer were to modify or add functionality to this program, the programmer would know from the definition of Error_Code what constants are valid for assigning to err.
Conversely, if the same example were to be applied using the symbolic constant method, the code would look like this:
void copy_file(char* source_file, char* dest_file)
{
int err;
if (drive_ready() != TRUE)
err = DRIVE_NOT_READY;
}
Looking at the preceding example, a programmer modifying or adding functionality to the copy_file() function would not immediately know what values are valid for assigning to the err variable. The programmer would need to search for the #define DRIVE_NOT_READY statement and hope that all relevant constants are defined in the same header file. This could make maintenance more difficult than it needs to be and make your programs harder to understand.
A third advantage to using enumeration constants is that some symbolic debuggers can print the value of an enumeration constant. Conversely, most symbolic debuggers cannot print the value of a symbolic constant. This can be an enormous help in debugging your program, because if your program is stopped at a line that uses an enum, you can simply inspect that constant and instantly know its value. On the other hand, because most debuggers cannot print #define values, you would most likely have to search for that value by manually looking it up in a header file.
8. How are portions of a program disabled in demo versions?
If you are distributing a demo version of your program, the preprocessor can be used to enable or disable portions of your program. The following portion of code shows how this task is accomplished, using the preprocessor directives #if and #endif:
int save_document(char* doc_name)
{
#if DEMO_VERSION
printf("Sorry! You can't save documents using the DEMO version of this program!\n");
return(0);
#endif
}
When you are compiling the demo version of your program, insert the line #define DEMO_VERSION and the preprocessor will include the conditional code that you specified in the save_document() function. This action prevents the users of your demo program from saving their documents.
As a better alternative, you could define DEMO_VERSION in your compiler options when compiling and avoid having to change the source code for the program.
This technique can be applied to many different situations. For instance, you might be writing a program that will support several operating systems or operating environments. You can create macros such as WINDOWS_VER,UNIX_VER, and DOS_VER that direct the preprocessor as to what code to include in your program depending on what operating system you are compiling for.
9. Is it better to use a macro or a function?
The answer depends on the situation you are writing code for. Macros have the distinct advantage of being more efficient (and faster) than functions, because their corresponding code is inserted directly into your source code at the point where the macro is called. There is no overhead involved in using a macro like there is in placing a call to a function. However, macros are generally small and cannot handle large, complex coding constructs. A function is more suited for this type of situation.
Additionally, macros are expanded inline, which means that the code is replicated for each occurrence of a macro. Your code therefore could be somewhat larger when you use macros than if you were to use functions.
Thus, the choice between using a macro and using a function is one of deciding between the tradeoff of faster program speed versus smaller program size. Generally, you should use macros to replace small, repeatable code sections, and you should use functions for larger coding tasks that might require several lines of code.
10. What is the best way to comment out a section of code that contains comments?
Most C compilers offer two ways of putting comments in your program. The first method is to use the /* and */ symbols to denote the beginning and end of a comment. Everything from the /* symbol to the */ symbol is considered a comment and is omitted from the compiled version of the program. This method is best for commenting out sections of code that contain many comments. For instance, you can comment out a paragraph containing comments like this:
/*
This portion of the program contains a comment that is several lines long and is not included in the compiled version of the program.
*/
The other way to put comments in your program is to use the // symbol. Everything from the // symbol to the end of the current line is omitted from the compiled version of the program. This method is best for one-line comments, because the // symbol must be replicated for each line that you want to add a comment to. The preceding example, which contains four lines of comments, would not be a good candidate for this method of commenting, as demonstrated here:
// This portion of the program contains
// a comment that is several lines long
// and is not included in the compiled
// version of the program.
You should consider using the /* and */ method of commenting rather than the // method, because the // method of commenting is not ANSI compatible. Many older compilers might not support the // comments.
11. What is the difference between #include and #include "file" ?
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 <file> 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.
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