The #line preprocessor directive is used to reset the values of the __LINE__ and __FILE__ symbols, respectively. This directive is commonly used in fourth-generation languages that generate C language source files. For instance, if you are using a fourth-generation language named "X," the 4GL compiler will generate C source code routines for compilation based on your 4GL source code. If errors are present in your 4GL code, they can be mapped back to your 4GL source code by using the #line directive. The 4GL code generator simply inserts a line like this into the generated C source:

#line 752, "XSOURCE.X"
void generated_code(void)
{
...
}

Now, if an error is detected anywhere in the generated_code() function, it can be mapped back to the original 4GL source file named XSOURCE.X. This way, the 4GL compiler can report the 4GL source code line that has the error in it.

When the #line directive is used, the __LINE__ symbol is reset to the first argument after the #line keyword (in the preceding example, 752), and the __FILE__ symbol is reset to the second argument after the #line keyword (in the preceding example, "XSOURCE.X"). All references hereafter to the __LINE__ and __FILE__ symbols will reflect the reset values and not the original values of __LINE__ and __FILE__.

The __DATE__ macro is used to insert the current compilation date in the form "mm dd yyyy" into your program. Similarly, the __TIME__ macro is used to insert the current compilation time in the form "hh:mm:ss" into your program. This date-and-time-stamp feature should not be confused with the current system date and time. Rather, these two macros enable you to keep track of the date and time your program was last compiled. This feature can come in very handy when you are trying to track different versions of your program. For instance, many programmers like to put a function in their programs that gives compilation information as to when the current module was compiled. This task can be performed as shown here:

#include <stdio.h>
void main(void);
void print_version_info(void);
void main(void)
{
print_version_info();
}
void print_version_info(void)
{
     printf("Date Compiled: %s\n", __DATE__);
     printf("Time Compiled: %s\n", __TIME__);
}

In this example, the function print_version_info() is used to show the date and time stamp of the last time this module was compiled.

The ANSI C standard provides a predefined symbol named __STDC__ that is set to 1 when the compiler is enforcing strict ANSI standard conformance. If you want your programs to be 100 percent ANSI conformant, you should ensure that the __STDC__ symbol is defined. If the program is being compiled with non-ANSI options, the __STDC__ symbol is undefined. The following code segment shows how this symbol can be checked:

#ifdef __STDC__
     printf("Congratulations! You are conforming perfectly to the ANSI
             standards!\n");
#else
     printf("Shame on you, you nonconformist anti-ANSI rabble-rousing programmer!\n");
#endif

You can use the #undef preprocessor directive to undefine (override) a previously defined macro. Many programmers like to ensure that their applications are using their own terms when defining symbols such as TRUE and FALSE. Your program can check to see whether these symbols have been defined already, and if they have, you can override them with your own definitions of TRUE and FALSE. The following portion of code shows how this task can be accomplished:

...
#ifdef TRUE             /* Check to see if TRUE has been defined yet */
#undef TRUE             /* If so, undefine it */
#endif
#define TRUE 1          /* Define TRUE the way we want it defined */
#ifdef FALSE            /* Check to see if FALSE has been defined yet */
#undef FALSE            /* If so, undefine it */
#endif
#define FALSE !TRUE     /* Define FALSE the way we want it defined */
...

In the preceding example, the symbols TRUE and FALSE are checked to see whether they have been defined yet. If so, they are undefined, or overridden, using the #undef preprocessor directive, and they are redefined in the desired manner. If you were to eliminate the #undef statements in the preceding example, the compiler would warn you that you have multiple definitions of the same symbol. By using this technique, you can avoid this warning and ensure that your programs are using valid symbol definitions.

You can use the #ifdef and #ifndef preprocessor directives to check whether a symbol has been defined (#ifdef) or whether it has not been defined (#ifndef). Many programmers like to ensure that their own version of NULL is defined, not someone else's. This task can be accomplished as shown here:

#ifdef NULL
#undef NULL
#endif
#define NULL (void*) 0

The first line, #ifdef NULL, checks to see whether the NULL symbol has been defined. If so, it is undefined using #undef NULL and the new definition of NULL is defined.

To check whether a symbol has not been defined yet, you would use the #ifndef preprocessor directive.