What is a pragma? in C programming

What is a pragma?

The #pragma preprocessor directive allows each compiler to implement compiler-specific features that can
be turned on and off with the #pragma statement. For instance, your compiler might support a feature called
loop optimization. This feature can be invoked as a command-line option or as a #pragma directive. To
implement this option using the #pragma directive, you would put the following line into your code:
#pragma loop_opt(on)

Conversely, you can turn off loop optimization by inserting the following line into your code:

#pragma loop_opt(off)

Sometimes you might have a certain function that causes your compiler to produce a warning such a Parameter xxx is never used in function yyy or some other warning that you are well aware of but choose to ignore. You can temporarily disable this warning message on some compilers by using a #pragma directive
to turn off the warning message before the function and use another #pragma directive to turn it back on after
the function. For instance, consider the following example, in which the function named insert_record() generates a warning message that has the unique ID of 100. You can temporarily disable this warning as
shown here:

#pragma warn -100 /* Turn off the warning message for warning #100 */
int insert_record(REC* r) /* Body of the function insert_record() */
{
/* insert_rec() function statements go here... */
}

#pragma warn +100 /* Turn the warning message for warning #100 back on */
Check your compiler’s documentation for a list of #pragma directives. As stated earlier, each compiler’s
implementation of this feature is different, and what works on one compiler almost certainly won’t work on
another. Nevertheless, the #pragma directives can come in very handy when you’re turning on and off some
of your compiler’s favorite (or most annoying) features.

Cross Reference:

V.2: What will the preprocessor do for a program?
V.23: What is #line used for?