What is ## operator in C?

In C, #pragma is a preprocessor directive that provides additional information to the compiler. It is a way to give special instructions to the compiler about how to process the code. The #pragma directive is followed by an identifier that indicates the specific action to be taken.

Regarding executing functions before and after the main function in a C program, you can use the #pragma startup and #pragma exit directives.

1. Before main Function:

#include <stdio.h>

void startup_function(void) {
    printf("Function executed before main\n");
}

#pragma startup startup_function

int main() {
    printf("Inside main function\n");
    return 0;
}

1. The #pragma startup directive is used to specify a function (startup_function in this case) that will be executed before the main function.

2. After main Function:

#include <stdio.h>

void exit_function(void) {
    printf("Function executed after main\n");
}

#pragma exit exit_function

int main() {
    printf("Inside main function\n");
    return 0;
}

1. The #pragma exit directive is used to specify a function (exit_function in this case) that will be executed after the main function.

It's important to note that the usage of #pragma directives is compiler-specific, and not all compilers support these directives. The examples provided here are based on compilers that support these specific directives, like Turbo C or certain versions of GCC. Always check the documentation of your compiler for the supported directives and their behavior.

To override an existing macro, we need to undefine existing macro using “#undef”. Then, we need to define same macro again with new value.

#ifdef pre-processor directive checks whether particular macro is defined or not in a C program.