Primary C Concepts: Main Function

1. So, what is the main function in C?

The main function in the C programming language is necessary for programs to run and to be compiled. In the main function program starts it's execution, making this function an entry point to your program. The main function, just like any other function in C, is opened and closed with curly braces. Within these curly braces, then, tasks are performed, such as calling of other functions, definitions of variables, printing of information and so on.

Return type of the portable main function is int, hence an integer has to be returned from this function.

Note: returned integer from the main function indicates exit status of the program.

2. What are the different types of the main function in C?

In the C programming language, there are quite a number of ways of defining the main function. Different types of main functions can be grouped into the following two categories:

• Portable main functions
• Non-portable main functions

2.1 Portable main functions in C

The portable main functions in C are supported by all operating systems. These main functions can be compiled and can run on any OS that has a C compiler.

int main(void);
int main(int argc, char *argv[]);

The difference between these two types of portable functions lays in the parameters' part. In the case of int main(void), a single parameter is void. Meaning that this main function does not receive any command-line arguments that are passed during the execution of the program.

In contrast with int main(int argc, char *argv[]). This type of portable main function receives two parameters from the command-line. First parameter is int argc, which indicates the number of arguments that were provided in the command-line during the execution. Assuming our program was executed in such a way: ./a.out "one argument" "another argument", then argc in the main program would receive the integer value of 3 (1. ./a.out, 2. "one argument", 3. "another argument").

Second parameter is char *argv[]. Which is an array of pointers that points to each argument written in the command-line during execution. Given your program is executed with ./a.out "one argument" "another argument", then argv would contain these four elements.

argv[0] - "./a.out"
argv[1] - "one argument"
argv[2] - "another argument"
argv[3] - NULL

2.2 Non-portable main functions in C

Non-portable main functions are those main functions in C that compile and run only on a limited number of operating systems and are not fully supported. Here are two examples of such functions below:

void main(void);
int main(int argc, char *argv[], char *env[]);

The void main(void) function does not return any data because the return type of this function is void nor does it receive any data from the command-line.

Note: return statement within functions with a return type of void can be used without specification of return value ( return ; )

The int main(int argc, char *argv[], char *env[]) function has three parameters. Two of which, we have already discussed above. Third parameter is mostly supported in UNIX systems. This parameter (envp) receives a two-dimensional array. This array, in turn, contains pointers to the environment variables and is terminated by a NULL pointer. To test if your operating system supports this type of main function and to print out environment variables, feel free to compile and run the code below.

Example 1:.

To summarize different types of the main function in C, we could assert that C offers multiple ways to enter programs. This can be utilized and tailored to the specific needs of your program.