Different mains

Basic main Function:

#include <stdio.h>

int main() {
    // Program logic goes here

    return 0;
}

Explanation:

• This is the simplest form of the main function.

• It does not accept any command-line arguments.

• The program logic is placed within the curly braces.

• It returns an integer value (0 in this case) to the operating system, indicating successful execution.

Below are examples of main function signatures that use argc, argv, and envp, along with sample code that demonstrates their usage. I'll also provide comments explaining each part of the code and show how to execute the program in a Bash environment, passing command-line arguments and setting environment variables.

1. main Function with argc and argv:

#include <stdio.h>

int main(int argc, char *argv[]) {
    // argc: Number of command-line arguments
    // argv: Array of command-line argument strings

    // Check if at least two command-line arguments are provided
    if (argc < 3) {
        printf("Usage: %s <arg1> <arg2>\n", argv[0]);
        return 1; // Return 1 to indicate an error
    }

    // Access command-line arguments
    printf("First argument: %s\n", argv[1]);
    printf("Second argument: %s\n", argv[2]);

    return 0;
}

Execution and Output in Bash:

$ gcc command_line_args.c -o command_line_args
$ ./command_line_args arg1_value arg2_value
First argument: arg1_value
Second argument: arg2_value

2. main Function with argc, argv, and envp:

#include <stdio.h>

int main(int argc, char *argv[], char *envp[]) {
    // argc: Number of command-line arguments
    // argv: Array of command-line argument strings
    // envp: Array of environment variable strings

    // Access command-line arguments
    printf("Command-line arguments:\n");
    for (int i = 0; i < argc; ++i) {
        printf("  %s\n", argv[i]);
    }

    // Access environment variables
    printf("\nEnvironment variables:\n");
    for (int i = 0; envp[i] != NULL; ++i) {
        printf("  %s\n", envp[i]);
    }

    return 0;
}

Execution and Output in Bash:

$ gcc env_args.c -o env_args
$ ./env_args arg1 arg2
Command-line arguments:
  ./env_args
  arg1
  arg2

Environment variables:
  PATH=...
  HOME=...
  ...

$ MY_ENV_VAR=123 ./env_args arg1 arg2
Command-line arguments:
  ./env_args
  arg1
  arg2

Environment variables:
  PATH=...
  HOME=...
  ...
  MY_ENV_VAR=123

These examples demonstrate how to use argc, argv, and envp in a C program, and the Bash commands show how to compile and execute the programs while providing command-line arguments and setting environment variables.