# Different mains

### Basic `main` Function:

```c
#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`:

```c
#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:**

```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`:

```c
#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:**

```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.