2.3 The main Function: The Entry Point

Execution of a Rust binary begins at the main function, just like in C. By convention, this function often resides in a file named src/main.rs within a Cargo project. A project can contain multiple .rs files organized into modules and potentially link against library crates.

2.3.1 A Minimal Rust Program

fn main() {
    println!("Hello, world!");
}
  • fn: Keyword to declare a function.
  • main: The special name for the program’s entry point.
  • (): Parentheses enclose the function’s parameter list (empty in this case).
  • {}: Curly braces enclose the function’s body.
  • println!: A macro (indicated by the !) for printing text to the standard output, followed by a newline.
  • ;: Semicolons terminate most statements.
  • Rust follows indentation conventions similar to those in C, but—as in C—this indentation is purely for readability and has no effect on the compiler.

2.3.2 Comparison with C

#include <stdio.h>

int main(void) { // Or int main(int argc, char *argv[])
    printf("Hello, world!\n");
    return 0; // Return 0 to indicate success
}
  • C’s main typically returns an int status code (0 for success).
  • Rust’s main function, by default, returns the unit type (), implicitly indicating success. It can be declared to return a Result type for more explicit error handling, as we’ll see later.