Rust for C-Programmers

23.10 Binary vs. Library Crates

Cargo distinguishes between packages that produce executable binary crates and those that produce library crates:

• Binary Packages/Crates: Compile to an executable file. They must have a src/main.rs file containing a fn main() function, which serves as the program’s entry point for the main binary crate. cargo new <name> creates a binary package by default, containing one binary crate.
• Library Packages/Crates: Compile to a Rust library file (.rlib or .dylib) intended to be used as a dependency by other packages. They typically have a src/lib.rs file as their crate root. cargo new <name> --lib creates a library package, containing one library crate.

A single package can contain both a library crate and one or more binary crates:

• Define the library crate in src/lib.rs.
• Define the main binary crate in src/main.rs.
• Define additional binary crates in src/bin/another_bin.rs, src/bin/yet_another.rs, etc.

Cargo will build the library crate and all specified binary crates within the package. This pattern is common for packages that provide both a reusable library API and a command-line tool interface (as separate crates within the same package).