curl

curl is a Rust crate that provides safe, idiomatic bindings to libcurl, the battle-tested C transfer library that powers countless applications. It lets Rust programs perform HTTP, HTTPS, FTP, and many other protocol transfers by wrapping libcurl’s easy (single transfer) and multi (concurrent, event-driven) interfaces.

Because it builds on libcurl, the crate inherits a mature feature set including TLS via the platform’s native stack or OpenSSL, cookies, proxies, authentication, and fine-grained transfer control. It is a foundational HTTP client in the Rust ecosystem, used directly and as a backend for higher-level libraries such as Cargo’s networking.

What You Get

• An Easy handle for performing individual HTTP(S) and other-protocol requests with callback-based I/O.
• A Multi interface for running many transfers concurrently under your own event loop.
• Access to libcurl’s rich options: headers, cookies, proxies, authentication, timeouts, and TLS configuration.
• Platform-native TLS (SChannel on Windows) or OpenSSL on Unix, plus an optional static build.
• The low-level curl-sys FFI crate underneath for direct libcurl access when needed.

Common Use Cases

• Making HTTP/HTTPS requests from a Rust application or CLI with a proven networking stack.
• Downloading or uploading files over FTP, HTTP, and other libcurl-supported protocols.
• Serving as the HTTP backend for higher-level Rust tools (e.g. Cargo) that need reliable, configurable transfers.

Under The Hood

Architecture - The crate is a safe wrapper over the curl-sys FFI bindings (a workspace member built via build.rs, which can vendor and compile libcurl). src/easy/ implements the Easy/Easy2 handles and a Handler trait for read/write/progress callbacks, src/multi.rs wraps libcurl’s multi interface for concurrent transfers, and src/error.rs maps libcurl return codes into typed Rust errors. A panic.rs module carefully manages unwinding across the FFI boundary from user callbacks.

Tech Stack - Rust 2018 edition, depending on libc, socket2, and the in-workspace curl-sys. TLS is platform-specific: schannel and windows-sys on MSVC targets, openssl-sys/openssl-probe on Unix. The build integrates with system or vendored libcurl through cc.

Code Quality - The repository carries an extensive tests/ suite (easy, multi, post, formdata, protocols, plus an in-repo test server) and a systest crate that validates the FFI signatures against libcurl’s headers. As a long-lived, widely depended-on crate (used by Cargo), it is conservatively maintained with strong CI across platforms.

API Design - The Easy API mirrors libcurl’s option-setting model: you configure a handle, register callback closures, and call perform(). This is powerful and complete but lower-level than convenience HTTP clients, so callers manage callbacks and error unwrapping themselves. The transfer() borrow pattern and the Multi interface add ergonomics for capturing output and running concurrent requests.