Table of contents

Can I use Reqwest in a no-std environment?

Unfortunately, Reqwest cannot be used directly in a no-std environment. Reqwest is built on top of Tokio and requires the standard library (std) for its core functionality, including async runtime support, TCP connections, and HTTP protocol implementation. However, there are several alternative approaches and workarounds for HTTP client functionality in no-std environments.

Why Reqwest Requires std

Reqwest depends on several standard library components that are not available in no-std environments:

  • Tokio Runtime: Requires std::thread and OS-level threading primitives
  • TCP Sockets: Uses std::net::TcpStream for network connections
  • Heap Allocation: Relies on std::collections and dynamic memory allocation
  • Error Handling: Uses std::error::Error trait extensively
  • Async Executor: Requires the standard library's async runtime support
// This will NOT compile in no-std
#![no_std]
use reqwest; // Error: reqwest requires std

Alternative HTTP Clients for no-std

1. embedded-svc with HTTP Client

For embedded systems, consider using embedded-svc with appropriate HTTP client implementations:

#![no_std]
#![no_main]

use embedded_svc::http::client::Client;
use embedded_svc::io::Write;
use esp_idf_svc::http::client::EspHttpConnection;

extern crate alloc;
use alloc::vec::Vec;

fn make_http_request() -> Result<Vec<u8>, Box<dyn core::error::Error>> {
    let connection = EspHttpConnection::new(&Default::default())?;
    let mut client = Client::wrap(connection);

    let response = client.get("https://api.example.com/data")?
        .submit()?;

    let mut body = Vec::new();
    response.read_to_end(&mut body)?;
    Ok(body)
}

2. Custom HTTP Implementation with heapless

Build a minimal HTTP client using heapless for stack-allocated data structures:

#![no_std]

use heapless::{String, Vec};
use nb_connect::{ConnectError, TcpConnect};

const MAX_RESPONSE_SIZE: usize = 1024;
const MAX_URL_SIZE: usize = 256;

struct NoStdHttpClient<T: TcpConnect> {
    tcp: T,
}

impl<T: TcpConnect> NoStdHttpClient<T> {
    pub fn new(tcp: T) -> Self {
        Self { tcp }
    }

    pub fn get(&mut self, url: &str) -> Result<Vec<u8, MAX_RESPONSE_SIZE>, HttpError> {
        let mut request: String<MAX_URL_SIZE> = String::new();
        request.push_str("GET ").unwrap();
        request.push_str(url).unwrap();
        request.push_str(" HTTP/1.1\r\n\r\n").unwrap();

        // Send request and receive response
        let mut response = Vec::new();
        // Implementation depends on your TCP stack
        Ok(response)
    }
}

#[derive(Debug)]
enum HttpError {
    ConnectionFailed,
    RequestTooLarge,
    ResponseTooLarge,
}

3. Using smoltcp for Network Stack

For a complete no-std networking solution, combine smoltcp with custom HTTP implementation:

#![no_std]

use smoltcp::wire::{IpAddress, IpCidr};
use smoltcp::iface::{InterfaceBuilder, NeighborCache, Routes};
use smoltcp::socket::{TcpSocket, TcpSocketBuffer};
use heapless::{Vec, consts::*};

fn create_http_client() {
    // Create network interface
    let mut routes_storage = [None; 1];
    let mut routes = Routes::new(&mut routes_storage[..]);

    let mut neighbor_cache_storage = [None; 8];
    let mut neighbor_cache = NeighborCache::new(&mut neighbor_cache_storage[..]);

    // TCP socket setup
    let tcp_rx_buffer = TcpSocketBuffer::new(vec![0; 1024]);
    let tcp_tx_buffer = TcpSocketBuffer::new(vec![0; 1024]);
    let tcp_socket = TcpSocket::new(tcp_rx_buffer, tcp_tx_buffer);

    // HTTP request implementation would go here
}

Feature Flags and Conditional Compilation

Some crates offer no-std compatibility through feature flags. While Reqwest doesn't support this, you can structure your code to conditionally use different HTTP clients:

#[cfg(feature = "std")]
use reqwest;

#[cfg(not(feature = "std"))]
use embedded_svc::http::client::Client;

#[cfg(feature = "std")]
async fn fetch_data(url: &str) -> Result<String, reqwest::Error> {
    let response = reqwest::get(url).await?;
    response.text().await
}

#[cfg(not(feature = "std"))]
fn fetch_data_no_std(url: &str) -> Result<heapless::Vec<u8, 1024>, MyError> {
    // Embedded HTTP client implementation
    unimplemented!()
}

Async Support in no-std

For async functionality without std, consider using embassy-executor or async-embedded:

#![no_std]
#![no_main]

use embassy_executor::Spawner;
use embassy_time::{Duration, Timer};
use embassy_net::{tcp::TcpSocket, Stack, StackResources};

#[embassy_executor::task]
async fn http_client_task(stack: &'static Stack<cyw43::NetDriver<'static>>) {
    let mut rx_buffer = [0; 1024];
    let mut tx_buffer = [0; 1024];
    let mut socket = TcpSocket::new(stack, &mut rx_buffer, &mut tx_buffer);

    // Connect and send HTTP request
    socket.connect(("httpbin.org", 80)).await.unwrap();
    socket.write_all(b"GET /ip HTTP/1.1\r\nHost: httpbin.org\r\n\r\n").await.unwrap();

    let mut response = [0; 512];
    let n = socket.read(&mut response).await.unwrap();
    // Process response
}

Memory Management Considerations

When working in no-std environments, carefully manage memory allocation:

Stack-Allocated Buffers

#![no_std]

use heapless::{Vec, String};

const BUFFER_SIZE: usize = 512;

fn parse_http_response(raw_response: &[u8]) -> Option<String<BUFFER_SIZE>> {
    // Find the start of HTTP body
    let body_start = raw_response
        .windows(4)
        .position(|window| window == b"\r\n\r\n")?
        + 4;

    let body = &raw_response[body_start..];
    let mut result = String::new();

    for &byte in body.iter().take(BUFFER_SIZE - 1) {
        if result.push(byte as char).is_err() {
            break;
        }
    }

    Some(result)
}

Using External Allocators

If you need dynamic allocation, consider linked_list_allocator or heapless:

#![no_std]
#![no_main]

extern crate alloc;
use alloc::{vec::Vec, string::String};
use linked_list_allocator::LockedHeap;

#[global_allocator]
static ALLOCATOR: LockedHeap = LockedHeap::empty();

fn init_heap() {
    use linked_list_allocator::LockedHeap;
    const HEAP_SIZE: usize = 1024 * 64; // 64KB heap
    static mut HEAP: [u8; HEAP_SIZE] = [0; HEAP_SIZE];

    unsafe { ALLOCATOR.lock().init(HEAP.as_ptr() as usize, HEAP_SIZE) }
}

Best Practices for no-std HTTP Clients

1. Minimize Dependencies

Keep your dependency tree small and focused:

[dependencies]
heapless = "0.7"
nb = "1.0"
embedded-hal = "0.2"

# Avoid heavy dependencies like:
# reqwest = "0.11"  # Requires std
# tokio = "1.0"     # Requires std

2. Error Handling

Implement lightweight error types:

#![no_std]

#[derive(Debug, Clone, Copy)]
enum HttpError {
    NetworkError,
    ParseError,
    BufferOverflow,
    InvalidResponse,
}

impl core::fmt::Display for HttpError {
    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
        match self {
            HttpError::NetworkError => write!(f, "Network connection failed"),
            HttpError::ParseError => write!(f, "Failed to parse HTTP response"),
            HttpError::BufferOverflow => write!(f, "Response too large for buffer"),
            HttpError::InvalidResponse => write!(f, "Invalid HTTP response format"),
        }
    }
}

3. Configuration Management

Use const generics for compile-time configuration:

#![no_std]

struct HttpClient<const BUFFER_SIZE: usize, const MAX_HEADERS: usize> {
    buffer: [u8; BUFFER_SIZE],
    headers: [(&'static str, &'static str); MAX_HEADERS],
}

impl<const BUFFER_SIZE: usize, const MAX_HEADERS: usize> 
    HttpClient<BUFFER_SIZE, MAX_HEADERS> 
{
    const fn new() -> Self {
        Self {
            buffer: [0; BUFFER_SIZE],
            headers: [("", ""); MAX_HEADERS],
        }
    }
}

// Usage
const CLIENT: HttpClient<1024, 10> = HttpClient::new();

Working with Different Embedded Platforms

ESP32 with embassy-net

#![no_std]
#![no_main]

use embassy_net::{Config, Stack, StackResources};
use embassy_esp_wifi::{initialize, EspWifiInitFor};

#[embassy_executor::main]
async fn main(spawner: Spawner) {
    let peripherals = esp32::Peripherals::take();
    let wifi = peripherals.WIFI;

    let (wifi_interface, controller) = 
        esp_wifi::wifi::new_with_mode(&init, wifi, WifiStaDevice)?;

    let config = Config::dhcpv4(Default::default());
    let seed = 1234; // Use real entropy source
    let stack = &*make_static!(Stack::new(
        wifi_interface,
        config,
        make_static!(StackResources::<3>::new()),
        seed
    ));

    spawner.spawn(connection_task(controller)).ok();
    spawner.spawn(net_task(stack)).ok();
    spawner.spawn(http_client_task(stack)).ok();
}

STM32 with Embassy

#![no_std]
#![no_main]

use embassy_stm32::{eth, peripherals::ETH, rng::Rng};
use embassy_net::{Stack, StackResources, Config};

#[embassy_executor::main]
async fn main(spawner: Spawner) -> ! {
    let p = embassy_stm32::init(Default::default());

    let eth = eth::Ethernet::new(
        make_static!(eth::PacketQueue::<16, 16>::new()),
        p.ETH,
        Irqs,
        p.PA1,
        p.PA2,
        // ... other pins
        eth::GenericSMI::new(0),
        make_static!([0u8; 1536]),
        LocalMac,
    );

    let config = Config::dhcpv4(Default::default());
    let stack = &*make_static!(Stack::new(
        eth,
        config,
        make_static!(StackResources::<3>::new()),
        seed
    ));

    spawner.spawn(net_task(stack)).unwrap();
    spawner.spawn(http_task(stack)).unwrap();

    loop {
        Timer::after(Duration::from_secs(1)).await;
    }
}

HTTP/1.1 Protocol Implementation

For complete control, implement a minimal HTTP/1.1 client:

#![no_std]

use heapless::{String, LinearMap};

const MAX_HEADERS: usize = 16;
const MAX_HEADER_SIZE: usize = 128;

pub struct HttpRequest<const N: usize> {
    method: &'static str,
    path: String<N>,
    headers: LinearMap<String<MAX_HEADER_SIZE>, String<MAX_HEADER_SIZE>, MAX_HEADERS>,
    body: Option<String<N>>,
}

impl<const N: usize> HttpRequest<N> {
    pub fn new(method: &'static str, path: &str) -> Result<Self, &'static str> {
        let mut path_string = String::new();
        path_string.push_str(path).map_err(|_| "Path too long")?;

        Ok(Self {
            method,
            path: path_string,
            headers: LinearMap::new(),
            body: None,
        })
    }

    pub fn add_header(&mut self, key: &str, value: &str) -> Result<(), &'static str> {
        let mut key_string = String::new();
        let mut value_string = String::new();

        key_string.push_str(key).map_err(|_| "Header key too long")?;
        value_string.push_str(value).map_err(|_| "Header value too long")?;

        self.headers.insert(key_string, value_string)
            .map_err(|_| "Too many headers")?;

        Ok(())
    }

    pub fn serialize(&self) -> Result<String<N>, &'static str> {
        let mut request = String::new();

        // Request line
        request.push_str(self.method).map_err(|_| "Request too large")?;
        request.push(' ').map_err(|_| "Request too large")?;
        request.push_str(&self.path).map_err(|_| "Request too large")?;
        request.push_str(" HTTP/1.1\r\n").map_err(|_| "Request too large")?;

        // Headers
        for (key, value) in &self.headers {
            request.push_str(key).map_err(|_| "Request too large")?;
            request.push_str(": ").map_err(|_| "Request too large")?;
            request.push_str(value).map_err(|_| "Request too large")?;
            request.push_str("\r\n").map_err(|_| "Request too large")?;
        }

        // Empty line before body
        request.push_str("\r\n").map_err(|_| "Request too large")?;

        // Body
        if let Some(ref body) = self.body {
            request.push_str(body).map_err(|_| "Request too large")?;
        }

        Ok(request)
    }
}

Testing no-std HTTP Clients

Unit Testing with std

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_http_request_creation() {
        let mut request = HttpRequest::<512>::new("GET", "/api/data").unwrap();
        request.add_header("Host", "example.com").unwrap();
        request.add_header("User-Agent", "no-std-client/1.0").unwrap();

        let serialized = request.serialize().unwrap();
        assert!(serialized.contains("GET /api/data HTTP/1.1"));
        assert!(serialized.contains("Host: example.com"));
    }
}

Integration Testing with Embedded Simulator

#[cfg(test)]
mod integration_tests {
    use std::net::TcpListener;
    use std::io::{Read, Write};
    use std::thread;

    #[test]
    fn test_http_client_with_mock_server() {
        let listener = TcpListener::bind("127.0.0.1:0").unwrap();
        let port = listener.local_addr().unwrap().port();

        thread::spawn(move || {
            for stream in listener.incoming() {
                let mut stream = stream.unwrap();
                let mut buffer = [0; 1024];
                stream.read(&mut buffer).unwrap();

                let response = "HTTP/1.1 200 OK\r\nContent-Length: 13\r\n\r\nHello, World!";
                stream.write_all(response.as_bytes()).unwrap();
            }
        });

        // Test your no-std HTTP client here
    }
}

Conclusion

While Reqwest cannot be used directly in no-std environments, there are several viable alternatives for HTTP client functionality. The choice depends on your specific requirements:

  • Embedded Systems: Use embedded-svc or embassy-net for ESP32/STM32 platforms
  • Custom Implementations: Build minimal HTTP clients with heapless and smoltcp
  • Hybrid Approaches: Use conditional compilation to support both std and no-std targets

For complex web scraping scenarios that require JavaScript execution, consider using browser automation tools for JavaScript-heavy websites in your development environment, then deploy simplified HTTP clients in production embedded systems.

The key is to balance functionality with the constraints of your no-std environment, focusing on essential HTTP features while maintaining memory efficiency and reliability. Consider implementing timeout handling and proper error recovery mechanisms to ensure robust operation in resource-constrained environments.

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Related Questions

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