Memory

LWHTTP takes all of its memory from the lwIP-CE heap, the same 42 KB pool that lwIP-CE uses for sockets, packet buffers and TLS. It has no static buffers of its own beyond a few bytes.

Per client

lwhttp_client_create() allocates one block:

Part	Size
Client state	about 200 bytes
Receive buffer	512 bytes
Send buffer	512 bytes
Header store	lwhttp_client_config_t.header_size, 4096 by default

The default client therefore needs about 5.3 KB of contiguous heap.

Per request

While a request runs, LWHTTP also holds a copy of the URL and host name and the serialized request head, together usually well under 1 KB. lwIP-CE adds its own socket state, a receive ring of 512 bytes to 4 KB and, for HTTPS, its TLS context.

Sizing the header store

The header store holds the response head and trailers, one field after another. Each field costs its name, its value and two bytes. 128 bytes always stay free for chunk-size and trailer lines, and the longest single header line must also fit.

Server	Response header section
Small APIs, httpbin.org	under 1.5 KB
duckduckgo.com	about 3 KB, with a 2.2 KB Content-Security-Policy line
github.com	about 5.5 KB, with a 4 KB line

When you only talk to your own server, 1 KB is often enough. The minimum is 256 bytes.

Recommendations

• Create one client per concurrent request and reuse it; creating and destroying clients repeatedly fragments the heap.

• Keep few clients alive at once, especially next to HTTPS connections.

• Stream large bodies with on_body instead of collecting them in memory.