HelenOS sources
http_headers_append 174 uspace/app/download/main.c rc = http_headers_append(&req->headers, "Host", uri->host);
http_headers_append 180 uspace/app/download/main.c rc = http_headers_append(&req->headers, "User-Agent", USER_AGENT);
http_headers_append 103 uspace/lib/http/include/http/http.h extern errno_t http_headers_append(http_headers_t *, const char *, const char *);
http_headers_append 301 uspace/lib/http/src/headers.c return http_headers_append(headers, name, value);
HelenOS homepage, sources at GitHub