file_body buffer size can be configured.

doc/qbk/03_core/8_conf_macros.qbk

@@ -35,6 +35,13 @@ failure to do so may result in violations of ODR (One Definition Rule).
         Enables the use of deprecated APIs within Beast.
     ]
 ]
+[
+    [
+        BOOST_BEAST_FILE_BUFFER_SIZE
+    ][
+        Sets the small buffer size for the file_body. Defaults to 4096.
+    ]
+]
 ]
 
 [endsect]

include/boost/beast/core/detail/config.hpp

@@ -113,4 +113,8 @@ namespace net = boost::asio;
 
 #endif
 
+#ifndef BOOST_BEAST_FILE_BUFFER_SIZE
+#define BOOST_BEAST_FILE_BUFFER_SIZE 4096
+#endif
+
 #endif

include/boost/beast/http/basic_file_body.hpp

@@ -234,9 +234,9 @@ size(value_type const& body)
 template<class File>
 class basic_file_body<File>::writer
 {
-    value_type& body_;      // The body we are reading from
-    std::uint64_t remain_;  // The number of unread bytes
-    char buf_[4096];        // Small buffer for reading
+    value_type& body_;                       // The body we are reading from
+    std::uint64_t remain_;                   // The number of unread bytes
+    char buf_[BOOST_BEAST_FILE_BUFFER_SIZE]; // Small buffer for reading
 
 public:
     // The type of buffer sequence returned by `get`.

include/boost/beast/http/impl/file_body_win32.hpp

@@ -124,9 +124,9 @@ struct basic_file_body<file_win32>
                 basic_file_body<file_win32>, Fields>& sr,
             error_code& ec);
 
-        value_type& body_;  // The body we are reading from
-        std::uint64_t pos_; // The current position in the file
-        char buf_[4096];    // Small buffer for reading
+        value_type& body_;                       // The body we are reading from
+        std::uint64_t pos_;                      // The current position in the file
+        char buf_[BOOST_BEAST_FILE_BUFFER_SIZE]; // Small buffer for reading
 
     public:
         using const_buffers_type =