The Symcall plugin

The symcall plugin (modifier 18) is a convenience plugin allowing you to write native uWSGI request handlers without the need of developing a full uWSGI plugin.

You tell it which symbol to load on startup and then it will run it at every request.


The “symcall” plugin is built-in by default in standard build profiles.

Step 1: preparing the environment

The uWSGI binary by itself allows you to develop plugins and libraries without the need of external development packages or headers.

The first step is getting the uwsgi.h C/C++ header:

uwsgi --dot-h > uwsgi.h

Now, in the current directory, we have a fresh uwsgi.h ready to be included.

Step 2: our first request handler:

Our C handler will print the REMOTE_ADDR value with a couple of HTTP headers.

(call it mysym.c or whatever you want/need)

#include "uwsgi.h"

int mysym_function(struct wsgi_request *wsgi_req) {

     // read request variables
     if (uwsgi_parse_vars(wsgi_req)) {
             return -1;

     // get REMOTE_ADDR
     uint16_t vlen = 0;
     char *v = uwsgi_get_var(wsgi_req, "REMOTE_ADDR", 11, &vlen);

     // send status
     if (uwsgi_response_prepare_headers(wsgi_req, "200 OK", 6)) return -1;
     // send content_type
     if (uwsgi_response_add_content_type(wsgi_req, "text/plain", 10)) return -1;
     // send a custom header
     if (uwsgi_response_add_header(wsgi_req, "Foo", 3, "Bar", 3)) return -1;

     // send the body
     if (uwsgi_response_write_body_do(wsgi_req, v, vlen)) return -1;

     return UWSGI_OK;

Step 3: building our code as a shared library

The uwsgi.h file is an ifdef hell (so it’s probably better not to look at it too closely).

Fortunately the uwsgi binary exposes all of the required CFLAGS via the –cflags option.

We can build our library in one shot:

gcc -fPIC -shared -o `uwsgi --cflags` mysym.c

you now have the library ready to be loaded in uWSGI

Final step: map the symcall plugin to the mysym_function symbol

uwsgi --dlopen ./ --symcall mysym_function --http-socket :9090 --http-socket-modifier1 18

With --dlopen we load a shared library in the uWSGI process address space.

The --symcall option allows us to specify which symbol to call when modifier1 18 is in place

We bind the instance to HTTP socket 9090 forcing modifier1 18.

Hooks and symcall unleashed: a TCL handler

We want to write a request handler running the following TCL script (foo.tcl) every time:

# call it foo.tcl
proc request_handler { remote_addr path_info query_string } {
     set upper_pathinfo [string toupper $path_info]
     return "Hello $remote_addr $upper_pathinfo $query_string"

We will define a function for initializing the TCL interpreter and parsing the script. This function will be called on startup soon after privileges drop.

Finally we define the request handler invoking the TCL proc and passign args to it

#include <tcl.h>
#include "uwsgi.h"

// global interpreter
static Tcl_Interp *tcl_interp;

// the init function
void ourtcl_init() {
     // create the TCL interpreter
     tcl_interp = Tcl_CreateInterp() ;
     if (!tcl_interp) {
             uwsgi_log("unable to initialize TCL interpreter\n");

     // initialize the interpreter
     if (Tcl_Init(tcl_interp) != TCL_OK) {
             uwsgi_log("Tcl_Init error: %s\n", Tcl_GetStringResult(tcl_interp));

     // parse foo.tcl
     if (Tcl_EvalFile(tcl_interp, "foo.tcl") != TCL_OK) {
             uwsgi_log("Tcl_EvalFile error: %s\n", Tcl_GetStringResult(tcl_interp));

     uwsgi_log("TCL engine initialized");

// the request handler
int ourtcl_handler(struct wsgi_request *wsgi_req) {

     // get request vars
     if (uwsgi_parse_vars(wsgi_req)) return -1;

     Tcl_Obj *objv[4];
     // the proc name
     objv[0] = Tcl_NewStringObj("request_handler", -1);
     objv[1] = Tcl_NewStringObj(wsgi_req->remote_addr, wsgi_req->remote_addr_len);
     // PATH_INFO
     objv[2] = Tcl_NewStringObj(wsgi_req->path_info, wsgi_req->path_info_len);
     objv[3] = Tcl_NewStringObj(wsgi_req->query_string, wsgi_req->query_string_len);

     // call the proc
     if (Tcl_EvalObjv(tcl_interp, 4, objv, TCL_EVAL_GLOBAL) != TCL_OK) {
             // ERROR, report it to the browser
             if (uwsgi_response_prepare_headers(wsgi_req, "500 Internal Server Error", 25)) return -1;
             if (uwsgi_response_add_content_type(wsgi_req, "text/plain", 10)) return -1;
             char *body = (char *) Tcl_GetStringResult(tcl_interp);
             if (uwsgi_response_write_body_do(wsgi_req, body, strlen(body))) return -1;
             return UWSGI_OK;

     // all fine
     if (uwsgi_response_prepare_headers(wsgi_req, "200 OK", 6)) return -1;
     if (uwsgi_response_add_content_type(wsgi_req, "text/plain", 10)) return -1;

     // write the result
     char *body = (char *) Tcl_GetStringResult(tcl_interp);
     if (uwsgi_response_write_body_do(wsgi_req, body, strlen(body))) return -1;
     return UWSGI_OK;

You can build it with:

gcc -fPIC -shared -o `./uwsgi/uwsgi --cflags` -I/usr/include/tcl ourtcl.c -ltcl

The only differences from the previous example are the -I and -l for adding the TCL headers and library.

So, let’s run it with:

uwsgi --dlopen ./ --hook-as-user call:ourtcl_init --http-socket :9090 --symcall ourtcl_handler --http-socket-modifier1 18

Here the only new player is --hook-as-user call:ourtcl_init invoking the specified function after privileges drop.


This code is not thread safe! If you want to improve this tcl library to support multithreading, best approach will be having a TCL interpreter for each pthread instead of a global one.


Since uWSGI 1.9.21, thanks to the --build-plugin option, developing uWSGI plugins has become really easy.

The symcall plugin is for tiny libraries/pieces of code, for bigger needs consider developing a full plugin.

The tcl example we have seen before is maybe the right example of “wrong” usage ;)