VMOD - Varnish Modules

For all you can do in VCL, there are things you can not do. Look an IP number up in a database file for instance. VCL provides for inline C code, and there you can do everything, but it is not a convenient or even readable way to solve such problems.

This is where VMODs come into the picture: A VMOD is a shared library with some C functions which can be called from VCL code.

For instance:

import std;

sub vcl_deliver {
        set resp.http.foo = std.toupper(req.url);

The "std" vmod is one you get with Varnish, it will always be there and we will put "boutique" functions in it, such as the "toupper" function shown above. The full contents of the "std" module is documented in vmod_std(7).

This part of the manual is about how you go about writing your own VMOD, how the language interface between C and VCC works etc. This explanation will use the "std" VMOD as example, having a varnish source tree handy may be a good idea.

The vmod.vcc file

The interface between your VMOD and the VCL compiler ("VCC") and the VCL runtime ("VRT") is defined in the vmod.vcc file which a python script called "vmod.py" turns into thaumathurgically challenged C data structures that does all the hard work.

The std VMODs vmod.vcc file looks somewhat like this:

Module std
Init init_function
Function VOID set_ip_tos(INT)

The first line gives the name of the module, nothing special there.

The second line specifies an optional "Init" function, which will be called whenever a VCL program which imports this VMOD is loaded. This gives a chance to initialize the module before any of the functions it implements are called.

The next three lines specify two functions in the VMOD, along with the types of the arguments, and that is probably where the hardest bit of writing a VMOD is to be found, so we will talk about that at length in a moment.

Notice that the third function returns VOID, that makes it a "procedure" in VCL lingo, meaning that it cannot be used in expressions, right side of assignments and such places. Instead it can be used as a primary action, something functions which return a value can not:

sub vcl_recv {

Running vmod.py on the vmod.vcc file, produces an "vcc_if.c" and "vcc_if.h" files, which you must use to build your shared library file.

Forget about vcc_if.c everywhere but your Makefile, you will never need to care about its contents, and you should certainly never modify it, that voids your warranty instantly.

But vcc_if.h is important for you, it contains the prototypes for the functions you want to export to VCL.

For the std VMOD, the compiled vcc_if.h file looks like this:

struct sess;
struct VCL_conf;
const char * vmod_toupper(struct sess *, struct vmod_priv *, const char *, ...);
const char * vmod_tolower(struct sess *, struct vmod_priv *, const char *, ...);
int meta_function(void **, const struct VCL_conf *);

Those are your C prototypes. Notice the vmod_ prefix on the function names and the C-types as return types and arguments.

VCL and C data types

VCL data types are targeted at the job, so for instance, we have data types like "DURATION" and "HEADER", but they all have some kind of C language representation. Here is a description of them, from simple to nasty.


C-type: int

An integer as we know and love them.


C-type: double

A floating point value


C-type: double

Units: seconds

A time interval, as in "25 minutes".


C-type: double

Units: seconds since UNIX epoch

An absolute time, as in "Mon Sep 13 19:06:01 UTC 2010".


C-type: const char *

A NUL-terminated text-string.

Can be NULL to indicate that the nonexistent string, for instance:


If there were no "foobar" HTTP header, the vmod_foo() function would be passed a NULL pointer as argument.

When used as a return value, the producing function is responsible for arranging memory management. Either by freeing the string later by whatever means available or by using storage allocated from the session or worker workspaces.


C-type: const char *, ...

A multi-component text-string. We try very hard to avoid doing text-processing in Varnish, and this is one way we do that, by not editing separate pieces of a sting together to one string, until we need to.

Consider this contrived example:

set bereq.http.foo = std.toupper(req.http.foo + req.http.bar);

The usual way to do this, would be be to allocate memory for the concatenated string, then pass that to toupper() which in turn would return another freshly allocated string with the modified result. Remember: strings in VCL are const, we cannot just modify the string in place.

What we do instead, is declare that toupper() takes a "STRING_LIST" as argument. This makes the C function implementing toupper() a vararg function (see the prototype above) and responsible for considering all the const char * arguments it finds, until the magic marker "vrt_magic_string_end" is encountered.

Bear in mind that the individual strings in a STRING_LIST can be NULL, as described under STRING, that is why we do not use NULL as the terminator.

Right now we only support STRING_LIST being the last argument to a function, we may relax that at a latter time.

If you don't want to bother with STRING_LIST, just use STRING and make sure your thread_pool_workspace param is big enough.

See below
See below

C-type: void

Can only be used for return-value, which makes the function a VCL procedure.


C-type: enum gethdr_e, const char *

XXX: explain me

XXX: these types are not released for use in vmods yet.

Private Pointers

It is often useful for library functions to maintain local state, this can be anything from a precompiled regexp to open file descriptors and vast data structures.

The VCL compiler supports two levels of private pointers, "per call" and "per VCL"

"per call" private pointers are useful to cache/store state relative to the specific call or its arguments, for instance a compiled regular expression specific to a regsub() statement or a simply caching the last output of some expensive lookup.

"per vcl" private pointers are useful for such global state that applies to all calls in this VCL, for instance flags that determine if regular expressions are case-sensitive in this vmod or similar.

The way it works in the vmod code, is that a struct vmod_priv * is passed to the functions where argument type PRIV_VCL or PRIV_CALL is specified.

This structure contains two members:

typedef void vmod_priv_free_f(void *);
struct vmod_priv {
        void                    *priv;
        vmod_priv_free_f        *free;

The "priv" element can be used for whatever the vmod code wants to use it for, it defaults to a NULL pointer.

The "free" element defaults to NULL, and it is the modules responsibility to set it to a suitable function, which can clean up whatever the "priv" pointer points to.

When a VCL program is discarded, all private pointers are checked to see if both the "priv" and "free" elements are non-NULL, and if they are, the "free" function will be called with the "priv" pointer as only argument. The "per vcl" pointers is guaranteed to be the last one inspected.

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