Skip to content
Back to top

# VMOD - Vinyl Cache Modules

For all you can do in VCL, there are things you cannot 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 Vinyl Cache, 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(3).

This part of the manual is about how you go about writing your own VMOD, how the language interface between C and VCC works, where you can find contributed VMODs etc. This explanation will use the “std” VMOD as example, having a Vinyl Cache source tree handy may be a good idea.

## VMOD Directory

The VMOD directory is an up-to-date compilation of maintained extensions written for Vinyl Cache:

> https://www.vinyl-cache.org/vmods

## Getting started writing VMODs

There are some projects which can help interested developers get started writing VMODs:

- For the C programming language, there is the Vinyl Cache Development Kit (VCDK)

- Rustacians might want to have a look at varnish-rs.

## 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 “vmodtool.py” turns into thaumaturgically challenged C data structures that do all the hard work.

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

    $ABI strict
    $Version my.version
    $Module std 3 "Vinyl Standard Module"
    $Event event_function
    $Function STRING toupper(STRANDS s)
    $Function STRING tolower(STRANDS s)
    $Function VOID set_ip_tos(INT)

The `$ABI` line is optional. Possible values are `strict` (default) and `vrt`. It allows to specify that a vmod is integrating with the blessed `vrt` interface provided by `vinyld` or go deeper in the stack.

As a rule of thumb you, if the VMOD uses more than the VRT (Vinyl RunTime), in which case it needs to be built for the exact Vinyl Cache version, use `strict`. If it complies to the VRT and only needs to be rebuilt when breaking changes are introduced to the VRT API, use `vrt`.

The `$Version` line is also optional. It specifies the version identifier compiled into the VMOD binary for later identification. If omitted, `PACKAGE_STRING` from an automake `Makefile` will be used, or `NOVERSION` otherwise.

The `$Module` line gives the name of the module, the manual section where the documentation will reside, and the description.

The `$Event` line specifies an optional “Event” function, which will be called whenever a VCL program which imports this VMOD is loaded or transitions to any of the warm, active, cold or discarded states. More on this below.

The `$Function` lines define three 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. Instead it can be used as a primary action, something functions which return a value cannot:

    sub vcl_recv {
            std.set_ip_tos(32);
    }

Running vmodtool.py on the vmod.vcc file, produces a “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:

    VCL_STRING vmod_toupper(VRT_CTX, VCL_STRANDS);
    VCL_STRING vmod_tolower(VRT_CTX, VCL_STRANDS);
    VCL_VOID vmod_set_ip_tos(VRT_CTX, VCL_INT);

    vmod_event_f event_function;

Those are your C prototypes. Notice the `vmod_` prefix on the function names, more on that in C symbols.

### Named arguments and default values

The basic vmod.vcc function declaration syntax introduced above makes all arguments mandatory for calls from vcl - which implies that they need to be given in order.

Naming the arguments as in:

    $Function BOOL match_acl(ACL acl, IP ip)

allows calls from VCL with named arguments in any order, for example:

    if (debug.match_acl(ip=client.ip, acl=local)) { # ...

Named arguments also take default values, so for this example from the debug vmod:

    $Function STRING argtest(STRING one, REAL two=2, STRING three="3",
                             STRING comma=",", INT four=4)

only argument one is required, so that all of the following are valid invocations from vcl:

    debug.argtest("1", 2.1, "3a")
    debug.argtest("1", two=2.2, three="3b")
    debug.argtest("1", three="3c", two=2.3)
    debug.argtest("1", 2.4, three="3d")
    debug.argtest("1", 2.5)
    debug.argtest("1", four=6);

The C interface does not change with named arguments and default values, arguments remain positional and default values appear no different to user specified values.

Note that default values have to be given in the native C-type syntax, see below. As a special case, `NULL` has to be given as `0`.

### Optional arguments

The vmod.vcc declaration also allows for optional arguments in square brackets like so:

    $Function VOID opt(PRIV_TASK priv, INT four = 4, [STRING opt])

With any optional argument present, the C function prototype looks completely different:

> - Only the `VRT_CTX` and object pointer arguments (only for methods) remain positional

> - All other arguments get passed in a struct as the last argument of the C function.

The argument struct is simple, vmod authors should check the vmodtool-generated `vcc_if.c` file for the function and struct declarations:

> - for each optional argument, a `valid_`argument member is used to signal the presence of the respective optional argument.

>   `valid_` argstruct members should only be used as truth values, irrespective of their actual data type.

> - named arguments are passed in argument struct members by the same name and with the same data type.

> - unnamed (positional) arguments are passed as `arg` n with n starting at 1 and incrementing with the argument’s position.

Optionally, the VCL and C argument names can be specified independently using the `<vclname>:<cname>` syntax. See C symbols for details.

### Objects and methods

Vinyl Cache also supports a simple object model for vmods. Objects and methods are declared in the vcc file as:

    $Object class(...)
    $Method .method(...)

For declared object classes of a vmod, object instances can then be created in `vcl_init { }` using the `new` statement:

    sub vcl_init {
            new foo = vmod.class(...);
    }

and have their methods called anywhere (including in `vcl_init {}` after the instantiation):

    sub somewhere {
            foo.method(...);
    }

Nothing prevents a method to be named like the constructor and the meaning of such a method is up to the vmod author:

    $Object foo(...)
    $Method .bar(...)
    $Method .foo(...)

Object instances are represented as pointers to vmod-implemented C structs. Vinyl Cache only provides space to store the address of object instances and ensures that the right object address gets passed to C functions implementing methods.

> - Objects’ scope and lifetime are the vcl

> - Objects can only be created in `vcl_init {}` and have their destructors called by Vinyl Cache after `vcl_fini {}` has completed.

vmod authors are advised to understand the prototypes in the vmodtool-generated `vcc_if.c` file:

> - For `$Object` declarations, a constructor and destructor function must be implemented

> - The constructor is named by the suffix `__init`, always is of `VOID` return type and has the following arguments before the vcc-declared parameters:

>   - `VRT_CTX` as usual

>   - a pointer-pointer to return the address of the created oject

>   - a string containing the vcl name of the object instance

> - The destructor is named by the suffix `__fini`, always is of `VOID` return type and has a single argument, the pointer-pointer to the address of the object. The destructor is expected clear the address of the object stored in that pointer-pointer.

> - Methods gain the pointer to the object as an argument after

>     the `VRT_CTX`.

As Vinyl Cache is in no way involved in managing object instances other than passing their addresses, vmods need to implement all aspects of managing instances, in particular their memory management. As the lifetime of object instances is the vcl, they will usually be allocated from the heap.

### Functions and Methods scope restriction

The `$Restrict` stanza offers a way to limit the scope of the preceding vmod function or method, so that they can only be called from restricted vcl call sites. It must only appear after a `$Method` or `$Function` and has the following syntax:

    $Restrict scope1 [scope2 ...]

Possible scope values are: `backend, client, housekeeping, vcl_recv, vcl_pipe, vcl_pass, vcl_hash, vcl_purge, vcl_miss, vcl_hit, vcl_deliver, vcl_synth, vcl_backend_fetch, vcl_backend_response, vcl_backend_error, vcl_init, vcl_fini`

### Deprecated Aliases

The `$Alias` stanza offers a mechanism to rename a function or an object’s method without removing the previous name. This allows name changes to maintain compatibility until the alias is dropped.

The syntax for a function is:

    $Alias deprecated_function original_function

    [description]

The syntax for a method is:

    $Alias .deprecated_method object.original_method

    [description]

The `$Alias` stanza can appear anywhere, this allows grouping them in a dedicated “deprecated” section of their manual. The optional description can be used to explain why a function was renamed.

## 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.

All but the PRIV types have typedefs: VCL_INT, VCL_REAL, etc.

Notice that most of the non-native (C pointer) types are `const`, which, if returned by a vmod function/method, are assumed to be immutable. In other words, a vmod must not modify any data which was previously returned.

When returning non-native values, the producing function is responsible for arranging memory management. Either by freeing the structure later by whatever means available or by using storage allocated from the client or backend workspaces.

ACL

  C-type: `const struct vrt_acl *`

  A type for named ACLs declared in VCL.

BACKEND

  C-type: `const struct director *`

  A type for backend and director implementations. See Writing a Director.

BLOB

  C-type: `const struct vmod_priv *`

  An opaque type to pass random bits of memory between VMOD functions.

BODY

  C-type: `const void *`

  A type only used on the LHS of an assignment that can take either a blob or an expression that can be converted to a string.

BOOL

  C-type: `unsigned`

  Zero means false, anything else means true.

BYTES

  C-type: `double`

  Unit: bytes.

  A storage space, as in 1024 bytes.

DURATION

  C-type: `double`

  Unit: seconds.

  A time interval, as in 25 seconds.

ENUM

  vcc syntax: ENUM { val1, val2, … }

  vcc example: `ENUM { one, two, three } number="one"`

  C-type: `const char *`

  Allows values from a set of constant strings. Note that the C-type is a string, not a C enum.

  Enums will be passed as fixed pointers, so instead of string comparisons, also pointer comparisons with `VENUM(name)` are possible.

HEADER

  C-type: `const struct gethdr_s *`

  These are VCL compiler generated constants referencing a particular header in a particular HTTP entity, for instance `req.http.cookie` or `beresp.http.last-modified`. By passing a reference to the header, the VMOD code can both read and write the header in question.

  If the header was passed as STRING, the VMOD code only sees the value, but not where it came from.

HTTP

  C-type: `struct http *`

  A reference to a header object as `req.http` or `bereq.http`.

INT

  C-type: `long`

  A (long) integer as we know and love them.

IP

  C-type: `const struct suckaddr *`

  This is an opaque type, see the `include/vsa.h` file for which primitives we support on this type.

PRIV_CALL

  See Private Pointers below.

PRIV_TASK

  See Private Pointers below.

PRIV_TOP

  See Private Pointers below.

PRIV_VCL

  See Private Pointers below.

PROBE

  C-type: `const struct vrt_backend_probe *`

  A named standalone backend probe definition.

REAL

  C-type: `double`

  A floating point value.

REGEX

  C-type: `const struct vre *`

  This is an opaque type for regular expressions with a VCL scope. The REGEX type is only meant for regular expression literals managed by the VCL compiler. For dynamic regular expressions or complex usage see the API from the `include/vre.h` file.

STRING

  C-type: `const char *`

  A NUL-terminated text-string.

  Can be NULL to indicate a nonexistent string, for instance in:

      mymod.foo(req.http.foobar);

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

STEVEDORE

  C-type: `const struct stevedore *`

  A storage backend.

STRANDS

  C-Type: `const struct strands *`

  Strands are a list of strings that gets passed in a struct with the following members:

  - `int n`: the number of strings

  - `const char **p`: the array of strings with n elements

  A VMOD should never hold onto strands beyond a function or method execution. See `include/vrt.h` for the details.

TIME

  C-type: `double`

  Unit: seconds since UNIX epoch.

  An absolute time, as in 1284401161.

VCL_SUB

  C-type: `const struct vcl_sub *`

  Opaque handle on a VCL subroutine.

  References to subroutines can be passed into VMODs as arguments and called later through `VRT_call()`. The scope strictly is the VCL: vmods must ensure that `VCL_SUB` references never be called from a different VCL.

  `VRT_call()` fails the VCL for recursive calls and when the `VCL_SUB` cannot be called from the current context (e.g. calling a subroutine accessing `req` from the backend side).

  For more than one invocation of `VRT_call()`, VMODs must check if `VRT_handled()` returns non-zero in-between calls: The called SUB may have returned with an action (any `return(x)` other than plain `return`) or may have failed the VCL, and in both cases the calling VMOD must return also, possibly after having conducted some cleanup. Note that undoing the handling through `VRT_handling()` is a bug.

  `VRT_check_call()` can be used to check if a `VRT_call()` would succeed in order to avoid the potential VCL failure. It returns `NULL` if `VRT_call()` would make the call or an error string why not.

VOID

  C-type: `void`

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

## C symbols

Through generation of `vcc_if.h`, `vmodtool.py` pre-defines the names of most symbols on the C side of the vmod interface, namely:

- function names as <prefix>_<function>

- event handler names as <prefix>_<handler>

- method names as <prefix>_<class>_<method>, with two special methods named

  - `_init` for the constructor and

  - `_fini` for the destructor

- class struct names as <prefix>_<vmod>_<class>

- argument struct names for support of optional arguments as arg_<prefix>_<vmod>_<function> for functions and arg_<prefix>_<vmod>_<class>_<method> for methods, with member names

  - valid_<argument> for the flag of optional arguments being present and

  - <argument> for the argument name

- enum values as enum_<prefix>_<vmod>_<value>

For the above, the <xxx> placeholders are defined as:

<prefix>

  The `$Prefix` stanza value, if defined in the `.vcc` file, or `vmod` by default.

<vmod>

  The vmod name fro the `$Module` stanza of the `.vcc` file.

<argument>

  The function or method argument cname or, if not given, vclname as specified using the <vclname>:<cname> syntax.

The other placeholders should be self-explanatory as the name of the respective function, class, method or handler.

In summary, symbol names can either be influenced by the vmod author globally using `$Prefix`, or using the <vclname>:<cname> syntax for argument names.

## 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 the following private pointers:

- `PRIV_CALL` “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 simply caching the most recent output of some expensive operation. These private pointers live for the duration of the loaded VCL.

- `PRIV_TASK` “per task” private pointers are useful for state that applies to calls for either a specific request or a backend request. For instance this can be the result of a parsed cookie specific to a client. Note that `PRIV_TASK` contexts are separate for the client side and the backend side, so use in `vcl_backend_*` will yield a different private pointer from the one used on the client side. These private pointers live only for the duration of their task.

- `PRIV_TOP` “per top-request” private pointers live for the duration of one request and all its ESI-includes. They are only defined for the client side. When used from backend VCL subs, a NULL pointer will potentially be passed and a VCL failure triggered. These private pointers live only for the duration of their top level request

- `PRIV_VCL` “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 `PRIV_VCL` object is the same object that is passed to the VMOD’s event function. This private pointer lives for the duration of the loaded VCL.

  The `PRIV_CALL` vmod_privs are finalized before `PRIV_VCL`.

The way it works in the vmod code, is that a `struct vmod_priv *` is passed to the functions where one of the `PRIV_*` argument types is specified.

This structure contains three members:

    struct vmod_priv {
            void                            *priv;
            long                            len;
            const struct vmod_priv_methods  *methods;
    };

The `.priv` and `.len` elements can be used for whatever the vmod code wants to use them for.

`.methods` can be an optional pointer to a struct of callbacks:

    typedef void vmod_priv_fini_f(VRT_CTX, void *);

    struct vmod_priv_methods {
            unsigned                        magic;
            const char                      *type;
            vmod_priv_fini_f                *fini;
    };

`.magic` has to be initialized to `VMOD_PRIV_METHODS_MAGIC`. `.type` should be a descriptive name to help debugging.

`.fini` will be called for a non-NULL `.priv` of the `struct vmod_priv` when the scope ends with that `.priv` pointer as its second argument besides a `VRT_CTX`.

The common case where a private data structure is allocated with malloc(3) would look like this:

    static void
    myfree(VRT_CTX, void *p)
    {
            CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
            free (p);
    }

    static const struct vmod_priv_methods mymethods[1] = {{
            .magic = VMOD_PRIV_METHODS_MAGIC,
            .type = "mystate",
            .fini = myfree
    }};

    // ....

    if (priv->priv == NULL) {
            priv->priv = calloc(1, sizeof(struct myfoo));
            AN(priv->priv);
            priv->methods = mymethods;
            mystate = priv->priv;
            mystate->foo = 21;
            ...
    } else {
            mystate = priv->priv;
    }
    if (foo > 25) {
            ...
    }

### Private Pointers Memory Management

The generic malloc(3) / free(3) approach documented above works for all private pointers. It is the simplest and less error prone (as long as allocated memory is properly freed though the fini callback), but comes at the cost of calling into the heap memory allocator.

Per-vmod constant data structures can be assigned to any private pointer type, but, obviously, free(3) must not be used on them.

Dynamic data stored in `PRIV_TASK` and `PRIV_TOP` pointers can also come from the workspace:

- For `PRIV_TASK`, any allocation from `ctx->ws` works, like so:

      if (priv->priv == NULL) {
              priv->priv = WS_Alloc(ctx->ws, sizeof(struct myfoo));
              if (priv->priv == NULL) {
                      VRT_fail(ctx, "WS_Alloc failed");
                      return (...);
              }
              priv->methods = mymethods;
              mystate = priv->priv;
              mystate->foo = 21;
              ...

- For `PRIV_TOP`, first of all keep in mind that it must only be used from the client context, so vmod code should error out for `ctx->req == NULL`.

  For dynamic data, the top request’s workspace must be used, which complicates things a bit:

      if (priv->priv == NULL) {
              struct ws *ws;

              CHECK_OBJ_NOTNULL(ctx->req, REQ_MAGIC);
              CHECK_OBJ_NOTNULL(ctx->req->top, REQTOP_MAGIC);
              CHECK_OBJ_NOTNULL(ctx->req->top->topreq, REQ_MAGIC);
              ws = ctx->req->top->topreq->ws;

              priv->priv = WS_Alloc(ws, sizeof(struct myfoo));
              // ... same as above for PRIV_TASK

Notice that allocations on the workspace do not need to be freed, their lifetime is the respective task.

### Private Pointers and Objects

`PRIV_TASK` and `PRIV_TOP` arguments to methods are not per object instance, but per vmod as for ordinary vmod functions. Thus, vmods requiring per-task / per top-request state for object instances need to implement other means to associate storage with object instances.

This is what `VRT_priv_task()` / `VRT_priv_task_get()` and `VRT_priv_top()` / `VRT_priv_top_get()` are for:

The non-get functions either return an existing `PRIV_TASK` / `PRIV_TOP` for a given `void *` argument or create one. They return `NULL` in case of an allocation failure.

The `_get()` functions do not create a `PRIV_*`, but return either an existing one or `NULL`.

By convention, private pointers for object instance are created on the address of the object, as in this example for a `PRIV_TASK`:

    VCL_VOID
    myvmod_obj_method(VRT_CTX, struct myvmod_obj *o)
    {
        struct vmod_priv *p;

        p = VRT_priv_task(ctx, o);

        // ... see above

The `PRIV_TOP` case looks identical except for calling `VRT_priv_top(ctx, o)` in place of `VRT_priv_task(ctx, o)`, but be reminded that the `VRT_priv_top*()` functions must only be called from client context (if `ctx->req != NULL`).

## Event functions

VMODs can have an “event” function which is called when a VCL which imports the VMOD is loaded or discarded. This corresponds to the `VCL_EVENT_LOAD` and `VCL_EVENT_DISCARD` events, respectively. In addition, this function will be called when the VCL temperature is changed to cold or warm, corresponding to the `VCL_EVENT_COLD` and `VCL_EVENT_WARM` events.

The first argument to the event function is a VRT context.

The second argument is the vmod_priv specific to this particular VCL, and if necessary, a VCL specific VMOD “fini” function can be attached to its “free” hook.

The third argument is the event.

If the VMOD has private global state, which includes any sockets or files opened, any memory allocated to global or private variables in the C-code etc, it is the VMODs own responsibility to track how many VCLs were loaded or discarded and free this global state when the count reaches zero.

VMOD writers are strongly encouraged to release all per-VCL resources for a given VCL when it emits a `VCL_EVENT_COLD` event. You will get a chance to reacquire the resources before the VCL becomes active again and be notified first with a `VCL_EVENT_WARM` event. Unless a user decides that a given VCL should always be warm, an inactive VMOD will eventually become cold and should manage resources accordingly.

An event function must return zero upon success. It is only possible to fail an initialization with the `VCL_EVENT_LOAD` or `VCL_EVENT_WARM` events. Should such a failure happen, a `VCL_EVENT_DISCARD` or `VCL_EVENT_COLD` event will be sent to the VMODs that succeeded to put them back in a cold state. The VMOD that failed will not receive this event, and therefore must not be left half-initialized should a failure occur.

If your VMOD is running an asynchronous background job you can hold a reference to the VCL to prevent it from going cold too soon and get the same guarantees as backends with ongoing requests for instance. For that, you must acquire the reference by calling `VRT_VCL_Prevent_Discard` when you receive a `VCL_EVENT_WARM` and later calling `VRT_VCL_Allow_Discard` once the background job is over. Receiving a `VCL_EVENT_COLD` is your cue to terminate any background job bound to a VCL.

You can find an example of VCL references in vmod-debug:

    priv_vcl->vclref = VRT_VCL_Prevent_Discard(ctx, "vmod-debug");
    ...
    VRT_VCL_Allow_Discard(&ctx, &priv_vcl->vclref);

In this simplified version, you can see that you need at least a VCL-bound data structure like a `PRIV_VCL` or a VMOD object to keep track of the reference and later release it. You also have to provide a description, it will be printed to the user if they try to warm up a cooling VCL:

    $ vinyladm vcl.list
    available  auto/cooling       0 vcl1
    active     auto/warm          0 vcl2

    $ vinyladm vcl.state vcl1 warm
    Command failed with error code 300
    Failed <vcl.state vcl1 auto>
    Message:
            VCL vcl1 is waiting for:
            - vmod-debug

In the case where properly releasing resources may take some time, you can opt for an asynchronous worker, either by spawning a thread and tracking it, or by using Vinyl Cache’s worker pools.

## When to lock, and when not to lock

Vinyl Cache is heavily multithreaded, so by default VMODs must implement their own locking to protect shared resources.

When a VCL is loaded or unloaded, the event and priv->free are run sequentially all in a single thread, and there is guaranteed to be no other activity related to this particular VCL, nor are there init/fini activity in any other VCL or VMOD at this time.

That means that the VMOD init, and any object init/fini functions are already serialized in sensible order, and won’t need any locking, unless they access VMOD specific global state, shared with other VCLs.

Traffic in other VCLs which also import this VMOD, will be happening while housekeeping is going on.

## Statistics Counters

Starting in Varnish 6.0, VMODs can define their own counters that appear in vinylstat.

If you’re using autotools, see the `VINYL_COUNTERS` macro in vinyl.m4 for documentation on getting your build set up.

Counters are defined in a .vsc file. The `VINYL_COUNTERS` macro calls vsctool.py to turn a foo.vsc file into VSC_foo.c and VSC_foo.h files, just like vmodtool.py turns foo.vcc into vcc_foo_if.c and vcc_foo_if.h files. Similarly to the VCC files, the generated VSC files give you a structure and functions that you can use in your VMOD’s code to create and destroy the counters your defined. The vsctool.py tool also generates a VSC_foo.rst file that you can include in your documentation to describe the counters your VMOD has.

The .vsc file looks like this:

    .. vinyl_vsc_begin:: xkey
            :oneliner:      xkey Counters
            :order:         70

            Metrics from vmod_xkey

    .. vinyl_vsc:: g_keys
            :type:          gauge
            :oneliner:      Number of surrogate keys

            Number of surrogate keys in use. Increases after a request that includes a new key in the xkey header. Decreases when a key is purged or when all cache objects associated with a key expire.

    .. vinyl_vsc_end:: xkey

Counters can have the following parameters:

type

  The type of metric this is. Can be one of `counter`, `gauge`, or `bitmap`.

ctype

  The type that this counter will have in the C code. This can only be `uint64_t` and does not need to be specified.

level

  The verbosity level of this counter. vinylstat will only show counters with a higher verbosity level than the one currently configured. Can be one of `info`, `diag`, or `debug`.

oneliner

  A short, one line description of the counter.

group

  I don’t know what this does.

format

  Can be one of `integer`, `bytes`, `bitmap`, or `duration`.

After these parameters, a counter can have a longer description, though this description has to be all on one line in the .vsc file.

You should call `VSC_*_New()` when your VMOD is loaded and `VSC_*_Destroy()` when it is unloaded. See the generated `VSC_*.h` file for the full details about the structure that contains your counters.

## Temporary Files

`vinyld` creates a directory named `worker_tmpdir` under the Vinyl Cache working directory (see `vinyld -n` argument) for read/write access by the worker process.

From the perspective of VMODs, the relative path is always `worker_tmpdir`.

This directory is intended (though not limited) to provide a place for VMODs to create temporary files using `mkstemp()` and related libc functions. VMODs are responsible for cleaning up files which are no longer required, and they will ultimately be removed when the `vinyld` worker process restarts. There is no isolation between VMODs (as is the case anyway).

A simple example for how to use it:

    #include <stdlib.h>
    #include <unistd.h>

    #include "vdef.h"
    #include "vas.h"

    static void
    tmpfile_example(void) {
        int fd;
        char name[] = "worker_tmpdir/myvmod.XXXXXX";

        fd = mkstemp(name);
        if (fd < 0) {
            // handle error
            return;
        }

        // hide file
        AZ(unlink(name));

        // use fd

        AZ(close(fd));
    }

## VMOD Version Information

Panic messages (see Troubleshooting Vinyl Cache for an introduction) are an invaluable information source to developers when analyzing crashes. To avoid uncertainties about the exact code base a VMOD was built from, Panics contain, if possible, a maintainer-controlled version and a “version” from a version control system (VCS, usually `git`).

The maintainer-controlled version is taken from the `$Version` stanza with a fallback to `PACKAGE_STRING` from the `Makefile`, see The vmod.vcc file.

The VCS version is intended to be more specific than the maintainer-controlled version and is thus automatically generated unless deliberately overwritten: If `vmodtool.py` finds itself invoked from a git repository, it uses the version output by `git rev-parse HEAD`. To also have proper VCS version information in builds from a dist archive (as created with `make dist` when using autotools), `vmodtool.py` also writes a VCS version it gets from git to `vmod_vcs_version.txt`. This file is intended to be added to the distribution, such that when `vmodtool.py` is run from a distribution archive, it also has the VCS version available.

### Supporting vmod_vcs_version.txt with autotools

To support `vmod_vcs_version.txt` with autotools, VMOD authors are advised to implement the following:

- add a dependency from `vmod_vcs_version.txt` to (one of) the files generated by `vmodtool.py`

- add `vmod_vcs_version.txt` to `DISTCLEANFILES` and `EXTRA_DIST`

`vmod_vcs_version.txt` should also be added to `.gitignore`

To illustrate, here is an example diff for a trivial vmod by the name blobsynth:

    diff --git a/src/Makefile.am b/src/Makefile.am
    index 63e4d55..d8cc659 100644
    --- a/src/Makefile.am
    +++ b/src/Makefile.am
    @@ -10,6 +10,10 @@ nodist_libvmod_blobsynth_la_SOURCES = \
            vcc_blobsynth_if.c \
            vcc_blobsynth_if.h

    +DISTCLEANFILES = vmod_vcs_version.txt
    +
    +vmod_vcs_version.txt: $(nodist_libvmod_blobsynth_la_SOURCES)
    +
     dist_man_MANS = vmod_blobsynth.3

     @BUILD_VMOD_BLOBSYNTH@
    @@ -25,4 +29,5 @@ TESTS = @VMOD_TESTS@

     EXTRA_DIST = \
            vmod_blobsynth.vcc \
    +       vmod_vcs_version.txt \
            $(VMOD_TESTS)

### Retrieving version information using the CLI

The `debug.vmod` CLI command outputs the version information, for example (edited for readability, the original output is on a single line):

    $ vinyladm debug.vmod
        1 dynamic (path=".../vmods/libvmod_dynamic.so",
                   version="libvmod-dynamic trunk",
                   vcs="1e4179430404aaf170530af7514fbecb1f38f8af")

### Reading the VCS version from the binary shared object

As a useful by-product, the version information can also be extracted from the vmod `.so` file on many platforms. On Linux using `readelf -p.vmod_vcs <file>`:

    $ readelf -p.vmod_vcs .../vmods/libvmod_dynamic.so

    String dump of section '.vmod_vcs':
      [     0]  1e4179430404aaf170530af7514fbecb1f38f8af

On MacOS:

    $ otool -s __TEXT vmod_vcs .../vmods/libvmod_debug.so |
      awk '/^[0-9a-f]/ {for(i=2;i<=NF;i++) printf "%s",$i}' |
      xxd -r -p | strings
    18e27f081788b5e5d44c480f6d9749c07d53ddb9