9. Server Protocol

A Futhark program can be compiled to a server executable. Such a server maintains a Futhark context and presents a line-oriented interface (over stdin/stdout) for loading and dumping values, as well as calling the entry points in the program. The main advantage over the plain executable interface is that program initialisation is done only once, and we can work with opaque values.

The server interface is not intended for human consumption, but is useful for writing tools on top of Futhark programs, without having to use the C API. Futhark’s built-in benchmarking and testing tools use server executables.

A server executable is started like any other executable, and supports most of the same command line options.

9.1. Basics

Each command is sent as a single line on standard input. A command consists of space-separated words. A word is either a sequence of non-space characters (foo), or double quotes surrounding a sequence of non-newline and non-quote characters ("foo bar").

The response is sent on standard output. The server will print %%% OK on a line by itself to indicate that a command has finished. It will also print %%% OK at startup once initialisation has finished. If initialisation fails, the process will terminate. If a command fails, the server will print %%% FAILURE followed by the error message, and then %%% OK when it is ready for more input. Some output may also precede %%% FAILURE, e.g. logging statements that occured before failure was detected. Fatal errors (that lead to server shutdown) may be printed to stderr.

9.2. Variables

Some commands produce or read variables. A variable is a mapping from a name to a Futhark value. Values can be both transparent (arrays and primitives), but they can also be opaque values. These can be produced by entry points and passed to other entry points, but cannot be directly inspected.

9.3. Types

All variables have types, and all entry points accept inputs and produce outputs of defined types. The notion of transparent and opaque types are the same as in the C API: primitives and array of primitives are directly supported, and everything else is treated as opaque. See also Value Mapping. When printed, types follow basic Futhark type syntax without sizes (e.g. [][]i32). Uniqueness is not part of the types, but is indicated with an asterisk in the inputs and outputs commands (see below).

9.4. Consumption and aliasing

Since the server protocol closely models the C API, the same rules apply to entry points that consume their arguments (see Consumption and Aliasing). In particular, consumed variables must still be freed with the free command - but this is the only operation that may be used on consumed variables.

9.5. Commands

The following commands are supported.

9.5.1. call entry o1 … oN i1 … iM

Call the given entry point with input from the variables i1 to iM. The results are stored in o1 to oN, which must not already exist.

9.5.2. restore file v1 t1 … vN tN

Load N values from file and store them in the variables v1 to vN of types t1 to tN, which must not already exist.

9.5.3. store file v1 … vN

Store the N values in variables v1 to vN in file.

9.5.4. free v1 … vN

Delete the given variables.

9.5.5. rename oldname newname

Rename the variable oldname to newname, which must not already exist.

9.5.6. inputs entry

Print the types of inputs accepted by the given entry point, one per line. If the given input is consumed, the type is prefixed by *.

9.5.7. outputs entry

Print the types of outputs produced by the given entry point, one per line. If the given output is guaranteed to be unique (does not alias any inputs), the type is prefixed by *.

9.5.8. clear

Clear all internal caches and counters maintained by the Futhark context. Corresponds to futhark_context_clear_caches().

9.5.9. pause_profiling

Corresponds to futhark_context_pause_profiling().

9.5.10. unpause_profiling

Corresponds to futhark_context_unpause_profiling().

9.5.11. report

Corresponds to futhark_context_report().

9.5.12. set_tuning_param

Corresponds to futhark_context_config_set_tuning_param().

9.6. Environment Variables

9.6.1. FUTHARK_COMPILER_DEBUGGING

Turns on debugging output for the server when set to 1.