erl_driver - API functions for an erlang driver

The driver calls back to the emulator, using the API functions declared in erl_driver.h. They are used for outputting data from the driver, using timers, etc.

A driver is a library with a set of function that the emulator calls, in response to erlang functions and message sending. There may be multiple instances of a driver, each instance is connected to an erlang port. Every port has a port owner process. Communication with the port is normally done through the port owner process.

Most of the functions takes the port handle as an argument. This identifies the driver instance. Note that this port handle must be stored by the driver, it is not given when the driver is called from the emulator (see driver_entry).

Some of the functions takes a parameter of type ErlDrvBinary, a driver binary. It should be both allocated and freed by the caller. Using a binary directly avoid one extra copying of data.

Many of the output functions has a "header buffer", with hbuf and hlen parameters. This buffer is sent as a list before the binary (or list, depending on port mode) that is sent. This is convenient when matching on messages received from the port. (Although in the latest versions of erlang, there is the binary syntax, that enables you to match on the beginning of a binary.)

All functions that a driver needs to do with erlang are performed through driver API functions. There are functions for the following functionality:

Timer functions: Timer functions are used to control the timer that a driver may use. The timer will have the emulator call the timeout entry function after a specified time. Only one timer is available for each driver instance.

Queue handling: Every driver has an associated queue. This queue is a SysIOVec that works as a buffer. It's mostly used for the driver to buffer data that should be written to a device, it is a byte stream. If the port owner process closes the driver, and the queue is not empty, the driver will not be closed. This enables the driver to flush its buffers before closing.

Output functions: With the output functions, the driver sends data back the emulator. They will be received as messages by the port owner process, see open_port/2. The vector function and the function taking a driver binary is faster, because thet avoid copying the data buffer. There is also a fast way of sending terms from the driver, without going through the binary term format.

Failure: The driver can exit and signal errors up to erlang. This is only for severe errors, when the driver can't possibly keep open.

Asynchronous calls: The latest erlang versions (R7B and later) has provision for asynchonous function calls, using a thread pool provided by erlang. There is also a select call, that can be used for asynchronous drivers.

Adding / remove drivers: A driver can add and later remove drivers.

ErlDrvBinary

Types

int orig_size

int refc

char orig_bytes[]

The ErlDrvBinary structure is a binary, as sent between the emulator and the driver. All binaries are reference counted; when driver_binary_free is called, the refc field is decremented, when it reaches zero, the binary is deallocated. The orig_size is the size of the binary, and orig_bytes is the buffer. The ErlDrvBinary does not have a fixed size, its size is orig_size + 2 * sizeof(int).

Some driver calls, such as driver_enq_binary, increments the driver ref-count, and others, such as driver_deq decrements it.

Using a driver binary instead of a normal buffer, is often faster, since the emulator doesn't need to copy the data, only the pointer is used.

A driver binary allocated in the driver, with driver_alloc_binary, should be freed in the driver, with driver_free_binary. (Note that this doesn't necessarily deallocate it, if the driver is still referred in the emulator, the ref-count will not go to zero.)

Driver binaries are used in the driver_output2 and driver_outputv calls, and in the queue. Also the driver call-back outputv uses driver binaries.

If the driver of some reason or another, wants to keep a driver binary around, in a static variable for instance, the ref-count in the refc field should be incremented, and the binary can later be freed in the stop call-back, with driver_free_binary.

Note that since a driver binary is shared by the driver and the emulator, a binary received from the emulator or sent to the emulator, shouldn't be changed by the driver.

ErlDrvData

The ErlDrvData is a handle to driver-specific data, passed to the driver call-backs. It is a pointer, and is most often casted to a specific pointer in the driver.

SysIOVec

This is a system I/O vector, as used by writev on unix and WSASend on Win32. It is used in ErlIOVec.

ErlIOVec

Types

int vsize

int size

SysIOVec* iov

ErlDrvBinary** binv

The I/O vector used by the emulator and drivers, is a list of binaries, with a SysIOVec pointing to the buffers of the binaries. It is used in driver_outputv and the outputv driver call-back. Also, the driver queue is an ErlIOVec.

int driver_output(ErlDrvPort port, char *buf, int len)

The driver_output function is used to send data from the driver up to the emulator. The data will be received as terms or binary data, depending on how the driver port was opened.

The data is queued in the port owner process' message queue. Note that this does not yield to the emulator. (Since the driver and the emulator runs in the same thread.)

The parameter buf points to the data to send, and len is the number of bytes.

The return value for all output functions is 0. (Unless the driver is used for distribution, in which case it can fail and return -1. For normal use, the output function always returns 0.)

int driver_output2(ErlDrvPort port, char *hbuf, int hlen, char *buf, int len)

The driver_output2 function first sends hbuf (length in hlen) data as a list, regardless of port settings. Then buf is sent as a binary or list. E.g. if hlen is 3 then the port owner process will receive [H1, H2, H3 | T].

The point of sending data as a list header, is to facilitate matching on the data received.

The return value is 0 for normal use.

int driver_output_binary(ErlDrvPort port, char *hbuf, int hlen, ErlDrvBinary* bin, int offset, int len)

This function sends data to port owner process from a driver binary, it has a header buffer (hbuf and hlen) just like driver_output2. The hbuf parameter can be NULL.

The parameter offset is an offset into the binary and len is the number of bytes to send.

Driver binaries are created with driver_alloc_binary.

The data in the header is sent as a list and the binary as an erlang binary in the tail of the list.

E.g. if hlen is 2, then the port owner process will receive [H1, H2 | <<T>>].

The return value is 0 for normal use.

Note that, using the binary syntax in erlang, the driver application can match the header directly from the binary, so the header can be put in the binary, and hlen can be set to 0.

int driver_outputv(ErlDrvPort port, char* hbuf, int hlen, ErlIOVec *ev, int skip)

This function sends data from an IO vector, ev, to the port owner process. It has a header buffer (hbuf and hlen), just like driver_output2.

The skip parameter is a number of bytes to skip of the ev vector from the head.

You get vectors of ErlIOVec type from the driver queue (see below), and the outputv driver entry function. You can also make them yourself, if you want to send several ErlDriverBinary buffers at once. Often it is faster to use driver_output or driver_output_binary.

E.g. if hlen is 2 and ev points to an array of three binaries, the port owner process will receive [H1, H2, <<B1>>, <<B2>> | <<B3>>].

The return value is 0 for normal use.

The comment for driver_output_binary applies for driver_outputv too.

int driver_vec_to_buf(ErlIOVec *ev, char *buf, int len)

This function collects several segments of data, referenced by ev, by copying them in order to the buffer buf, of the size len.

If the data is to be sent from the driver to the port owner process, it is faster to use driver_outputv.

The return value is the space left in the buffer, i.e. if the ev contains less than len bytes it's the difference, and if ev contains len bytes or more, it's 0. This is faster if there is more than one header byte, since the binary syntax can construct integers directly from the binary.

int driver_set_timer(ErlDrvPort port, unsigned long time)

This function sets a timer on the driver, which will count down and call the driver when it is timed out. The time parameter is the time in milliseconds before the timer expires.

When the timer reaches 0 and expires, the driver entry function timeout is called.

Note that there is only one timer on each driver instance; setting a new timer will replace an older one.

Return value i 0 (-1 only when the timeout driver function is NULL).

int driver_cancel_timer(ErlDrvPort port)

This function cancels a timer set with driver_set_timer.

The return value is 0.

int driver_read_timer(ErlDrvPort port, unsigned long *time_left)

This function reads the current time of a timer, and places the result in time_left. This is the time in milliseconds, before the timeout will occur.

The return value is 0.

int driver_select(ErlDrvPort port, ErlDrvEvent event, int mode, int on)

The driver_select is used by the driver to provide the emulator with an event to check for. This enables the emulator to call the driver when something has happened asynchronously.

The event parameter is used in the emulator cycle in a select call. If the event is set then the driver is called. The mode parameter can be either ON_READ or ON_WRITE, and specifies whether ready_output or ready_input will be called when the event is fired. Note that this is just a convention, they don't have to read or write anything.

The on parameter should be 1 for adding the event and 0 for removing it.

On unix systems, the function select is used. The event must be a socket or pipe (or other object that select can use).

On windows, the Win32 API function WaitForMultipleObjects is used. This places other restriction on the event. Refer to the Win32 SDK documentation.

The return value is 0 (Failure, -1, only if the ready_input/ready_output is NULL.

void *driver_alloc(size_t size)

This function allocates a memory block of the size specified in size, and returns it. This only fails on out of memory, in that case NULL is returned. (This is most often a wrapper for malloc).

Memory allocated must be explicitly freed. Every driver_alloc call must have a corresponding driver_free.

void *driver_realloc(void *ptr, size_t size)

This function resizes a memory block, either in place, or by allocating a new block, copying the data and freeing the old block. A pointer is returned to the reallocated memory. On failure (out of memory), NULL is returned. (This is most ofthen a wrapper for realloc.)

void driver_free(void *ptr)

This function frees the memory pointed to by ptr. The memory should have been allocated with driver_alloc. All allocated memory should be deallocated, just once. There is no garbage collection in drivers.

ErlDrvBinary* driver_alloc_binary(int size)

This function allocates a driver binary with a memory block of at least size bytes, and returns a pointer to it, or NULL on failure (out of memory). When a driver binary has been sent to the emulator, it shouldn't be altered. Every allocated binary should be freed.

Note that a driver binary has an internal reference counter, this means that calling driver_free_binary it may not actually dispose of it. If it's sent to the emulator, it may be referenced there.

The driver binary has a field, orig_bytes, which marks the start of the data in the binary.

ErlDrvBinary* driver_realloc_binary(ErlDrvBinary *bin, int size)

This function resizes a driver binary, while keeping the data. The resized driver binary is returned. On failure (out of memory), NULL is returned.

void driver_free_binary(ErlDrvBinary *bin)

This function frees a driver binary bin, allocated previously with driver_alloc_binary. Since binaries in erlang are reference counted, the binary may still be around. Every call to driver_alloc_binary should have a matching call to driver_free_binary.

int driver_enq(ErlDrvPort port, char* buf, int len)

This function enqueues data in the driver queue. The data in buf is copied (len bytes) and placed at the end of the driver queue. The driver queue is normally used in a FIFO way.

The driver queue is available to queue output from the emulator to the driver (data from the driver to the emulator is queued by the emulator in normal erlang message queues). This can be useful if the driver has to wait for slow devices etc, and wants to yield back to the emulator. The driver queue is implemented as an ErlIOVec.

When the queue contains data, the driver won't close, until the queue is empty.

The return value is 0.

int driver_pushq(ErlDrvPort port, char* buf, int len)

This function puts data at the head of the driver queue. The data in buf is copied (len bytes) and placed at the beginning of the queue.

The return value is 0.

int driver_deq(ErlDrvPort port, int size)

This function dequeues data by moving the head pointer forward in the driver queue by size bytes. The data in the queue will be dealloced.

The return value is 0.

int driver_sizeq(ErlDrvPort port)

This function returns the number of bytes currently in the driver queue.

int driver_enq_bin(ErlDrvPort port, ErlDrvBinary *bin, int offset, int len)

This function enqueues a driver binary in the driver queue. The data in bin at offset with length len is placed at the end of the queue. This function is most often faster than driver_enq, because the data doesn't have to be copied.

The return value is 0.

int driver_pushq_bin(ErlDrvPort port, ErlDrvBinary *bin, int offset, int len)

This function puts data in the binary bin, at offset with length len at the head of the driver queue. It is most often faster than driver_pushq, because the data doesn't have to be copied.

The return value is 0.

SysIOVec* driver_peekq(ErlDrvPort port, int *vlen)

This function retrieves the driver queue as a pointer to an array of SysIOVecs. It also returns the number of elements in vlen. This is the only way to get data out of the queue.

Nothing is remove from the queue by this function, that must be done with driver_deq.

The returned array is suitable to use with the unix system call writev.

int driver_enqv(ErlDrvPort port, ErlIOVec *ev, int skip)

This function enqueues the data in ev, skipping the first skip bytes of it, at the end of the driver queue. It is faster than driver_enq, because the data doesn't have to be copied.

The return value is 0.

int driver_pushqv(ErlDrvPort port, ErlIOVec *ev, int skip)

This function puts the data in ev, skipping the first skip bytes of it, at the head of the driver queue. It is faster than driver_pushq, because the data doesn't have to be copied.

The return value is 0.

void add_driver_entry(ErlDrvEntry *de)

This function adds a driver entry to the list of drivers known by erlang. The init function of the de parameter is called.

int remove_driver_entry(ErlDrvEntry *de)

This function removes a driver entry de previously added with add_driver_entry.

char* erl_errno_id(int error)

This function returns the atom name of the erlang error, given the error number in error. Error atoms are: einval, enoent, etc. It can be used to make error terms from the driver.

void set_busy_port(ErlDrvPort port, int on)

This function set and resets the busy status of the port. If on is 1, the port is set to busy, if it's 0 the port is set to not busy.

When the port is busy, sending to it with Port ! Data or port_command/2, will block the port owner process, until the port is signaled as not busy.

void set_port_control_flags(ErlDrvPort port, int flags)

This function sets flags for how the control driver entry function will return data to the port owner process. (The control function is called from port_control/3 in erlang.)

Currently there are only two meaningful values for flags: 0 means that data is returned in a list, and PORT_CONTROL_FLAG_BINARY means data return from control is sent to the port owner process.

int driver_failure_eof(ErlDrvPort port)

This function signals to erlang that the driver has encountered an EOF and should be closed, unless the port was opened with the eof option, in that case eof is sent to the port. Otherwise, the port is close and an 'EXIT' message is sent to the port owner process.

The return value is 0.

int driver_failure_atom(ErlDrvPort port, char *string)

int driver_failure_posix(ErlDrvPort port, int error)

int driver_failure(ErlDrvPort port, int error)

These functions signal to erlang that the driver has encountered an error and should be closed. The port is closed and the tuple {'EXIT', error, Err}, is sent to the port owner process, where error is an error atom (driver_failure_atom and driver_failure_posix), or an integer (driver_failure).

The driver should fail only when in severe error situations, when the driver cannot possibly kepp open, for instance buffer allocation gets out of memory. Normal errors is more appropriate to handle with sending error codes with driver_output.

The return value is 0.

ErlDriverTerm driver_connected(ErlDrvPort port)

This function returns the port owner process.

ErlDriverTerm driver_caller(ErlDrvPort port)

This function returns the process that made the current call to the driver. This can be used with driver_send_term to send back data to the caller. (This is the process that called one of erlang:send/2, erlang:port_command/2 or erlang:port_control/3).

int driver_output_term(ErlDrvPort port, ErlDriverTerm* term, int n)

This functions sends data in the special driver term format. This is a fast way to deliver term data to from a driver. It also needs no binary conversion, so the port owner process receives data as normal erlang terms.

The term parameter points to an array of ErlDriverTerm, with n elements. This array contains terms described in the driver term format. Every term consists of one to four elements in the array. The term first has a term type, and then arguments.

Tuple and lists (with the exception of strings, see below), are built in reverse polish notation, so that to build a tuple, the elements are given first, and then the tuple term, with a count. Likewise for lists.

A tuple must be specified with the number of elements. (The elements precedes the ERL_DRV_TUPLE term.)

A list must be specified with the number of elements, including the tail, which is the last term preceding ERL_DRV_LIST.

The special term ERL_DRV_STRING_CONS is used to "splice" in a string in a list, a string given this way is not a list per se, but the elements are elements of the surrounding list.

Term type            Argument(s)
===========================================
ERL_DRV_NIL          None
ERL_DRV_ATOM         driver_mk_atom(string)
ERL_DRV_INT          int
ERL_DRV_PORT         driver_mk_port(ix)
ERL_DRV_BINARY       ErlDriverBinary*, int len, int offset
ERL_DRV_STRING       char*, int len
ERL_DRV_TUPLE        int size
ERL_DRV_LIST         int size
ERL_DRV_PID          driver_connected() or driver_caller()
ERL_DRV_STRING_CONS  char*, int len
ERL_DRV_FLOAT        double*

To build the tuple {tcp, Port, [100 | Binary]}, the following call could be made.

    ErlDriverBinary* bin = ...
    ErlDriverPort port = ...
    ErlDriverTerm spec[] = {
        ERL_DRV_ATOM, driver_mk_atom("tcp"),
        ERL_DRV_PORT, driver_mk_port(port),
            ERL_DRV_INT, 100,
            ERL_DRV_BINARY, bin, 50, 0,
            ERL_DRV_LIST, 2,
        ERL_DRV_TUPLE, 3,
    };
    driver_output_term(port, spec, sizeof(spec) / sizeof(spec[0]));

Where bin is a driver binary of length at least 50 and port is a port handle. Note that the ERL_DRV_LIST comes after the elements of the list, likewise the ERL_DRV_TUPLE.

The term ERL_DRV_STRING_CONS is a way to construct strings. It works differently from how ERL_DRV_STRING works. ERL_DRV_STRING_CONS builds a string list in reverse order, (as opposed to how ERL_DRV_LIST works), concatenating the strings added to a list. The tail must be given before ERL_DRV_STRING_CONS.

The ERL_DRV_STRING constructs a string, and ends it. (So it's the same as ERL_DRV_NIL followed by ERL_DRV_STRING_CONS.)

    /* to send [x, "abc", y] to the port: */
    ErlDriverTerm spec[] = {
        ERL_DRV_ATOM, driver_mk_atom("x"),
        ERL_DRV_STRING, (ErlDriverTerm)"abc", 3,
        ERL_DRV_ATOM, driver_mk_atom("y"),
        ERL_DRV_NIL,
        ERL_DRV_LIST, 4
    };
    driver_output_term(port, spec, sizeof(spec) / sizeof(spec[0]));

    /* to send "abc123" to the port: */
    ErlDriverTerm spec[] = {
        ERL_DRV_NIL,        /* with STRING_CONS, the tail comes first */
        ERL_DRV_STRING_CONS, (ErlDriverTerm)"123", 3,
        ERL_DRV_STRING_CONS, (ErlDriverTerm)"abc", 3,
    };
    driver_output_term(port, spec, sizeof(spec) / sizeof(spec[0]));

ErlDriverTerm driver_mk_atom(char* string)

This function returns an atom given a name string. The atom is created and won't change, so the return value may be saved and reused, which is faster than looking up the atom several times.

ErlDriverTerm driver_mk_port(ErlDrvPort port)

This function converts a port handle to the erlang term format, usable in the driver_output_send function.

int driver_send_term(ErlDrvPort port, ErlDriverTerm receiver, ErlDriverTerm* term, int n)

This function is the only way for a driver to send data to other processes than the port owner process. The receiver parameter specifies the process to receive the data.

The parameters term and n does the same thing as in driver_output_term.

long driver_async (ErlDrvPort port, unsigned int* key, void (*async_invoke)(void*), void* async_data, void (*async_free)(void*))

This function performs an asynchronous call. The function async_invoke is invoked in a thread separate from the emulator thread. This enables the driver to perform time-consuming, blocking operations without blocking the emulator.

Normally, erlang is started without a thread pool. A start argument to the emulator, specifies how many threads that should be available (e.g. +A 5, gives five extra driver threads). If no thread pool is available, the call is made synchronously, in the emulator thread.

If there is a thread pool available, a thread will be used. If the key argument is null, the threads from the pool are used in a round-robin way, each call to driver_async uses the next thread in the pool. With the key argument set, this behaviour is changed. The two same values of *key always get the same thread.

To make sure that a driver instance always uses the same thread, the following call can be used:

  
    r = driver_async(myPort, (unsigned char*)&myPort, myData, myFunc);

If a thread is already working, the calls will be queued up and executed in order. Using the same thread for each driver instance ensures that the calls will be made in sequence.

The async_data is the argument to the functions async_invoke and async_free. It's typically a pointer to a structure that contains a pipe or event that can be used to signal that the async operation completed. The data should be freed in async_free, because it's called if driver_async_cancel is called.

When the async operation is done, ready_async driver entry function is called. If async_ready is null in the driver entry, the async_free function is called instead.

The return value is a handle to the asynchronous task, which can be used as argument to driver_async_cancel.

int driver_async_cancel(long id)

This function cancels an asynchronous operation, by removing it from the queue. Only functions in the queue can be cancelled; if a function is executing, it's too late to cancel it. The async_free function is also called.

The return value is 1 if the operation was removed from the queue, otherwise 0.

driver_entry(3), erl_ddll(3), erlang(3)

An Alternative Distribution Driver (ERTS User's Guide Ch. 3)

Kenneth Lundin - support@erlang.ericsson.se
Jakob Cederlund - support@erlang.ericsson.se