The driver-entry structure used by erlang drivers.


Use this functionality with extreme care!

A driver callback is executed as a direct extension of the native code of the VM. Execution is not made in a safe environment. The VM can not provide the same services as provided when executing Erlang code, such as preemptive scheduling or memory protection. If the driver callback function doesn't behave well, the whole VM will misbehave.

A driver callback that crash will crash the whole VM.

An erroneously implemented driver callback might cause a VM internal state inconsistency which may cause a crash of the VM, or miscellaneous misbehaviors of the VM at any point after the call to the driver callback.

A driver callback that do lengthy work before returning will degrade responsiveness of the VM, and may cause miscellaneous strange behaviors. Such strange behaviors include, but are not limited to, extreme memory usage, and bad load balancing between schedulers. Strange behaviors that might occur due to lengthy work may also vary between OTP releases.

As of erts version 5.9 (OTP release R15B) the driver interface has been changed with larger types for the callbacks output, control and call. See driver version management in erl_driver.


Old drivers (compiled with an erl_driver.h from an earlier erts version than 5.9) have to be updated and have to use the extended interface (with version management ).

The driver_entry structure is a C struct that all erlang drivers define. It contains entry points for the erlang driver that are called by the erlang emulator when erlang code accesses the driver.

The erl_driver driver API functions need a port handle that identifies the driver instance (and the port in the emulator). This is only passed to the start function, but not to the other functions. The start function returns a driver-defined handle that is passed to the other functions. A common practice is to have the start function allocate some application-defined structure and stash the port handle in it, to use it later with the driver API functions.

The driver call-back functions are called synchronously from the erlang emulator. If they take too long before completing, they can cause timeouts in the emulator. Use the queue or asynchronous calls if necessary, since the emulator must be responsive.

The driver structure contains the name of the driver and some 15 function pointers. These pointers are called at different times by the emulator.

The only exported function from the driver is driver_init. This function returns the driver_entry structure that points to the other functions in the driver. The driver_init function is declared with a macro DRIVER_INIT(drivername). (This is because different OS's have different names for it.)

When writing a driver in C++, the driver entry should be of "C" linkage. One way to do this is to put this line somewhere before the driver entry: extern "C" DRIVER_INIT(drivername);.

When the driver has passed the driver_entry over to the emulator, the driver is not allowed to modify the driver_entry.


Do not declare the driver_entry const. This since the emulator needs to modify the handle, and the handle2 fields. A statically allocated, and const declared driver_entry may be located in read only memory which will cause the emulator to crash.



typedef struct erl_drv_entry {
    int (*init)(void);          /* called at system start up for statically
                                   linked drivers, and after loading for
                                   dynamically loaded drivers */ 

#ifndef ERL_SYS_DRV
    ErlDrvData (*start)(ErlDrvPort port, char *command);
                                /* called when open_port/2 is invoked.
                                   return value -1 means failure. */
    ErlDrvData (*start)(ErlDrvPort port, char *command, SysDriverOpts* opts);
                                /* special options, only for system driver */
    void (*stop)(ErlDrvData drv_data);
                                /* called when port is closed, and when the
                                   emulator is halted. */
    void (*output)(ErlDrvData drv_data, char *buf, ErlDrvSizeT len);
                                /* called when we have output from erlang to 
                                   the port */
    void (*ready_input)(ErlDrvData drv_data, ErlDrvEvent event); 
                                /* called when we have input from one of 
                                   the driver's handles */
    void (*ready_output)(ErlDrvData drv_data, ErlDrvEvent event);  
                                /* called when output is possible to one of 
                                   the driver's handles */
    char *driver_name;          /* name supplied as command 
                                   in open_port XXX ? */
    void (*finish)(void);       /* called before unloading the driver -
                                   DYNAMIC DRIVERS ONLY */
    void *handle;               /* Reserved -- Used by emulator internally */
    ErlDrvSSizeT (*control)(ErlDrvData drv_data, unsigned int command,
                            char *buf, ErlDrvSizeT len,
			    char **rbuf, ErlDrvSizeT rlen);
                                /* "ioctl" for drivers - invoked by 
                                   port_control/3 */
    void (*timeout)(ErlDrvData drv_data);        /* Handling of timeout in driver */
    void (*outputv)(ErlDrvData drv_data, ErlIOVec *ev);
                                /* called when we have output from erlang
                                   to the port */
    void (*ready_async)(ErlDrvData drv_data, ErlDrvThreadData thread_data);
    void (*flush)(ErlDrvData drv_data);
                                /* called when the port is about to be 
                                   closed, and there is data in the 
                                   driver queue that needs to be flushed
                                   before 'stop' can be called */
    ErlDrvSSizeT (*call)(ErlDrvData drv_data, unsigned int command,
                         char *buf, ErlDrvSizeT len,
			 char **rbuf, ErlDrvSizeT rlen, unsigned int *flags);
                                /* Works mostly like 'control', a synchronous
                                   call into the driver. */
    void (*event)(ErlDrvData drv_data, ErlDrvEvent event,
                  ErlDrvEventData event_data);
                                /* Called when an event selected by 
                                   driver_event() has occurred */
    int extended_marker;        /* ERL_DRV_EXTENDED_MARKER */
    int major_version;          /* ERL_DRV_EXTENDED_MAJOR_VERSION */
    int minor_version;          /* ERL_DRV_EXTENDED_MINOR_VERSION */
    int driver_flags;           /* ERL_DRV_FLAGs */
    void *handle2;              /* Reserved -- Used by emulator internally */
    void (*process_exit)(ErlDrvData drv_data, ErlDrvMonitor *monitor);
                                /* Called when a process monitor fires */
    void (*stop_select)(ErlDrvEvent event, void* reserved);
                                /* Called to close an event object */
 } ErlDrvEntry;

int (*init)(void)

This is called directly after the driver has been loaded by erl_ddll:load_driver/2. (Actually when the driver is added to the driver list.) The driver should return 0, or if the driver can't initialize, -1.

ErlDrvData (*start)(ErlDrvPort port, char* command)

This is called when the driver is instantiated, when open_port/2 is called. The driver should return a number >= 0 or a pointer, or if the driver can't be started, one of three error codes should be returned:

ERL_DRV_ERROR_GENERAL - general error, no error code

ERL_DRV_ERROR_ERRNO - error with error code in erl_errno

ERL_DRV_ERROR_BADARG - error, badarg

If an error code is returned, the port isn't started.

void (*stop)(ErlDrvData drv_data)

This is called when the port is closed, with port_close/1 or Port ! {self(), close}. Note that terminating the port owner process also closes the port. If drv_data is a pointer to memory allocated in start, then stop is the place to deallocate that memory.

void (*output)(ErlDrvData drv_data, char *buf, ErlDrvSizeT len)

This is called when an erlang process has sent data to the port. The data is pointed to by buf, and is len bytes. Data is sent to the port with Port ! {self(), {command, Data}}, or with port_command/2. Depending on how the port was opened, it should be either a list of integers 0...255 or a binary. See open_port/3 and port_command/2.

void (*ready_input)(ErlDrvData drv_data, ErlDrvEvent event)
void (*ready_output)(ErlDrvData drv_data, ErlDrvEvent event)

This is called when a driver event (given in the event parameter) is signaled. This is used to help asynchronous drivers "wake up" when something happens.

On unix the event is a pipe or socket handle (or something that the select system call understands).

On Windows the event is an Event or Semaphore (or something that the WaitForMultipleObjects API function understands). (Some trickery in the emulator allows more than the built-in limit of 64 Events to be used.)

To use this with threads and asynchronous routines, create a pipe on unix and an Event on Windows. When the routine completes, write to the pipe (use SetEvent on Windows), this will make the emulator call ready_input or ready_output.

Spurious events may happen. That is, calls to ready_input or ready_output even though no real events are signaled. In reality it should be rare (and OS dependant), but a robust driver must nevertheless be able to handle such cases.

char *driver_name

This is the name of the driver, it must correspond to the atom used in open_port, and the name of the driver library file (without the extension).

void (*finish)(void)

This function is called by the erl_ddll driver when the driver is unloaded. (It is only called in dynamic drivers.)

The driver is only unloaded as a result of calling unload_driver/1, or when the emulator halts.

void *handle

This field is reserved for the emulator's internal use. The emulator will modify this field; therefore, it is important that the driver_entry isn't declared const.

ErlDrvSSizeT (*control)(ErlDrvData drv_data, unsigned int command, char *buf, ErlDrvSizeT len, char **rbuf, ErlDrvSizeT rlen)

This is a special routine invoked with the erlang function port_control/3. It works a little like an "ioctl" for erlang drivers. The data given to port_control/3 arrives in buf and len. The driver may send data back, using *rbuf and rlen.

This is the fastest way of calling a driver and get a response. It won't make any context switch in the erlang emulator, and requires no message passing. It is suitable for calling C function to get faster execution, when erlang is too slow.

If the driver wants to return data, it should return it in rbuf. When control is called, *rbuf points to a default buffer of rlen bytes, which can be used to return data. Data is returned different depending on the port control flags (those that are set with set_port_control_flags).

If the flag is set to PORT_CONTROL_FLAG_BINARY, a binary will be returned. Small binaries can be returned by writing the raw data into the default buffer. A binary can also be returned by setting *rbuf to point to a binary allocated with driver_alloc_binary. This binary will be freed automatically after control has returned. The driver can retain the binary for read only access with driver_binary_inc_refc to be freed later with driver_free_binary. It is never allowed to alter the binary after control has returned. If *rbuf is set to NULL, an empty list will be returned.

If the flag is set to 0, data is returned as a list of integers. Either use the default buffer or set *rbuf to point to a larger buffer allocated with driver_alloc. The buffer will be freed automatically after control has returned.

Using binaries is faster if more than a few bytes are returned.

The return value is the number of bytes returned in *rbuf.

void (*timeout)(ErlDrvData drv_data)

This function is called any time after the driver's timer reaches 0. The timer is activated with driver_set_timer. There are no priorities or ordering among drivers, so if several drivers time out at the same time, any one of them is called first.

void (*outputv)(ErlDrvData drv_data, ErlIOVec *ev)

This function is called whenever the port is written to. If it is NULL, the output function is called instead. This function is faster than output, because it takes an ErlIOVec directly, which requires no copying of the data. The port should be in binary mode, see open_port/2.

The ErlIOVec contains both a SysIOVec, suitable for writev, and one or more binaries. If these binaries should be retained, when the driver returns from outputv, they can be queued (using driver_enq_bin for instance), or if they are kept in a static or global variable, the reference counter can be incremented.

void (*ready_async)(ErlDrvData drv_data, ErlDrvThreadData thread_data)

This function is called after an asynchronous call has completed. The asynchronous call is started with driver_async. This function is called from the erlang emulator thread, as opposed to the asynchronous function, which is called in some thread (if multithreading is enabled).

ErlDrvSSizeT (*call)(ErlDrvData drv_data, unsigned int command, char *buf, ErlDrvSizeT len, char **rbuf, ErlDrvSizeT rlen, unsigned int *flags)

This function is called from erlang:port_call/3. It works a lot like the control call-back, but uses the external term format for input and output.

command is an integer, obtained from the call from erlang (the second argument to erlang:port_call/3).

buf and len provide the arguments to the call (the third argument to erlang:port_call/3). They can be decoded using ei functions.

rbuf points to a return buffer, rlen bytes long. The return data should be a valid erlang term in the external (binary) format. This is converted to an erlang term and returned by erlang:port_call/3 to the caller. If more space than rlen bytes is needed to return data, *rbuf can be set to memory allocated with driver_alloc. This memory will be freed automatically after call has returned.

The return value is the number of bytes returned in *rbuf. If ERL_DRV_ERROR_GENERAL is returned (or in fact, anything < 0), erlang:port_call/3 will throw a BAD_ARG.

void (*event)(ErlDrvData drv_data, ErlDrvEvent event, ErlDrvEventData event_data)

Intentionally left undocumented.

int extended_marker

This field should either be equal to ERL_DRV_EXTENDED_MARKER or 0. An old driver (not aware of the extended driver interface) should set this field to 0. If this field is equal to 0, all the fields following this field also have to be 0, or NULL in case it is a pointer field.

int major_version

This field should equal ERL_DRV_EXTENDED_MAJOR_VERSION if the extended_marker field equals ERL_DRV_EXTENDED_MARKER.

int minor_version

This field should equal ERL_DRV_EXTENDED_MINOR_VERSION if the extended_marker field equals ERL_DRV_EXTENDED_MARKER.

int driver_flags

This field is used to pass driver capability information to the runtime system. If the extended_marker field equals ERL_DRV_EXTENDED_MARKER, it should contain 0 or driver flags (ERL_DRV_FLAG_*) ored bitwise. Currently the following driver flags exist:

The runtime system will use port level locking on all ports executing this driver instead of driver level locking when the driver is run in a runtime system with SMP support. For more information see the erl_driver documentation.
Marks that driver instances can handle being called in the output and/or outputv callbacks even though a driver instance has marked itself as busy (see set_busy_port()). Since erts version 5.7.4 this flag is required for drivers used by the Erlang distribution (the behaviour has always been required by drivers used by the distribution).
void *handle2

This field is reserved for the emulator's internal use. The emulator will modify this field; therefore, it is important that the driver_entry isn't declared const.

void (*process_exit)(ErlDrvData drv_data, ErlDrvMonitor *monitor)

This callback is called when a monitored process exits. The drv_data is the data associated with the port for which the process is monitored (using driver_monitor_process) and the monitor corresponds to the ErlDrvMonitor structure filled in when creating the monitor. The driver interface function driver_get_monitored_process can be used to retrieve the process id of the exiting process as an ErlDrvTermData.

void (*stop_select)(ErlDrvEvent event, void* reserved)

This function is called on behalf of driver_select when it is safe to close an event object.

A typical implementation on Unix is to do close((int)event).

Argument reserved is intended for future use and should be ignored.

In contrast to most of the other call-back functions, stop_select is called independent of any port. No ErlDrvData argument is passed to the function. No driver lock or port lock is guaranteed to be held. The port that called driver_select might even be closed at the time stop_select is called. But it could also be the case that stop_select is called directly by driver_select.

It is not allowed to call any functions in the driver API from stop_select. This strict limitation is due to the volatile context that stop_select may be called.


erl_driver(3), erl_ddll(3), erlang(3), kernel(3)