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Allow read/write callbacks to respond with modbus exception codes #1

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Allow read/write callbacks to respond with modbus exception codes #1

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21 changes: 11 additions & 10 deletions doc/modbus_reply_callback.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -24,7 +24,7 @@ typedef struct {
DESCRIPTION
-----------
*modbus_set_reply_callbacks()* has to be used to set the callback-structure into the
libmodbus-context. An optional parameter _user_ctx_ can be suppied which will be passed to the
libmodbus-context. An optional parameter _user_ctx_ can be supplied which will be passed to the
callback function, typically for context purpose.

The *modbus_reply_callback()* function receives the request-byte-buffer and its byte-length as
Expand All @@ -43,16 +43,16 @@ a RTU-slave, the additional *accept_rtu_slave*-callback has to be provided.
At different stages *modbus_reply_callback()* calls these callbacks by passing different arguments and
the user-context-pointer _user_ctx_ as the first one.

The first callback invoked by modbus_reply_callback() is *accept_rtu_slave* if the user implements a
The first callback invoked by modbus_reply_callback() is *accept_rtu_slave* if the user implements an
RTU-slave. In its implementation the user has to check whether the slave-id, _slave_, which was
decoded from the request, should be answered to or not and returning TRUE if so, otherwise FALSE.
Returning FALSE will make *modbus_reply_callback()* exit immedialty and return 0.
Returning FALSE will make *modbus_reply_callback()* exit immediately and return 0.

All callbacks hereafter receive the following arguments:

* _slave_ still indicating the RTU-slave-id if the modbus-instance is in RTU-mode otherwise it has
no meaning.
* _function_ containting the value of the modbus-function decoded from the request.
* _function_ containing the value of the modbus-function decoded from the request.
* _address_, the first register or coil-address.
* _nb_, the number of values to be handled.
* _bytes[]_, the byte-buffer containing the raw-data (write-request) or where the raw-data has to
Expand All @@ -63,7 +63,7 @@ All callbacks hereafter receive the following arguments:
*modbus_reply_callback()* is then doing some basic sanitizing on standard-specific parameters before
calling the second callback *verify*.

This is done to verify whether the access is valid for this instance in regards to the address-range
This is done to verify whether the access is valid for this instance in regard to the address-range
or the modbus-function of the request. The user has to return 0 if the range defined by _address_
and _nb_ is inside the device's range and the modbus-function can be handled.

Expand All @@ -80,11 +80,12 @@ modbus-specification and needs to be encoded, or decoded, properly inside the ca
Coils are encoded as up to 8 coils per byte, registers are 16-bit wide and thus consume 2 bytes of the
buffer per value. See below.

The return value of the read-callback. Negative numbers or zero are interpreted as no response
should be send. To, for example, trigger a timeout on the other side.

The write-callbacks has to return 0 to signal success or a negative number to make the library
not send any response.
The read-callback should return the length (bytes) of data written if successful.
The write-callback should return 0 to signal success.
The read and write callbacks may return a negated error code within the range of modbus protocol
exception error codes (-EMBXILFUN to -EMBXGTAR) to send an exception response. Any other negative
numbers or zero are interpreted as no response should be sent. To, for example, trigger a timeout
on the other side.

The library does not do any differentiation regarding the actual modbus-function and the appropriate
invoked callback. All MODBUS_READ-functions are handled by the read-callback, all
Expand Down
17 changes: 16 additions & 1 deletion src/modbus-reply.c
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Original file line number Diff line number Diff line change
Expand Up @@ -306,7 +306,7 @@ int modbus_reply_callback(modbus_t *ctx, const uint8_t *req, int req_length)

/* user verification was successful */

int rc;
int rc = 0;
rsp_length = ctx->backend->build_response_basis(&sft, rsp);

switch (function) {
Expand Down Expand Up @@ -410,6 +410,21 @@ int modbus_reply_callback(modbus_t *ctx, const uint8_t *req, int req_length)
}

send_response:
/* Check if a user read/write callback returned a negated error code within the
* range of modbus protocol exceptions, if so respond with that exception */
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Alternatively could get the user to set errno in the callback, instead of returning the negated error number if you think that's better

if (rc < 0) {
int modbus_errno = -rc;
int protocol_exception = modbus_errno - MODBUS_ENOBASE;
if (protocol_exception > 0 && protocol_exception < MODBUS_EXCEPTION_MAX) {
rsp_length = response_exception(
ctx, &sft,
protocol_exception, rsp, FALSE,
"%s: 0x%0X (%s)\n",
modbus_strerror(modbus_errno),
function, function_name);
}
}

if ((ctx->backend->backend_type == _MODBUS_BACKEND_TYPE_RTU &&
slave == MODBUS_BROADCAST_ADDRESS) ||
rsp_length == 0) /* this indicates that the user does not want us to send response,
Expand Down