Product : ISaGRAF V3.23

Date    : 1-October-1998

File    : NulInMsg.Managing C Function or C FBloc that use messages

          with null character.htm

Subject : Managing C Function or C Function Block that use messages

          with null character

Keywords: C function - C function block - null character - message




If it possible to manage messages that include 0 (NULL) character?


Yes, it’s possible since 3.23 release of ISaGRAF kernel.

Here below is described the behavior of ISaGRAF kernel

before 3.23 release:

All ISaGRAF C functions, created on the workbench by using the library

and then modified and linked to an ISaGRAF kernel, have following


1st character of the message located inside the kernel data-base

(the function receives MESSAGE, with variable type T_MSG = char*)

But if that received pointer MESSAGE is decreased (-1), it’s then possible

to obtain current length of the application variable

(declared in ISaGRAF dictionary) used as an input parameter.

a temporary buffer with maximum size (=255 or 256).

When the content of the temporary buffer is copied into a variable

(= one that has been declared inside the dictionary), then

if the variable maximum length (the length allocated inside the dictionary)

isn’t large enough to contain what is in the temporary buffer, only a part

of the temporary buffer is copied into the output variable


What has been changed in ISaGRAF since 3.23 release, is that the kernel has been

modified and C function calling mechanism has been enhanced.

One of the kernel modification consists in allocating 3 additional characters,

for each temporary buffer allocated for copying in it the result of a function

(result with message type only).

The first allocated character (called USE_CL on schema here below) is a flag that

indicates if yes or no the function has set a valid value into the

« Current Length » character. This flag has to be set by the C function itself.

The second allocated character is maximum length.

The third allocated character has to be filled by the function itself, and contains

the current length of the message output parameter.

Examples of C function block using messages with null characters:

Example 1:

The following C function returns a message variable that is made only of

3 NULL characters.

Function input parameter : none

Function output parameter : variable Q with message type

void USP_null3msg (str_arg *arg)


char* p_UseCL = Q-3 ;




Q[3] = '\0'; /* Null character again. */

/*This is just to remind returned string needs to be ended by Null character */

/* strores string current length to 3 */

p_UseCL[2] = 3;

/* Indicates that current length has been changed by setting USE_CL flag */

p_UseCL[0] = 1;


Example 2:

The following C function returns the current length of a message variable that

may include NULL character inside.

Function input parameter : msg_wn with ‘message’ type

Function output parameter : length with ‘integer’ type

void ISADLL USP_len_wn (str_arg *arg)


arg->length = (int32) (*((arg->msg_wn) -1));



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