Using C preproc to uniquely identify structure members

I could use some advice for a problem that I've had on my mind for 
years. I'm looking for a black box function that will examine a memory 
block of a known structure and output a list of IDs that point to any 
members that have changed. I have a strong feeling that this problem has 
been solved by many people before me but I've never seen any solution. 
I'm looking for an elegant solution that requires little to no source 
code changes when a structure member is changed, added, or removed. 
Please follow my problem description below to get an idea of what I'm 
looking for.

I have a largish C structure containing data members of various types 
and sizes. The structure definition will probably have to change over 
the course of project development. Data packaged in the structure will 
arrive at some processing function. The processing function will always 
keep a copy of the previously processed structure so that a compare 
operation can be used to identify any structure members that have 
changed. There is such a large number of members that my quest is to 
automate as much as possible the ID and comparison operations. Not 
automating would require the programmer to redo the comparison which 
would be a giant if/elseif/else statement with customized comparison for 
each structure member. That would invite mistakes if not done accurately.

Below is what I've hacked out so far using the C preprocessor. It 
quickly points to various problems that makes the problem seem like too 
much work for the preprocessor (like, for example, it won't work for 
c[3]). Maybe a code generation step using perl or python would work 
better. I thought I'd check with the experts before working on it further.

#define	MEMBER(t,v) t v; size_t size_ ## v; size_t id_ ## v

typedef struct
{
   MEMBER(int,i);
   MEMBER(char,c);
   MEMBER(float,f);
} STRUCT;

STRUCT s;

s.size_i = sizeof(s.i); s.id_i = offsetof(STRUCT,i);
s.size_c = sizeof(s.c); s.id_c = offsetof(STRUCT,c);
s.size_f = sizeof(s.f); s.id_f = offsetof(STRUCT,f);

printf("I: Size = %d, ID = %d\n",s.size_i,s.id_i);
printf("C: Size = %d, ID = %d\n",s.size_c,s.id_c);
printf("F: Size = %d, ID = %d\n",s.size_f,s.id_f);

Thanks - JJS

On 4/10/2021 10:51 AM, John Speth wrote:
> I could use some advice for a problem that I've had on my mind for years. I'm 
> looking for a black box function that will examine a memory block of a known 
> structure and output a list of IDs that point to any members that have changed. 
> I have a strong feeling that this problem has been solved by many people before 
> me but I've never seen any solution. I'm looking for an elegant solution that 
> requires little to no source code changes when a structure member is changed, 
> added, or removed. Please follow my problem description below to get an idea of 
> what I'm looking for.
> 
> I have a largish C structure containing data members of various types and 
> sizes. The structure definition will probably have to change over the course of 
> project development. Data packaged in the structure will arrive at some 
> processing function. The processing function will always keep a copy of the 
> previously processed structure so that a compare operation can be used to 
> identify any structure members that have changed. There is such a large number 
> of members that my quest is to automate as much as possible the ID and 
> comparison operations. Not automating would require the programmer to redo the 
> comparison which would be a giant if/elseif/else statement with customized 
> comparison for each structure member. That would invite mistakes if not done 
> accurately.
> 
> Below is what I've hacked out so far using the C preprocessor. It quickly 
> points to various problems that makes the problem seem like too much work for 
> the preprocessor (like, for example, it won't work for c[3]). Maybe a code 
> generation step using perl or python would work better. I thought I'd check 
> with the experts before working on it further.
> 
> #define    MEMBER(t,v) t v; size_t size_ ## v; size_t id_ ## v
> 
> typedef struct
> {
>    MEMBER(int,i);
>    MEMBER(char,c);
>    MEMBER(float,f);
> } STRUCT;
> 
> STRUCT s;
> 
> s.size_i = sizeof(s.i); s.id_i = offsetof(STRUCT,i);
> s.size_c = sizeof(s.c); s.id_c = offsetof(STRUCT,c);
> s.size_f = sizeof(s.f); s.id_f = offsetof(STRUCT,f);
> 
> printf("I: Size = %d, ID = %d\n",s.size_i,s.id_i);
> printf("C: Size = %d, ID = %d\n",s.size_c,s.id_c);
> printf("F: Size = %d, ID = %d\n",s.size_f,s.id_f);

How do you plan on (automatically?) building the *code* that
checks each successive member, given that new members will
arise?  E.g., the printf()'s in your example, above.

Possibly concatenate member names into a list?  Then, "resolve"
that list?  What, besides noting member X has changed, do
you expect that code to do for you, given that you *likely*
want to treat each member slightly different from the others?

I.e., are you really making the code easier to maintain?
Or, more cryptic?

Am 10.04.2021 um 19:51 schrieb John Speth:
> I could use some advice for a problem that I've had on my mind for 
> years. I'm looking for a black box function that will examine a memory 
> block of a known structure and output a list of IDs that point to any 
> members that have changed.

The key flaw in that plan, as executed so far, is that you're trying to 
use a native C struct for this job.  Arbitrary data of varying amount 
and composition doesn't live comfortably in such a rigidly structured 
format.  Among others, forget about ever loading a piece of memory 
stored by a programming referring to one definition of that struct and 
having it interpreted sensibly by a program referring to a changed 
definition.  C structs are not at all meant for such work.

This is a job for a low-level type of database, or at the very least 
something like a tagged record-based data file.

> There is such a large number of members that my quest is to 
> automate as much as possible the ID and comparison operations. 

Automate what: the work of doing the actual comparison, or the work of 
creating/updating the code that does the comparison?

> Not 
> automating would require the programmer to redo the comparison 

I don't see a re-do, just an update to the existing comparison.

> #define    MEMBER(t,v) t v; size_t size_ ## v; size_t id_ ## v
> 
> typedef struct
> {
>    MEMBER(int,i);
>    MEMBER(char,c);
>    MEMBER(float,f);
> } STRUCT;

This design also violates an important principle: do not mix constant 
and variable data in the same structure.  This is even more important in 
smallish, bare-metal embedded work, where you want the constants to 
actually be in (flash) PROM, not waste space in RAM.

Worse than that, they wouldn't even contribute to the solution of the 
actual problem.  If these data remain strictly inside a realm of 
executable code using the exact same definition of that struct (on 
binary compatible platforms), you don't need the extra fields for 
anything.  If they can cross the boundary of that realm, having those 
elements doesn't help with the actual problem --- you then need a proper 
interface specification, which will _not_ be a C struct definition, for 
the transfer of those data, and serialization/deserialization functions 
to handle the translation between internal and external formats.

On Sat, 10 Apr 2021 10:51:18 -0700, John Speth <johnspeth@yahoo.com>
wrote:

>I could use some advice for a problem that I've had on my mind for 
>years. I'm looking for a black box function that will examine a memory 
>block of a known structure and output a list of IDs that point to any 
>members that have changed. I have a strong feeling that this problem has 
>been solved by many people before me but I've never seen any solution. 
>I'm looking for an elegant solution that requires little to no source 
>code changes when a structure member is changed, added, or removed. 
>Please follow my problem description below to get an idea of what I'm 
>looking for.
>
>I have a largish C structure containing data members of various types 
>and sizes. The structure definition will probably have to change over 
>the course of project development. Data packaged in the structure will 
>arrive at some processing function. The processing function will always 
>keep a copy of the previously processed structure so that a compare 
>operation can be used to identify any structure members that have 
>changed. There is such a large number of members that my quest is to 
>automate as much as possible the ID and comparison operations. Not 
>automating would require the programmer to redo the comparison which 
>would be a giant if/elseif/else statement with customized comparison for 
>each structure member. That would invite mistakes if not done accurately.
>
>Below is what I've hacked out so far using the C preprocessor. It 
>quickly points to various problems that makes the problem seem like too 
>much work for the preprocessor (like, for example, it won't work for 
>c[3]). Maybe a code generation step using perl or python would work 
>better. I thought I'd check with the experts before working on it further.
>
>#define	MEMBER(t,v) t v; size_t size_ ## v; size_t id_ ## v
>
>typedef struct
>{
>   MEMBER(int,i);
>   MEMBER(char,c);
>   MEMBER(float,f);
>} STRUCT;
>
>STRUCT s;
>
>s.size_i = sizeof(s.i); s.id_i = offsetof(STRUCT,i);
>s.size_c = sizeof(s.c); s.id_c = offsetof(STRUCT,c);
>s.size_f = sizeof(s.f); s.id_f = offsetof(STRUCT,f);
>
>printf("I: Size = %d, ID = %d\n",s.size_i,s.id_i);
>printf("C: Size = %d, ID = %d\n",s.size_c,s.id_c);
>printf("F: Size = %d, ID = %d\n",s.size_f,s.id_f);
>
>Thanks - JJS


While I agree with Don and Hans-Bernhard re: maintainability, I did
something similar in one of my projects.

In my case, I was translating messages between machine format (C
structs) and readable text format (to be sent via TCP or logged to a
file, depending).

I used a table-driven solution shown below.  Note this is just one
example - there were hundreds of such message structures in the
application, and keeping everything consistent was a chore.  The C
structs were used by many tasks, but only one needed to translate
them.

YMMV,
George


  *****************************************


typedef struct
    {
    DWORD        jobId;
    float        position;          /* position for _first_ exposure
*/
    int          slices;
    long         H1exposureTime;    /* ms total */
    long         H2exposureTime;    /* ms total for all - this may be
a guesstimate */
    int          H2s;               /* number of H2s */
    char*        magazineId;
    RECTANGLE    border;            /* image border */
    char*        patientName;
    char*        printDate;
    }
    MSG_JOB_PARAMETERS;


  *****************************************

typedef struct 
    {
    BOOLEAN      use;
    char*        text;
    t_Parameter  type;
    int          offset;
    }
    MESSAGE_FORMAT;


static MESSAGE_FORMAT  JobParametersFormat[] =  
    {
    { 1, "[EXPJ]"                  , PARAM_NONE   , 0 },
    { 1, "%JobId="                 , PARAM_INTEGER, offsetof(
MSG_JOB_PARAMETERS, jobId          ) },
    { 1, "%StartPosition="         , PARAM_FLOAT  , offsetof(
MSG_JOB_PARAMETERS, position       ) },
    { 1, "%NumberOfSlabs="         , PARAM_INTEGER, offsetof(
MSG_JOB_PARAMETERS, slices         ) },
    { 1, "%TotalH1ExposeTime="     , PARAM_INTEGER, offsetof(
MSG_JOB_PARAMETERS, H1exposureTime ) },
    { 1, "%NumberOfH2s="           , PARAM_INTEGER, offsetof(
MSG_JOB_PARAMETERS, H2s            ) },
    { 1, "%TotalH2ExposeTimeGuess=", PARAM_INTEGER, offsetof(
MSG_JOB_PARAMETERS, H2exposureTime ) },
    { 1, "%MagazineId="            , PARAM_STRING , offsetof(
MSG_JOB_PARAMETERS, magazineId     ) },
    { 1, "%HotspotX1="             , PARAM_INTEGER, offsetof(
MSG_JOB_PARAMETERS, border.left    ) },
    { 1, "%HotspotY1="             , PARAM_INTEGER, offsetof(
MSG_JOB_PARAMETERS, border.top     ) },
    { 1, "%HotspotX2="             , PARAM_INTEGER, offsetof(
MSG_JOB_PARAMETERS, border.right   ) },
    { 1, "%HotspotY2="             , PARAM_INTEGER, offsetof(
MSG_JOB_PARAMETERS, border.bottom  ) },
    { 1, "%PatientName="           , PARAM_STRING , offsetof(
MSG_JOB_PARAMETERS, patientName    ) },
    { 1, "%PrintDateTime="         , PARAM_STRING , offsetof(
MSG_JOB_PARAMETERS, printDate      ) },
    { 1, "[]"                      , PARAM_NONE   , 0 },
    { 0, NULL                      , PARAM_NONE   , 0 }
    };



static BOOLEAN 
LM_Encode( LM_ConnectionInfo* connection,  BYTE* msg, MESSAGE_FORMAT*
format )
{
char    buffer[64];
int     retcode;
int     index;
int     length;
int     i;

int*    iptr;
float*  fptr;
char**  sptr;


    retcode = LM_OK;
    

    index = 0;
    for ( i = 0; format->use == TRUE; ++i )
        {
        if ( i == 1) 
            {
            length = sprintf( buffer, "%%ConnectionId=%d\r\n",
connection->socketId );
            }
        else            
            {
            iptr = (int*)   (msg + format->offset);
            fptr = (float*) (msg + format->offset);
            sptr = (char**) (msg + format->offset);

            switch ( format->type )
                {
                case PARAM_NONE   : length = sprintf( buffer, "%s\r\n"
, format->text );                       break;
                case PARAM_INTEGER: length = sprintf( buffer,
"%s%d\r\n", format->text, *iptr );                break;
                case PARAM_FLOAT  : length = sprintf( buffer,
"%s%f\r\n", format->text, *fptr );                break;
                case PARAM_STRING : length = sprintf( buffer,
"%s%s\r\n", format->text, *sptr ? *sptr : "" );   break;
                }
            format++;              
            }

        if ((index + length) < connection->sendLimit)
            {
            strcpy( &connection->sendBuffer[index], buffer );
            index += length;
            }
        else
            {
            retcode = LM_BAD_MESSAGE_SIZE;
            break;
            }
        }


    return ( retcode );
}

On 4/10/2021 3:12 PM, George Neuner wrote:
> On Sat, 10 Apr 2021 10:51:18 -0700, John Speth <johnspeth@yahoo.com>
> wrote:
> 
>> I could use some advice for a problem that I've had on my mind for
>> years. I'm looking for a black box function that will examine a memory
>> block of a known structure and output a list of IDs that point to any
>> members that have changed. I have a strong feeling that this problem has
>> been solved by many people before me but I've never seen any solution.
>> I'm looking for an elegant solution that requires little to no source
>> code changes when a structure member is changed, added, or removed.
>> Please follow my problem description below to get an idea of what I'm
>> looking for.
>>
>> I have a largish C structure containing data members of various types
>> and sizes. The structure definition will probably have to change over
>> the course of project development. Data packaged in the structure will
>> arrive at some processing function. The processing function will always
>> keep a copy of the previously processed structure so that a compare
>> operation can be used to identify any structure members that have
>> changed. There is such a large number of members that my quest is to
>> automate as much as possible the ID and comparison operations. Not
>> automating would require the programmer to redo the comparison which
>> would be a giant if/elseif/else statement with customized comparison for
>> each structure member. That would invite mistakes if not done accurately.
>>
>> Below is what I've hacked out so far using the C preprocessor. It
>> quickly points to various problems that makes the problem seem like too
>> much work for the preprocessor (like, for example, it won't work for
>> c[3]). Maybe a code generation step using perl or python would work
>> better. I thought I'd check with the experts before working on it further.
>>
>> #define	MEMBER(t,v) t v; size_t size_ ## v; size_t id_ ## v
>>
>> typedef struct
>> {
>>    MEMBER(int,i);
>>    MEMBER(char,c);
>>    MEMBER(float,f);
>> } STRUCT;
>>
>> STRUCT s;
>>
>> s.size_i = sizeof(s.i); s.id_i = offsetof(STRUCT,i);
>> s.size_c = sizeof(s.c); s.id_c = offsetof(STRUCT,c);
>> s.size_f = sizeof(s.f); s.id_f = offsetof(STRUCT,f);
>>
>> printf("I: Size = %d, ID = %d\n",s.size_i,s.id_i);
>> printf("C: Size = %d, ID = %d\n",s.size_c,s.id_c);
>> printf("F: Size = %d, ID = %d\n",s.size_f,s.id_f);
>>
>> Thanks - JJS
> 
> 
> While I agree with Don and Hans-Bernhard re: maintainability, I did
> something similar in one of my projects.
> 
> In my case, I was translating messages between machine format (C
> structs) and readable text format (to be sent via TCP or logged to a
> file, depending).
> 
> I used a table-driven solution shown below.  Note this is just one
> example - there were hundreds of such message structures in the
> application, and keeping everything consistent was a chore.  The C
> structs were used by many tasks, but only one needed to translate
> them.

But you're essentially (and unconditionally) doing the same thing
with each member.

The OP's comment suggests he wants to act on CHANGED values.
What's not specified is what those actions are likely to be
I'm assuming if member X changes, he may want to do X_action()
while if Y has changed, he may want to do Y_action().  When
member Z is added, he still relies on the developer to
create Z_action().

Also, your application benefits directly from the definition
of the members -- in terms of defining which are present in
the "messages" as well as the order in which they are
to be emitted (which may not be significant).  Nothing is ever
ignored (if it's in the table, it's emitted).  The OP's
comment suggests if there is no *change*, then none of the
*_action()'s are invoked (?)

Thanks for the alternative thoughts you have provided. I admit my idea 
is only half thought out. My quest is to be able specify a structure 
definition, run make, and get some code of some sort that will output a 
list of changed structure members when a new copy of the structure 
arrives during run time. The details are all TBD by way of this discussion.

After reading the replies and thinking it through further, I think a 
perl or python assisted code generation step in the makefile will be the 
best way to go. The C preproc alone is insufficient. The script will 
take the structure definition as input. Script output will be a list of 
#defines that uniquely identify each structure member and a processing 
function that scans the incoming memory block for changes. There could 
be more than that for output after all is designed and implemented.

The proposed output above is something that could be coded manually but 
I see that something like this could have a high degree of re-usability 
and worth the investment.

JJS

On 4/11/2021 7:42 AM, John Speth wrote:
> Thanks for the alternative thoughts you have provided. I admit my idea is only 
> half thought out. My quest is to be able specify a structure definition, run 
> make, and get some code of some sort that will output a list of changed 
> structure members when a new copy of the structure arrives during run time. The 
> details are all TBD by way of this discussion.
> 
> After reading the replies and thinking it through further, I think a perl or 
> python assisted code generation step in the makefile will be the best way to 
> go. The C preproc alone is insufficient. The script will take the structure 
> definition as input. Script output will be a list of #defines that uniquely 
> identify each structure member and a processing function that scans the 
> incoming memory block for changes. There could be more than that for output 
> after all is designed and implemented.

You may wish to create your own "header" file, of sorts.  The tool can
take the struct definition (in some form that is convenient for you
to specify *there*) and generate the appropriate *.h for your code
to include.

It can, then, allow you to augment each member "declaration" (in your
special header file) with a stanza (or two) of code describing what
to do when a delta is detected.  This would then get #included into
the real source at the appropriate point.

This allows the details of the struct (not just its members but, also,
how the code interprets those deltas) to be maintained outside of
the "real" code.

I do this with my IDL compiler and C front-end.  It lets me define
the interface to object classes in a clean manner (that C won't
accommodate) along with the client- and server- -side stubs for
each method.

So, much of the actual code is built just by #including bits of this
cruft, where appropriate (e.g., a server is just a skeleton that
parses messages and dispatches to the appropriate server-side stubs...
specified in that file)

> The proposed output above is something that could be coded manually but I see 
> that something like this could have a high degree of re-usability and worth the 
> investment.

The downside is you have yet another tool that you have to maintain.
And, if others are using your codebase (and toolchain), another tool
to formally *define*.  (i.e., what happens if the tool evolves but you
later need to revisit an *old* release of the codebase?)

On Sat, 10 Apr 2021 18:20:30 -0700, Don Y
<blockedofcourse@foo.invalid> wrote:

>On 4/10/2021 3:12 PM, George Neuner wrote:
>> On Sat, 10 Apr 2021 10:51:18 -0700, John Speth <johnspeth@yahoo.com>
>> wrote:
>> 
>>> :
>>> I have a largish C structure containing data members of various types
>>> and sizes. The structure definition will probably have to change over
>>> the course of project development. Data packaged in the structure will
>>> arrive at some processing function. The processing function will always
>>> keep a copy of the previously processed structure so that a compare
>>> operation can be used to identify any structure members that have
>>> changed. There is such a large number of members that my quest is to
>>> automate as much as possible the ID and comparison operations. Not
>>> automating would require the programmer to redo the comparison which
>>> would be a giant if/elseif/else statement with customized comparison for
>>> each structure member. That would invite mistakes if not done accurately.
>>> :
>>
>> I used a table-driven solution ...
>
>But you're essentially (and unconditionally) doing the same thing
>with each member.
>
>The OP's comment suggests he wants to act on CHANGED values.
>What's not specified is what those actions are likely to be
>I'm assuming if member X changes, he may want to do X_action()
>while if Y has changed, he may want to do Y_action().  When
>member Z is added, he still relies on the developer to
>create Z_action().
>
>Also, your application benefits directly from the definition
>of the members -- in terms of defining which are present in
>the "messages" as well as the order in which they are
>to be emitted (which may not be significant).  Nothing is ever
>ignored (if it's in the table, it's emitted).  The OP's
>comment suggests if there is no *change*, then none of the
>*_action()'s are invoked (?)

All true, however I said I did something "similar" - not the same. The
first problem to be solved here is how to iterate over the struct
members - and I've shown a fairly simple way to do that.

My approach easily can be modified to detect changes in member values.
It's simple to cache the last value seen for each member, or in the
case of a string to keep a hash value so as not to have to copy and
store characters.

What I don't have a good solution for is automating construction of
the tables.  In C++ it might be doable with some template wizardry (I
haven't actually tried it), but in plain C its up to the programmer to
keep the tables congruent with their target structs.

YMMV,
George

On 4/12/2021 10:06 AM, George Neuner wrote:
> On Sat, 10 Apr 2021 18:20:30 -0700, Don Y
> <blockedofcourse@foo.invalid> wrote:
> 
>> On 4/10/2021 3:12 PM, George Neuner wrote:
>>> On Sat, 10 Apr 2021 10:51:18 -0700, John Speth <johnspeth@yahoo.com>
>>> wrote:
>>>
>>>> :
>>>> I have a largish C structure containing data members of various types
>>>> and sizes. The structure definition will probably have to change over
>>>> the course of project development. Data packaged in the structure will
>>>> arrive at some processing function. The processing function will always
>>>> keep a copy of the previously processed structure so that a compare
>>>> operation can be used to identify any structure members that have
>>>> changed. There is such a large number of members that my quest is to
>>>> automate as much as possible the ID and comparison operations. Not
>>>> automating would require the programmer to redo the comparison which
>>>> would be a giant if/elseif/else statement with customized comparison for
>>>> each structure member. That would invite mistakes if not done accurately.
>>>> :
>>>
>>> I used a table-driven solution ...
>>
>> But you're essentially (and unconditionally) doing the same thing
>> with each member.
>>
>> The OP's comment suggests he wants to act on CHANGED values.
>> What's not specified is what those actions are likely to be
>> I'm assuming if member X changes, he may want to do X_action()
>> while if Y has changed, he may want to do Y_action().  When
>> member Z is added, he still relies on the developer to
>> create Z_action().
>>
>> Also, your application benefits directly from the definition
>> of the members -- in terms of defining which are present in
>> the "messages" as well as the order in which they are
>> to be emitted (which may not be significant).  Nothing is ever
>> ignored (if it's in the table, it's emitted).  The OP's
>> comment suggests if there is no *change*, then none of the
>> *_action()'s are invoked (?)
> 
> All true, however I said I did something "similar" - not the same. The
> first problem to be solved here is how to iterate over the struct
> members - and I've shown a fairly simple way to do that.

Yes.  But, what you're ULTIMATELY doing with the results (creating a
message) is uniform across all members.  You're not doing one thing
with magazineIDs and something different with printDates.

My point was the OP seems to be wanting to automate the "easy
stuff"; the hard stuff is likely "member-dependent".  I don't
see how he's going to add that in a way that makes it resilient
to developer errors (if he can't expect developers to get
comparison operations correct!)

So, consider how to focus on the *actions* that are associated
with each "detected change" instead of just automating the
detection of the change.

> My approach easily can be modified to detect changes in member values.
> It's simple to cache the last value seen for each member, or in the
> case of a string to keep a hash value so as not to have to copy and
> store characters.
> 
> What I don't have a good solution for is automating construction of
> the tables.  In C++ it might be doable with some template wizardry (I
> haven't actually tried it), but in plain C its up to the programmer to
> keep the tables congruent with their target structs.

In my IDL compiler, I just copy member function names to a
"foo.h" as I parse the interface (with appropriate command line
switch).  The developer dutifully #includes that where needed.

Similarly, building the "dispatch tables" for each object class.
(Of course, I'm in the same boat as the OP when it comes to
actually crafting those stubs; there's no way to tell a tool
what you want the code to do -- other than writing the code
to do what you want it to do!  :> )

So, having one place where the interface is defined ensures everything
that needs to coincide with that definition gets defined properly
(as an output of the IDL compiler).  The goal being to ensure
the REAL compiler throws an error if you've forgotten to do something
that *only* you can do (like fleshing out the stubs or failing to #include
a header file, etc.).

But, this comes at the expense of developing a tool to do that work
for you.

I have no idea how you could do this in the C preprocessor.
M4 may be able to lend a hand.  But, in general, most (old)
assemblers had more flexible "macro languages" where you could
actually parse arguments, etc.

And, you have to also consider how robust the tool/technique will
be.  What if the input isn't what you'd expected?  Will you throw
an (compile) error?  Or, silently generate gobbledygook?

In my world, the C compiler doesn't provide any effective type
checking/enforcement.  E.g., a foo_t and a bar_t are both the
same underlying basic (C) type.  So, no way of ensuring
that you're actually invoking a member defined for a particular
object type *on* an instance of that type!  (i.e., you'd have
to rely on run-time error detection instead of catching it
at compile-time; rolling a tool gives you that added benefit)

Again, I repeat the observation that you are now in the tools
business once you head down this path.  And, perpetually
obligated to ensure your tools track the needs of the project
AT ANY POINT IN ITS DEVELOPMENT *history*.  You're now
developing, testing and maintaining a tool instead of just
"your code".

On Mon, 12 Apr 2021 11:13:42 -0700, Don Y
<blockedofcourse@foo.invalid> wrote:

>On 4/12/2021 10:06 AM, George Neuner wrote:
>>
>Yes.  But, what you're ULTIMATELY doing with the results (creating a
>message) is uniform across all members.  You're not doing one thing
>with magazineIDs and something different with printDates.

Detecting changes AND doing something with them (the next part below)
can all be table driven.


>My point was the OP seems to be wanting to automate the "easy
>stuff"; the hard stuff is likely "member-dependent".  I don't
>see how he's going to add that in a way that makes it resilient
>to developer errors (if he can't expect developers to get
>comparison operations correct!)
>
>So, consider how to focus on the *actions* that are associated
>with each "detected change" instead of just automating the
>detection of the change.

Include in each table entry a (pointer to a) function to be called
with the value(s) if/when a change is detected.


>> My approach easily can be modified to detect changes in member values.
>> It's simple to cache the last value seen for each member, or in the
>> case of a string to keep a hash value so as not to have to copy and
>> store characters.
>> 
>> What I don't have a good solution for is automating construction of
>> the tables.  In C++ it might be doable with some template wizardry (I
>> haven't actually tried it), but in plain C its up to the programmer to
>> keep the tables congruent with their target structs.
>
>In my IDL compiler, I just copy member function names to a
>"foo.h" as I parse the interface (with appropriate command line
>switch).  The developer dutifully #includes that where needed.
>
>Similarly, building the "dispatch tables" for each object class.
>(Of course, I'm in the same boat as the OP when it comes to
>actually crafting those stubs; there's no way to tell a tool
>what you want the code to do -- other than writing the code
>to do what you want it to do!  :> )
>
>So, having one place where the interface is defined ensures everything
>that needs to coincide with that definition gets defined properly
>(as an output of the IDL compiler).  The goal being to ensure
>the REAL compiler throws an error if you've forgotten to do something
>that *only* you can do (like fleshing out the stubs or failing to #include
>a header file, etc.).
>
>But, this comes at the expense of developing a tool to do that work
>for you.

Back to the maintenance issue.  

However, the OP himself raised the possibility of custom preprocessing
tools.  If you really need to work in C, a tool that reads struct
declarations and generates code for walking them probably IS the best
generic solution.

However, if the input is limited to legal C syntax, associating
arbitrary struct members with arbitrary functions will be ... let's
call it "really, really hard" and leave it there.


>I have no idea how you could do this in the C preprocessor.
>M4 may be able to lend a hand.  But, in general, most (old)
>assemblers had more flexible "macro languages" where you could
>actually parse arguments, etc.

The complexity will depend on whether structs/unions recursively can
contain other structs/unions.  The more general, the more complex.

Personally, I wouldn't want to try doing it with M4 - it would be
easier to use a real parser tool and grab/modify the struct handling
code from an already written C parser.  There are a number of them
available.


>And, you have to also consider how robust the tool/technique will
>be.  What if the input isn't what you'd expected?  Will you throw
>an (compile) error?  Or, silently generate gobbledygook?

The input should be legal C structs, so if a custom preprocessor
fails, so should the C compiler.  Not a problem if fails silently.

Similarly, if a preprocessor produces illegal C code, the C compiler
should catch that later.

The worrisome case is that the preprocessor produces legal code that
is incorrect given the input.  Only inspection will catch this.


>In my world, the C compiler doesn't provide any effective type
>checking/enforcement.  E.g., a foo_t and a bar_t are both the
>same underlying basic (C) type.  So, no way of ensuring
>that you're actually invoking a member defined for a particular
>object type *on* an instance of that type!  (i.e., you'd have
>to rely on run-time error detection instead of catching it
>at compile-time; rolling a tool gives you that added benefit)
>
>Again, I repeat the observation that you are now in the tools
>business once you head down this path.  And, perpetually
>obligated to ensure your tools track the needs of the project
>AT ANY POINT IN ITS DEVELOPMENT *history*.  You're now
>developing, testing and maintaining a tool instead of just
>"your code".

Yup.  Tools to make tools.


YMMV,
George