Re: Basic c question again - type casting

On 14/06/2008, at 2:29 AM, Lim Hock-Chai wrote:

I'm a bit lost. You say my RPG prototype should be 10A, not *string.
But your example showing RPG protype with *string.

I really prefer to do
typedef struct {
char value[10];
} logECD_STP3_piModuleName_T;

But I don't want to have to do below in c just to call a function:
memset(&piModuleName, ' ', sizeof(piModuleName));
memcpy(&piModuleName, "MYMODULE",8);
logECDSTP3(piModuleName...)

You have to do that because C provides no help with fixed-length variables. Note that C itself also provides no help with null- terminated strings. All the string functions used throughout C are library functions and part of the LANGUAGE specification. Therefore you could provide your own library of functions to deal with fixed- length data. This would have the effect of hiding the memset/memcpy pairs and if written properly could handle literal values too.

Simple pre-processor macros might help too:

#define CLEAR( s ) \
memset( (void *)&(s), 0x40, sizeof(s) )
#define COPY_STR( s, t ) \
memcpy( (void *)&(t), (void*)&(s), strlen(s) )
#define COPY_FXD( s, t ) \
memcpy( (void *)&(t), (void*)&(s), sizeof(s) )

You could combine these to get copy-with-pad functions.

Note: Not tested to use at own risk.

However, you can only do so much with macros and a set of fixed- length library functions would be better.

I want to do this, hence, I changed it to use *string
logECDSTP3("MYMODULE", ...)

You can do this by casting the literal to the correct type:

#include <stdio.h>
typedef struct {
char msg[10];
} Msg_T;
void func( Msg_T* );
void main( void )
{
Msg_T* text = {(Msg_T*)"test data"};
func( text );
func( (Msg_T*)"more test data" );
return;
}
void func( Msg_T* data )
{
printf( "%s\n", data );
return;
}

Anyone notice the defects in this code?

It works but it's not correct because:
a) printf() prints more than it should for the second call to func()
b) data has a null terminator on the first call because "test data" is shorter than the 10 bytes defined in text.msg so it's not fixed- length
c) data has no null terminator on the second call because "more test data" is longer than the 10 bytes defined thus printf() reads beyond the data.msg area

It's also ugly because of the casts between types. Sometimes necessary but should be avoided if possible.

In addition there are problems introduced by mixing fixed-length character data with functions expecting null-terminated strings. Specifying something like:

printf(%*.*s\n", sizeof(data->msg), sizeof(data->msg), data->msg);

may work but again is ugly.

Clear as mud?

Oh yes indeed.

Regards,
Simon Coulter.
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