Testing to see if something is chnull in Unix

consider this definition:

Code:

#define chnull (char *)0
  
typedef struct 
  {
      int month,day,year;
  }DATE;
  
  typedef struct Key  KEY;
  typedef struct Key* KEYS;

  struct Key 
  {
      char *sender;
      char *addressee;
      char *regarding;
      DATE date;
      int id;
      char *fname;
      KEYS next;
  };

if I set any of the char strings to chnull...

Code:

...inside some function...
  //declare stuff 
  KEY temp;

  temp.fname=(char*)malloc(sizeof(20));
  temp.fname=chnull;

I cannot successfully test for a null value by doing

Code:

if( ! temp.fname )
nor by
if( temp.fname==chnull )

but I can test for a good value

Code:

if( temp.fname )

So in this case, what is the proper way to test that
temp.fname = chnull so that the test will evaluate to true?

Thanks in advance,
Mike

I take it you're trying to generate an empty string

Well then
> #define chnull (char *)0
#define chnull '\0'

> temp.fname=(char*)malloc(sizeof(20));
> temp.fname=chnull;
temp.fname=malloc( sizeof(char) * 20 );
temp.fname[0]=chnull;
1. sizeof(20) is sizeof(int) - 4 usually, and certainly not 20
2. don't cast the result of malloc - make sure you include stdlib.h - this isn't C++

> if( temp.fname==chnull )
if ( temp.fname[0] == chnull )

I can't change the define. This is a project where changing
the defines are not allowed.


I did make the change on the unix machine to test for chnull.
if ( temp.fname[0] == chnull )
the compiler complained. I got a warning:

bash-2.03$ cc -c -g search.c
cc: Warning: search.c, line 82: In this statement, "ki.fname[0]" of type "signed char", is being converted to "pointer to signed char". (cvtdifty
pes)
if(ki.fname[0]==chnull && ki.id==0)
---------------------------^
bash-2.03$

So I changed it to ki.fhame[0]==*chnull;
the compiler didn't complain, but I (still) crash when I execute the line
of code that contains that expression.

thanks

> I can't change the define.
Oh, one of those problems

Well, in reference to your original post then...

You initialise the pointer to chnull, then presumably at some later time actually allocate some memory

temp.fname=chnull;
....
temp.fname = malloc( 20 );

I can't see anything wrong with your if statements

if( ! temp.fname )
if( temp.fname==chnull )
Both these are true, if temp.fname is chnull

if( temp.fname )
if( temp.fname != chnull )
These are true if temp.fname != chnull