pointer problems, help!

[Jody Hagins]

In article <1991Mar22.082225.24948 at bronze.ucs.indiana.edu>, mitchemt at silver.ucs.indiana.edu (Terry Mitchem) writes:
|> 
|>         I am having some major problems getting a piece of code to work.
|> It seems that everytime I change the contents of one string, it affects
|> another one. For example, when I build the filename below, it gets wiped
|> out when I null the members of "target_player". The code is below, and below
|> that is the datafile I am using.
|> 
|> void edit_category()
|> {
|>   struct
|>   {
|>     char *card_number;
|>     char *quantity;
|>     char *first_name;
|>     char *last_name;
|>     char *price_mint;
|>     char *price_ex;
|>   } target_player;
|> 
|>   char filename[80],carriage_return[5];
     ^^^^^^^^^^^^^^^^^

This means that you are setting aside enough space for 80 chars, and
that memory has a starting address.  The starting address is stored in
the variable <filename>.


|>   int infile,bytes;
|> 
|>   *filename=NULL;
|> 
|>   strcat(filename,".\\"); strcat(filename,category.brand);
|>   strcat(filename,"\\");  strcat(filename,category.type);
|>   strcat(filename,"\\");  strcat(filename,category.year);
|>   strcat(filename,"\\");  strcat(filename,category.other);
|>   strcat(filename,"\\data");


Ever heard of sprintf()?


|> 
|>   *target_player.card_number=NULL; *target_player.quantity=NULL;
|>   *target_player.first_name=NULL; *target_player.last_name=NULL;
|>   *target_player.price_mint=NULL; *target_player.price_ex=NULL;


Here you are initializing a piece of memory to NULL.

Yeh, I know, I'm setting the first character to null, just like filename
above.  (I think you think that's what you are doing)

Well, no.  See, target_player.card_number (as well as the rest of the members
of this structure) is a char pointer.  That means it POINTS TO A CHAR.  It
takes up enough space in memory to store the address of a char data type.
However, it's value is not initialized, and therefore could be pointing out
in space to anywhere.  When you say *target_player.card_number = NULL, you are
saying "set the char that target_player.card_number points to, to NULL.  However,
target_player.card_number doesn't point to anything that we know of.  Therefore
you are setting some unknown place in memory (probably 0, but that's another
story) to NULL.


|> 
|>   infile=open(filename,O_RDONLY);
|>   if (infile==-1) exit(1);
|> 
|>   read(infile,target_player.card_number,5);
|>   read(infile,target_player.quantity,3);
|>   read(infile,target_player.first_name,21);
|>   read(infile,target_player.last_name,21);
|>   read(infile,target_player.price_mint,7);
|>   read(infile,target_player.price_ex,7);


Again, you obviously do not understand the concept of pointers.  I suggest
that you read through the section on pointers in a good C ref. book.

But hey, Jody, what are you talking about?  read() is supposed to read a
certain number of bytes into the char * passed as the 2nd arg.

Yeh, you got that right!

Huh?

Well, think of it this way.  read() reads a certain number of bytes, and
puts the result into the buffer pointed to by the 2nd arg.  However, you
have not allocated space for a buffer.  What's more, you have uninitialized
pointers, so you are reading into "unknown" memory areas (again, probably 0).
Remember that allocating a pointer is not the same as allocating the memory
that the pointer points to!  You need to have some space in which to read the
data.


|> }
|> ----------------------------------------------------------------------------
|> Here is the datafile:
|> 
|> 8    2  joe                  montana              .75    .40
|> 9    2  christian            okoye                .20    .10    
|> 
|>         Any and all help is appreciated. I don't seem to be able to get
|> anything useful out of K&R to help me. I am compiling with turbo-c.
|>                         Thanks in advance
|>                                 Terry
|> 



Seriously, this is not a flame.  However, I do think you need to go back to
your text, and instructor, and get a grasp on the difference between a pointer
and the data the pointer is referencing.


-- 

Jody Hagins             
hagins at gamecock.rtp.dg.com    
Data General Corp.      
62 Alexander Dr.        
RTP, N.C.  27709        
(919) 248-6035          

Nothing I ever say reflects the opinions of DGC.