NetNews Usenet Archive 1992 #31/NN_1992_31.iso/spool/comp/lang/c/18833

Internet Message Format | 1992-12-23 | 2.0 KB

Path: sparky!uunet!cs.utexas.edu!sun-barr!olivea!charnel!sifon!thunder.mcrcim.mcgill.edu!mouse
From: [e-mail] (der Mouse)
Newsgroups: comp.lang.c
Subject: Re: allocating 2-d array
Message-ID: <[e-mail]>
Date: 23 Dec 92 17:40:26 GMT
References: <[e-mail]>
Organization: McGill Research Centre for Intelligent Machines
Lines: 46

In article <[e-mail]>, [e-mail] (Joseph Delotto) writes:

> I want to dynamically allocate a 2-d array for a variable defined as:

> int    **myarray;

> where the dimensions = nrows,ncols.  Currently I'm using the
> following code:

> myarray = (int **) malloc ((unsigned) nrows*sizeof(int*));
> for(i=0;i<=nrows;i++)
>      myarray[i] = (int *) malloc ((unsigned) ncols*sizeof(int));

> which seems to work ok but looks awfully sloppy.  Is there any way to
> get the whole thing done in one shot?

No, not unless it's "char **myarray", because only chars are known to
have no alignment restrictions.  But you can do it with only two
mallocs:

myarray = (int **) malloc(nrows*sizeof(int *));
myarray[0] = (int *) malloc(nrows*ncols*sizeof(int));
for (i=1;i<nrows;i++) myarray[i] = myarray[i-1] + ncols;

(Note that your initial code ran 0..nrows, not 0..nrows-1.  Thus, you
were writing into more memory than your first malloc allocated.)

If it *is* chars, you can do

 { char *tmp;
   tmp = malloc((nrows*sizeof(char *))+(nrows*ncols));
   myarray = (char **) tmp;
   tmp += nrows * sizeof(char *);
   for (i=0;i<nrows;i++) { myarray[i] = tmp; tmp += ncols; }
 }

but this depends on the alignment of char * being acceptable for char.
For no other type is the analogous restriction known a priori to be
satisfied, which is why this can be done only for chars.  (Unless, of
course, it's in machine-specific code where you don't mind depending on
the alignment behavior of your particular machine.)

                    der Mouse

                [e-mail]