I have char **pptochar and i want to allocate a memory segment
like pp[3][5] array in the memory |-----|-----|-----|
please help me if it is possible and tell me,
a book or tutorial about mastering pointers.

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most likely your problem is due to the memory |-----|-----|-----| being
previously allocated by the operating system.

it might also be of interest that pptochar is a pointer to (a) pointer(s),
not any sort of array to characters.  let me draw you a diagram:

        here's what you want:
                         pp[0]  |  pp[1]  |  pp[2]
                       _____________________________
                pp -> | | | | | | | | | | | | | | | |
                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

                where each of the cute wittle boxen are characters

        here's what you'll get:

                                _________
                pp +> pp[0] -> | | | | | |
                   |            ^^^^^^^^^
                   |            _________
                   +> pp[1] -> | | | | | |
                   |            ^^^^^^^^^
                   |            _________
                   +> pp[2] -> | | | | | |
                                ^^^^^^^^^

there is a huge distinction.

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Look up malloc, and do something like this:

allocate pp enough memory for 3 pointers to char
for each of the three pointers, allocate enough memory for 5 chars.

thats it
Mark McIntyre

C- FAQ: http://www.eskimo.com/~scs/C-faq/top.html

array and pointer is processed internally in the same way,
(but array name is not same object with the point name)

so first, you allocate (column)*(row)*(object type size) to
pointer
then use array form as you like

example
----------------------------
#define COLUMN 3
#define ROW 5
typedef OBJ_TYPE char
.
.
.
OBJ_TYPE *ch;
ch=malloc(COLUMN*ROW*sizeof(OBJ_TYPE));

ch[0][0]='1';
ch[0][1]='\0';
ch[1]="ARM";          // not exceed ROW
.
.
.

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i have tried
*pp=(char *)malloc(n*sizeof(char))
*pp=(char *)calloc(3,5)

,but both of them does not work
while assigning a character to pp
like a two dimensional. it gives Seg.Fault.

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To arrive at the two dimensional array laid out in memory as you
illustrate then you will need a pointer of a type different from
char ** to reference the region.
In words you need a pointer to an array of 5 characters; which
can be somewhat confusing to code directly so let us define a type:

  typedef char ARRAY5[5];

now we define pptochar as:

  ARRAY5 *pptochar;

now to get the allocation:

  pptochar=malloc(3*sizeof(*pptochar));
  if(pptochar==NULL) {
     /*error recovery or message and abort*/
  }

This should allocate 15 contiguous bytes of memory addressable
in the form pptochar[i][j];

Malcolm Kay

You are trying to store a pointer returned by malloc/calloc at
the position in memory pointed to by pp. But pp doesn't yet
point to anywhere in particular.

If you carefully read Mark's suggestion and study how malloc works
you will end up with something along along the lines of:

   char **pp;
   int i;

   pp=malloc(3*sizeof(*pp)); /* sets up space for 3 pointers */
   if(pp==NULL) /* handle error */;
   for(i=0;i<3;i++) /* set each pointer to point to 5 char space */
   {  pp[i]=malloc(5*sizeof(**pp));
      if(pp[i]==NULL) /* handle error */;
   }

Note that casting the return from malloc is frowned upon in C
as it hides programming errors. If your compiler objects you've
done something wrong.

Note that this approach does not cause pp to point to 15 bytes
of contiguous allocated memory, but points to 3 pointers that each
point to a region of 5 allocted characters.

The allocated space can be accessed with pp[i][j] but the physical
layout in memory is not that which you asked for.

Malcolm Kay

Thank you very much. I have been trying to find
how do it for 1 week.

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You're in the right area but you need to reread what I said

0) Assuming you defined pp correctly:
        char **pp=NULL;

1) allocate pp enough memory for 3 pointers to char
        pp =malloc(3 * sizeof(char*));

2) for each of the three pointers, allocate enough memory for 5 chars.
        pp[0] = malloc(5);
        pp[1] = malloc(5);
        pp[1] = malloc(5);

Mark McIntyre

C- FAQ: http://www.eskimo.com/~scs/C-faq/top.html

Why define a new type for char? Is this some form
of misdirection to make later errors appear more
plausible?

In reality ch is a pointer to char. Thus it can ony have a single
subscript (or array index if you prefer).
In any case how does the compiler know to attribute ROW as the
dimension of the second index.

ch[1] only has space for a character say 'A', not a string
or a string pointer.

Malcolm Kay

Is there any tutorial or book about mastering pointers in C ?.

Thanks in advice.

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Before you buy.

I don't think a book specifically on pointers in C would be a very
good idea. Pointers are at the very core of the C language concept
and shouldn't (in my opinion) be separated from the rest of the C
language. I guess any of the well reputed books on the language are
OK. My principal introduction to C was via K&R 1 i.e.
The C Programming Language by Kernigan and Richie first edition;
(I hope I've spelt the names correctly) which I found extremely
readable. That first edition is now seriously out of date but the
second edition (K&R 2) is still one of the best C tutorials and
references.

Understanding pointers is mostly about studying examples and thinking
through the underlying meaning of each statement.
In my opinion a large part of the understanding comes from recognising
what information the compiler needs to turn the C source code into
assembly or machine code. (At times I've claimed only partly tongue in
cheek that if you can write it unambiguously in C it must be OK)

Become a code reader - read and try to understand the sources of other
people's programs that are known to work well. You don't need to come to
grips with all the source of a large application; if it is well written
you'll find that there are many modules which are  understandable more
or less in isolation. But don't ever believe that anyone always produces
good or even correct code.

Malcolm Kay

Groovy hepcat Polat was jivin' on Sat, 01 Apr 2000 21:20:52 GMT in
comp.lang.c.
memory allocation for pointer to pointer to char's a cool scene! Dig
it!

  Have a look at my 2D allocation functions which I posted here
recently.
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============= Ain't I'm a dawg!! ==============

I will let you know a book to master pointers...Here it is...
"Pointers using C" by Yeshwant Kanetkar.
(The book is authored by an Indian person)

---Amar :-)

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