beginner: structures...

[rory]

Cna anyone point me in the right direction, I have a struture in my .h
file:

typedef struct mdata
{
int names[500];
int dates[500];
int ages[500];
}m_Data;

and in my .c file I declare a pointer to this strucutre and put aside
memory to hold it:

m_Data* firstlist;
firstlist = (m_Data*)malloc (sizeof(m_Data) );

What I'm wondering about it is if there is a way that I can change each
of the elements from being a fixed size of 500 to a being user defined
size? I'm guessing I can make the elements pointers but do I use malloc
on each of the elemnnts, and if so isn't that changing the size of the
struture in which I have already put aside an initial memory of
'malloc(sizeof( m_data))'. As you can see I'm a little confused, any
ideas?

regards,
Rory.

[Ravi Uday]

Given your orig. structure below there is no way to change the
memory already
allocated to the structural elements. Its fixed at '500'

If you want to change the mem. allocated to the structure elements at
runtime you need to define the original structural elements
as pointers first .
like -

typedef struct mdata
{
int *names;
int *dates;
int *ages;
}m_Data;

then in your program you can allocate memory (yes, using malloc) to
those elements depending on its usage scenario.
Ex: my_Data->name = malloc (sizeof (m_Data->name));
[given my_Data is declared as a pointer to m_Data.]

Get a C-book and read the section on pointers !

- Ravi

Cna anyone point me in the right direction, I have a struture in my .h
file:

typedef struct mdata
{
int names[500];
int dates[500];
int ages[500];
}m_Data;

and in my .c file I declare a pointer to this strucutre and put aside
memory to hold it:

m_Data* firstlist;
firstlist = (m_Data*)malloc (sizeof(m_Data) );

What I'm wondering about it is if there is a way that I can change each
of the elements from being a fixed size of 500 to a being user defined
size? I'm guessing I can make the elements pointers but do I use malloc
on each of the elemnnts, and if so isn't that changing the size of the
struture in which I have already put aside an initial memory of
'malloc(sizeof( m_data))'. As you can see I'm a little confused, any
ideas?

regards,
Rory.

[ajm]

Hi Rory,

If you want variable sized members within your structure then you have
no choice but to use pointers (i.e., malloc and co.) so you have:

typedef struct mdata
{
int *names;
int *dates;
int *ages;
} m_Data;

it might be good to define a function to dimension your structure,
e.g., with prototype

m_Data * mdata_init(size _t names_sz, size_t dates_sz, size_t ages_sz);

which uses the *_sz arguments to perform the necessary mallocs and
return the
pointer to the caller. you still use the (m_Data *)malloc call before
returning the pointer
to the caller since this is accounts for the memory cost of holding
your int pointers and
so can be determined consistently.

you might also want to write a mdata_destroy to perform the necessary
frees too.

if in doubt just write a couple of short programs to try out the ideas
and post any
queries you still have.

hth,
ajm.

[pete]

rory wrote:

Cna anyone point me in the right direction, I have a struture in my .h
file:

typedef struct mdata
{
int names[500];
int dates[500];
int ages[500];
}m_Data;

and in my .c file I declare a pointer to this strucutre and put aside
memory to hold it:

m_Data* firstlist;
firstlist = (m_Data*)malloc (sizeof(m_Data) );
#include <stdlib.h>
firstlist = malloc(sizeof *firstlist);

The reference to m_Data is not needed with the malloc call.
The cast can hide your failure to #include <stdlib.h>

What I'm wondering about it is
if there is a way that I can change each
of the elements from being a fixed size of 500
to a being user defined size?
Yes.
I'm guessing I can make the elements pointers but do I use malloc
on each of the elemnnts,
Yes.
and if so isn't that changing the size of the
struture in which I have already put aside an initial memory of
'malloc(sizeof( m_data))'.
No.
The structure will only hold three pointers.
As you can see I'm a little confused, any
ideas?

Use pointers like you were thinking.

--
pete

[rory]

Cheers, I am now defining the original structure as you suggested with
pointers, and I try to allocate the space for each like this

m_Data* list1;
list1 = (m_Data*)malloc (sizeof(m_Data) );
list1->note = (m_Data->names*)malloc( sizeof(m_Data->names)*50);
list1->channel = (m_Data->dates*)malloc( sizeof(m_Data->dates)*50);
list1->duration = (m_Data->ages*)malloc(s izeof(m_Data->ages)*50);

however I get errors on the lines that I try to allocate memory to the
elements on:

example.c:23: parse error before '->' token
example.c:24: parse error before '->' token
example.c:25: parse error before '->' token

I hope it isn't a simple syntax error, if so I apologise in advance!

[ajm]

you don't need the m_Data-> etc. in the sizeof() calls and you can
replace the cast with (int *) since your "cast" is not a type.

Ravi's comment (jibe ;) to read the pointers section of a C book is not
bad advice...

hth
ajm.

[pete]

rory wrote:

Cheers, I am now defining the original structure as you suggested with
pointers, and I try to allocate the space for each like this

m_Data* list1;
list1 = (m_Data*)malloc (sizeof(m_Data) );
list1->note = (m_Data->names*)malloc( sizeof(m_Data->names)*50);
list1->channel = (m_Data->dates*)malloc( sizeof(m_Data->dates)*50);
list1->duration = (m_Data->ages*)malloc(s izeof(m_Data->ages)*50);

list1 = malloc(sizeof *list1);
list1 -> note = malloc(50 * sizeof *list1 -> note);
list1 -> channel = malloc(50 * sizeof *list1 -> channel);
list1 -> duration = malloc(50 * sizeof *list1 -> duration);

--
pete

[Walter Roberson]

In article <11************ **********@z14g 2000cwz.googleg roups.com>,
rory <ro*******@gmai l.com> wrote:

Cheers, I am now defining the original structure as you suggested with
pointers, and I try to allocate the space for each like this m_Data* list1;
list1 = (m_Data*)malloc (sizeof(m_Data) );
list1->note = (m_Data->names*)malloc( sizeof(m_Data->names)*50);
list1->channel = (m_Data->dates*)malloc( sizeof(m_Data->dates)*50);
list1->duration = (m_Data->ages*)malloc(s izeof(m_Data->ages)*50); however I get errors on the lines that I try to allocate memory to the
elements on: example.c:23 : parse error before '->' token

The parse error is in the second use of -> on each line.
m_Data is a structure type, not a pointer (not even a pointer
to a structure type -- it is list1 which is the pointer to a structure),
so m_Data->names is not valid.

You also appear to be doing something funky with what you are
trying to take the size of. The left hand side of your assignments
implies that list1 is a pointer to a structure which has elements
'note', 'channel', and 'duration', but the sizeof() part of
your expression implies that the structure has elements named
'names', 'dates', and 'ages' instead. Are you trying to imply
that 'note' is an array of structures of type 'names' ??

--
Okay, buzzwords only. Two syllables, tops. -- Laurie Anderson

[rory]

Cheers guys that's sorted it, I'm off now to the library to get a few
pointers on C programming (pun fully intended!).

Rory.

[Alex Fraser]

"rory" <ro*******@gmai l.com> wrote in message
news:11******** **************@ g49g2000cwa.goo glegroups.com.. .

Cna anyone point me in the right direction, I have a struture in my .h
file:

typedef struct mdata
{
int names[500];
int dates[500];
int ages[500];
}m_Data;

and in my .c file I declare a pointer to this strucutre and put aside
memory to hold it:

m_Data* firstlist;
firstlist = (m_Data*)malloc (sizeof(m_Data) );
Firstly, the cast here is unnecessary. Unnecessary casts can mask errors; in
this case, forgetting to #include <stdlib.h>. If you forget, the compiler
assumes malloc() returns an int. The result is undefined behaviour, but the
cast typically avoids any warning.

Secondly, it makes more sense to specify the size of the object rather than
the object's type. That way, if the type changes you don't need to change
the call to malloc().

With these changes, the above is written:

firstlist = malloc(sizeof *firstlist);
What I'm wondering about it is if there is a way that I can change each
of the elements from being a fixed size of 500 to a being user defined
size?
Sure.
I'm guessing I can make the elements pointers but do I use malloc on each
of the elemnnts,
Yes; make the members pointers and allocate space for them with malloc(). It
is often useful to add members to the struct to hold the size of each
allocated array, eg:

typedef struct mdata {
int *names;
size_t names_size;
int *dates;
size_t dates_size;
int *ages;
size_t ages_size;
} m_Data;

If all the sizes are the same, you could have just a single size member, or
perhaps an array of structs would make more sense.
and if so isn't that changing the size of the struture in which I have
already put aside an initial memory of 'malloc(sizeof (m_data))'.

It increases the amount of memory "associated with" the structure, but
sizeof(m_Data) is just the memory required to store the members, and doesn't
include the memory that any pointer members point to.

Alex