question about dynamic memory allocation

[Fernando Barsoba]

Hi all,

I have a simple question regarding dynamic memory allocation.. is there
a way to create a variable for which we don't know its size using the
'stack' instead of the 'heap'? For instance, I have the following:

#define MESG_LENGTH 12 unsigned char packet[sizeof(struct ip) + MESG_LENGTH];

But what if I don't know beforehand the size of MESG_LENGTH? Can I have
#define a variable taking its value from outside the code? for instance,
from a file or a parameter?

I know that for these situations one usually uses the 'heap' (malloc,
etc).. but I was wondering if there is a way to do it as asked... just
curiosity..

Thanks,

FBM

[Alex Fraser]

"Fernando Barsoba" <fb******@verizon.net> wrote in message
news:Sltcf.11829$Bx.4727@trnddc01...

I have a simple question regarding dynamic memory allocation.. is there
a way to create a variable for which we don't know its size using the
'stack' instead of the 'heap'?

With a C99 compiler, you may use variable length arrays. OT: on many
systems, a function alloca() exists which can achieve a similar effect.

Alex

[Fernando Barsoba]

Alex Fraser wrote:

"Fernando Barsoba" <fb******@verizon.net> wrote in message
news:Sltcf.11829$Bx.4727@trnddc01...

I have a simple question regarding dynamic memory allocation.. is there
a way to create a variable for which we don't know its size using the
'stack' instead of the 'heap'?

With a C99 compiler, you may use variable length arrays. OT: on many
systems, a function alloca() exists which can achieve a similar effect.

Alex

Is this good programming style? Any problems doing this?

[pete]

Fernando Barsoba wrote:

Alex Fraser wrote:

"Fernando Barsoba" <fb******@verizon.net> wrote in message
news:Sltcf.11829$Bx.4727@trnddc01...

I have a simple question regarding dynamic memory allocation.. is there
a way to create a variable for which we don't know its size using the
'stack' instead of the 'heap'?

With a C99 compiler, you may use variable length arrays. OT: on many
systems, a function alloca()
exists which can achieve a similar effect.

Alex

Is this good programming style? Any problems doing this?

malloc is more portable than alloc.

--
pete

[Jordan Abel]

On 2005-11-09, Fernando Barsoba <fb******@verizon.net> wrote:

Alex Fraser wrote:

"Fernando Barsoba" <fb******@verizon.net> wrote in message
news:Sltcf.11829$Bx.4727@trnddc01...

I have a simple question regarding dynamic memory allocation.. is there
a way to create a variable for which we don't know its size using the
'stack' instead of the 'heap'?

With a C99 compiler, you may use variable length arrays. OT: on many
systems, a function alloca() exists which can achieve a similar effect.

Alex

Is this good programming style? Any problems doing this?

alloca() is never good programming style. There are many problems with
doing it - In particular, many historical implementations are quite
buggy and in particular cannot be safely called in the middle of an
argument list.

[tedu]

Fernando Barsoba wrote:

Hi all,

I have a simple question regarding dynamic memory allocation.. is there
a way to create a variable for which we don't know its size using the
'stack' instead of the 'heap'? For instance, I have the following:

> #define MESG_LENGTH 12

> unsigned char packet[sizeof(struct ip) + MESG_LENGTH];

But what if I don't know beforehand the size of MESG_LENGTH? Can I have
#define a variable taking its value from outside the code? for instance,
from a file or a parameter?

#define MAX_MESG_LENGTH 1024
is the easiest way to go in many cases.

[Fernando Barsoba]

Alex Fraser wrote:

"Fernando Barsoba" <fb******@verizon.net> wrote in message
news:Sltcf.11829$Bx.4727@trnddc01...

I have a simple question regarding dynamic memory allocation.. is there
a way to create a variable for which we don't know its size using the
'stack' instead of the 'heap'?

With a C99 compiler, you may use variable length arrays.

So, do you mean by variable length arrays that this is valid?

int size_array = sizeof(struct ip) + cnf->msg_length;
/* cnf->msg_length returns an 'int' */
unsigned char packet[size_array];

FBM