Thread: I have a Question about memory usage for C.

oh sorry, I read your post wrong... thought you were saying that C compilers would error without a cast... my mistake...

I ran your code:

#include <stdlib.h>
int main()
{
char *p;
p=(char *)malloc(30);
if(p==NULL)
{
printf("Oops, not enought mem.");
return EXIT_FAILURE;
}
free(p);
return 0;
}

through the compiler with your recommended switches:

gcc -W -Wall -pedantic -ansi test.c

and got:

test.c: In function 'main':
test.c:9: warning: implicit declaration of function 'printf'

I then included a cast:

p=(char *)malloc(30);

and got:

test.c: In function 'main':
test.c:9: warning: implicit declaration of function 'printf'

CONCLUSION: Including or not including the cast makes no difference in the case of no errors.

I then made a mistake (changed the type of the variable p) with the cast still in place:

char **p;

and got:

test.c: In function 'main':
test.c:6: warning: assignment from incompatible pointer type
test.c:9: warning: implicit declaration of function 'printf'

I then removed the cast:

p=malloc(30);

and got:

test.c: In function 'main':
test.c:9: warning: implicit declaration of function 'printf'

My point is that I find warnings such as:
test.c:6: warning: assignment from incompatible pointer type
quite useful. Maybe it's a matter of style.

Originally posted by Unregistered

To reiterate again(so it gets through your thick skull)

Sorry if my posting offended you. You are right about not using sizeof() being more efficient. (I do think that any ANSI compiler worth its salt would recognize '*sizeof(char)' as 'multiply by 1' and optimize it - without optimizations on!)

CONCLUSION: I know nothing about good programming practice! :-)

well the implicit printf problem could be solved by including stdio.h... sorry I forgot about that. By "through your thick skulls" I was being general. I meant those who had thick skulls(the ones who beleive something without proving it even though the opposite is proven to them) sorry if I offended you too much

Oh I just looked back at your example and realized that you say that a warning about allocating memory for a char ** with a cast of (char *) is helpful... well if you leave the cast out like so
char **c;

c=malloc(30);

you will end up with 30 pointers to char (char **)... what is wrong with that? If you want only 30 chars then you should be more watchful of your coding.

True quzah, I was rushed when I typed out that post(by a jack .......... friend ) but I think my point was still made...

sorry to be one of those idiots who posts little thoughts all in short posts, but you could also say
char **p;

p=malloc(30*sizeof *p);

Originally posted by quzah
char **c;

c = malloc( 30 ); //WRONG

This only allocates 30 BYTES, not 30 pointers.

c = malloc( 30 * sizeof( char* ) );

This allocates 30 character pointers.

Quzah.

Whoops! This is why I like to use sizeof() for all my malloc()s. :-)

My point was that by casting the compiler checks the type of the memory that I *think* I'm allocating. Without the cast the compiler doesn't warn me.

i.e.

char **c;

/* no compiler warning */
c = malloc( 30 * sizeof( char* ) );

/* compiler warning */
c = (char *)malloc( 30 * sizeof( char* ) );

then why waste CPU time calculating the value of a
constant? Not only do you waste time by calculating the constant value of the size of char, you also waste more time multiplying it by how many elements you want.

sizeof is an operator determined at compile time. That is why you can write something like

Code:

int main(void)
{
    int A[50*sizeof(int)];
    return 0;
}

50*sizeof(int) isn't calculated by your program but by the compiler.

Originally posted by Agrajag
And omakoo you are missing the point entirely, but I will give it a rest because I have tried to explain it to you about 3 times and quzah about the same...

Sorry that you've given up because I still don't understand. I did some tests that I posted earlier in the thread that show that there is no difference between casting and not casting when there are no errors. I still don't see any benefit from leaving out the cast. Sorry to be a pain but can anyone shed any light on the reason for leaving out the cast?