malloc(), um.. yeah. This should work, but it isn't.

**knave** · 03-11-2002

Compiler : Microsoft Visual C++ (6.0)
OS : Windows XP Professional
--

I was just wanting to test the malloc() function.. Well, I got an error when trying to compile the following.
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The program is supposed to allocate memory on-the-fly for a string of 35(includeing the \0) chars or less, then assigns the alphabet(Uppercase) to the memory location, and then prints it to the screen.
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Code:

#include "stdio.h"
#include "stdlib.h"

int main()
{
	char count;
	char * ptr, *p;

	ptr = malloc(35 * sizeof(char));

	p = ptr;

	for(count = 65; count < 91; count++)
		*p++ = count;

	*p = '\0';

	puts(ptr);

	return 0;
}

Compiler Output
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error C2440: '=' : cannot convert from 'void *' to 'char *' Conversion from 'void*' to pointer to non-'void' requires an explicit cast
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I disagree with this, I don't see anything wrong with the above code... Its very frustrating to look for an error in code, when there is none. Can anyone tell me whats wrong with this?

--
The code above came from SAMS Teach Yourself C in 21 Days (Fifth Edition)
--

Yeah, i know this is a boring question/thread.

**hk_mp5kpdw** · 03-11-2002

Well, try what it suggests, use an explicit cast:

Code:

ptr = (char*) malloc( 35*sizeof(char) );

**-KEN-** · 03-11-2002

It's because it's compiling it as C++ code. Change the file extension (manually) to .c, and recompile.

Either that, or change this:

ptr = malloc(35 * sizeof(char));

to this:

ptr = (char *)malloc(35 * sizeof(char));

**C_Coder** · 03-11-2002

Compiles and executes with no problems for me, using msvc 6.

ptr = malloc(35 * sizeof(char));

isnt a char always 1 byte? So just the 35 would do?
Also check for success

Code:

ptr = malloc(35);
if(!ptr)
   printf("No more memory");

**knave** · 03-11-2002

ptr = (char*) malloc( 35*sizeof(char) ); // worked perfect.

>>It's because it's compiling it as C++ code. Change the file extension (manually) to .c, and recompile.

so...
ptr = malloc(35 * sizeof(char)); is for C++

and...
ptr = (char *)malloc(35 * sizeof(char)); is for C

hm.. I didn't know that.
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>>isnt a char always 1 byte? So just the 35 would do?
Well for portability reasons, you would use the sizeof() macro(i think its a macro).
--

Thanks to all.

**quzah** · 03-11-2002

No, actually you have it backwards. C does not require you to typecast void pointers, C++ does.

Quzah.

Thread: malloc(), um.. yeah. This should work, but it isn't.

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