Thread: Need help on malloc and realloc

Hello,
I am a C newbie working on a program using malloc and realloc functions. The problem is after compiling and running the program, the output of the array is garbage values. I would really appreciate it if someone could point out what's wrong with my code. Thanks a lot.

Code:

#include <iostream>

using std::cout;
using std::cin;
using std::endl;

#include <cstdlib>  // for malloc and realloc memory functions

int main() {
	int *listPtr;   // pointer to dynamically allocated array
	int temp_input; // value which will be copied into listPtr array
	int count=0;    // count value used to reallocate a new size for the integer
	                // array on each integer input

	listPtr = (int *) malloc(2 * sizeof(int)); 
	// allocate memory locations for a minimum of 2 integers

	do {
		++count;
		cout << "Enter integer " << count << ": ";
		cin >> temp_input;

		listPtr = (int *) realloc(listPtr, count); 
		// reallocate the array increasing its size by 1 since
		// one more integer is entered each time
		
		if (temp_input != -1) { 
			*(listPtr + (count-1)) = temp_input; // count-1 so first space is used up
		}
		// assign subsequent integer into next free array spot
		// make sure an array is pointing to the next free spot since realloc()
		// function points to beginning of the array each time

	} while (temp_input != -1);

	for (int a=0; a < count; a++) {  // output the dynamically allocated array
		cout << *(listPtr + a) << ' ';
	}
	cout << endl;

return 0;
}

Justin

YevGenius.

Please decide whether you're going to code this in C or C++.

Once decided place the code in the appropriate forum.

Originally posted by YevGenius
Hi,
I don't understand. I think I have written everything according to C standards. Is it the cin and couts that make you think this is a C++ program? Can you please tell me why you can't tell if this is a C or C++ program?

Justin

me too

Originally posted by YevGenius
Hi,
I don't understand. I think I have written everything according to C standards. Is it the cin and couts that make you think this is a C++ program? Can you please tell me why you can't tell if this is a C or C++ program?

Justin

Code:

#include <iostream>

using std::cout;
using std::cin;
using std::endl;

#include <cstdlib>  // for malloc and realloc memory functions

Nothing in your program is C if any part of it is C++. Your C compiler can not compile any code with any C++ in it. If it compiles, it's because you're compiling it as C++ and not C.

Quzah.

> listPtr = (int *) malloc(2 * sizeof(int));
There is no point with an initial memory allocation, you can just start with
listPtr = NULL;
realloc handles extending memory from nothing.

> listPtr = (int *) realloc(listPtr, count);
3 things wrong here
1. You're compiling with C++ - yeah, the other posters are right. In C, we don't cast the return result of malloc type calls. C++ forces you to do this. The cout calls also give the game away.
2. you forgot the sizeof(int) scaling factor
3. you don't check that the realloc call succeeded. Worse still, if it failed, you lose everything

Code:

void *temp = realloc( listPtr, count * sizeof *listPtr );
if ( temp != NULL ) {
    /* success, update our real pointer */
    listPtr = temp;
} else {
    /* oh no, save whats in listPtr and back out */
    /* save and quit, or just report can't do that */
}

> if (temp_input != -1)
do this before extending the array.

> *(listPtr + (count-1)) = temp_input;
Unless this is also an exercise in using pointer notation, its clearer to write
listPtr[count-1] = temp_input;