malloc problem

[RicOz6]

This is supposed to be a basic programme to see how malloc works, it compiles and runs giving the correct output but then I get "DEBUG error, heap corruption detected" and I don't understand what I've done wrong.

#include <stdio.h>
#include <stdlib.h>

int main(void)
{
	char *p;

	p = (char *)malloc(sizeof(char));

	if (p==0)

	{
		printf("\nERROR. Not enough memory\n");
		return 1;
	}

	else{

	printf("\nEnter your six digit library card no. \n");
	scanf("%s", p);
	printf("value = %s\n", p);

	free(p);

	}


	return 0;

}

[rags_to_riches]

You're only allocating enough memory for a single character. You need to malloc enough space to hold the number of characters you are expecting plus one for the null terminator character.

[CommonTater]

You only allocated 1 character with malloc() and then tried to stuff 6 characters into it with scanf().

Also, there's no reason to typecast the return value of malloc ... p = malloc( ); is good enough.

[cas]

You're allocating a single byte (that's what sizeof(char) means). Thus the only string your pointer can point to is a single 0, or the empty string. But you're asking the user to enter sig digits. A string of 6 digits would require at least 7 bytes (6 for the digits, 1 for the null character). So you end up writing past the end of your buffer.

You can do something like:

p = malloc(50 * sizeof *p);

Now you have 50 bytes, which can still be overflowed, but at least you've got a fighting chance of working.

The multiplication by sizeof *p is not necessary, since it's 1 by definition, but some consider it good style anyway, the idea being that you can change the type of “p” and still get the allocation right. I don't subscribe to this notion, because char* is almost always used for strings, which coerce you into using char*; even if you switch to wchar_t, you'd have to change all sorts of other things as well, not just the declaration of “p”.

[kona49er]

first mistake, sizeof(char) is 1

[RicOz6]

thanks all, I think I understand a bit better how it works now.