Error with 'free(pointer)'

**dunxton** · 03-04-2009

Hello,

I am writing a simple function which removes non numeric characters in a string. Here I use a char pointer by allocating memory to it. However, when I try to free the allocated memory, the program fails at runtime . Kindly let me know why this is happening.

Code:

char * FilterNumerics(char * Input, int iLength)
	{
	int i=0,j=0;
	char *temp;
	temp = (char *)malloc(iLength + 2);
	if(!temp)
		{
		printf("Memory Allocation Failed\n");
		return(1);
		}
	memset(temp,'\0',strlen(temp));
	for (i = 0; i < iLength; i++) 
		{    
		if (Input[i] >= '0' && Input[i] <= '9') 
			{        
			temp[j] = Input[i];
			j++;
			}
		}
	free(temp);
	return(0);
	}

Also please let me know if there is a better way to achieve the above functionality.

**Meldreth** · 03-04-2009

you corrupt the memory with memset. strlen looks for a '\0' character but you haven't copied anything into the memory for temp. use the same method for initializing the memory that you did for creating it.

Code:

memset(temp, '\0', iLength+2);

**laserlight** · 03-04-2009

The problem could lie here:

Code:

memset(temp,'\0',strlen(temp));

Since temp points to uninitialised memory (hence the point of the memset), what strlen(temp) returns is anyone's guess. You probably meant to write iLength + 2 instead of strlen(temp), but in that case perhaps just using calloc() instead of malloc() and memset() would be better.

**dwks** · 03-04-2009

Also:

Code:

if (Input[i] >= '0' && Input[i] <= '9')

Consider isdigit().

**itCbitC** · 03-04-2009

Can you post the error you are getting?

**dunxton** · 03-04-2009

Yeah the problem was indeed with memset. After correction it is working fine now.Thanks everyone.

However, one doubt I still have is , when I executed the same code in main, it worked fine. But in a function call it was errorring out. Any ideas why it happened that way?

The error I WAS getting was :
dbgheap.c line 1010: "Expression: _crtisvalidheappointer(puserdata)"

**itCbitC** · 03-04-2009

Using malloc() and strlen() in main() like you have now is bound to have problems. As laserlight pointed out, strlen() of temp is unknown.
Post the code as it appears in main() and someone might be able to help out.

**Elysia** · 03-05-2009

Doing these kinds of things invokes undefined behavior, which is, well... undefined. So it can do anything. It can work, it can fail, it can blow up your house, or even launch nuclear missiles.

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