Setting a buffer/pointer to null - c++

Question

I am trying to constantly read data into a buffer of type unsigned char* from different files. However, I can't seem to set the buffer to NULL prior to reading in the next file.
Here is only the relevant code:
#include <stdio.h>
#include <fstream>
int
main (int argc, char** argv) {
FILE* dataFile = fopen("C:\\File1.txt", "rb");
unsigned char *buffer = NULL;
buffer = (unsigned char*)malloc(1000);
fread(buffer,1,1000,dataFile);
fclose(dataFile);
dataFile = fopen("C:\\File2.txt", "rb");
buffer = NULL;
fread(buffer,1,1000,dataFile);
fclose(dataFile);
system("pause");
return 0;
}
The error I run into is at the second occurrence of this line: fread(buffer,1,1000,dataFile);
The error I get is:
Debug Assertion Failed!
Expression: (buffer != NULL)
It points me to Line 147 of fread.c which is basically:
/* validation */
_VALIDATE_RETURN((buffer != NULL), EINVAL, 0);
if (stream == NULL || num > (SIZE_MAX / elementSize))
{
if (bufferSize != SIZE_MAX)
{
memset(buffer, _BUFFER_FILL_PATTERN, bufferSize);
}
_VALIDATE_RETURN((stream != NULL), EINVAL, 0);
_VALIDATE_RETURN(num <= (SIZE_MAX / elementSize), EINVAL, 0);
}
I did Google for ways to get the buffer pointer to NULL and tried the various suggestions, but none seem to work. Anyone can clarify what is the right way to set it to NULL?

Answer 1

Your buffer is a pointer.
When you do this:
buffer = (unsigned char*)malloc(1000);
you allocate some space in memory, and assign its starting position to buffer. Remember, buffer holds the address of the beginning of the space, that's all. When you do this:
buffer = NULL;
you have thrown away that address.
EDIT:
C++ style, without dynamic memory:
#include <fstream>
using std:: string;
using std:: ifstream;
void readFromFile(string fname)
{
char buffer[1000];
ifstream fin(fname.c_str());
fin.read(buffer, sizeof(buffer));
// maybe do things with the data
}
int main ()
{
readFromFile("File1.txt");
readFromFile("File2.txt");
return 0;
}
There's no need to erase the contents of the buffer. If the cost of allocating and deallocating the buffer with each call is too much, just add static:
static char buffer[1000];
It will be overwritten each time.

Answer 2

You can't say buffer = NULL because fread wil try to dereference it. Dereferencing NULL is one of the things that are certainly and completely illegal in C++. In effect you're losing what you got from malloc. Perhaps you're looking for memset and trying to zero the buffer:
memset(buffer, 0, 1000);
However, you don't need to do this before calling fread. There's simply no reason since fread will write the buffer anyway: it doesn't care if it's zeroed or not.
As a side note: you're writing very C-ish code in what I suspect is C++ (given your fstream header). There are better-suited I/O options for C++.