C51: Null Pointer Compare Fails With MALLOC and CALLOC

Information in this article applies to:

• C51 All Versions
• CX51 All Versions

QUESTION

The pointer NULL comparison in the following code never becomes true. However, the loop should allocate all available memory.

void *p;

while (1)  {
  p = malloc (100);
  if (p == NULL)  return;
}

Is there a mistake in my code?

ANSWER

The malloc function returns a memory specific pointer to xdata. When you convert the pointer to a generic pointer, the pointer value is 0x10000. Therefore, this is not a NULL pointer anymore.

When you define the pointer as a memory typed pointer, the code works:

void xdata *p;

while (1)  {
  p = malloc (100);
  if (p == NULL)  return;
}

SEE ALSO

• C51: Null Pointer Compare Fails When Mixing Memory Types

Last Reviewed: Thursday, February 25, 2021