A nice 64-bit error in C

19.11.2009 Andrey Karpov

In C language, you may use functions without defining them. Pay attention that I speak about C language, not C++. Of course, this ability is very dangerous. Let us have a look at an interesting example of a 64-bit error related to it. Below is the correct code that allocates and uses three arrays, 1 GB each:

#include <stdlib.h>
void test()
{
  const size_t Gbyte = 1024 * 1024 * 1024;
  size_t i;
  char *Pointers[3];
  // Allocate
  for (i = 0; i != 3; ++i)
    Pointers[i] = (char *)malloc(Gbyte);
  // Use
  for (i = 0; i != 3; ++i)
    Pointers[i][0] = 1;
  // Free
  for (i = 0; i != 3; ++i)
    free(Pointers[i]);
}

This code correctly allocates memory, writes one into the first item of each array and frees the allocated memory. The code is absolutely correct on a 64-bit system.

Now delete or comment the line "#include <stdlib.h>". The code still compiles but the program crashes after the launch. As the header file "stdlib.h" is disabled, the C compiler considers that malloc function will return int type. The first two allocations are most likely to be successful. After the third call, malloc function will return the array's address outside the range of the first two Gbyte. As the compiler considers the function's result to have int type, it interprets the result incorrectly and saves the incorrect value of the pointer in Pointers array.

To make it clearer, let us consider an assembler code generated by Visual C++ compiler for the 64-bit Debug version. At first look at the correct code generated when malloc function is defined (i.e. the file "stdlib.h" is included):

Pointers[i] = (char *)malloc(Gbyte);
mov   rcx,qword ptr [Gbyte]
call  qword ptr [__imp_malloc (14000A518h)]
mov    rcx,qword ptr [i]
mov    qword ptr Pointers[rcx*8],rax

Now consider the variant of the incorrect code when malloc function is not defined:

Pointers[i] = (char *)malloc(Gbyte);
mov    rcx,qword ptr [Gbyte]
call   malloc (1400011A6h)
cdqe
mov    rcx,qword ptr [i]
mov    qword ptr Pointers[rcx*8],rax

Consider the CDQE instruction (Convert doubleword to quadword). The compiler supposed the result to be kept in eax registers and extended it to a 64-bit value to write into Pointers array. Respectively, the high-order bits of rax register are lost. Even if the address of the allocated memory is inside the range of the first 4 GB, we still get the incorrect result when the high-order bit of eax register equals 1. For example, the address 0x81000000 turns into 0xFFFFFFFF81000000.

Fortunately, this type of errors is easy to define. For example, Visual C++ compiler generates two warnings informing about a potential problem:

warning C4013: 'malloc' undefined; assuming extern returning int
warning C4312: 'type cast' : conversion from 'int' to 'char *' of greater size

And PVS-Studio 3.40 analyzer generates the warning "error V201: Explicit type conversion. Type casting to memsize.".