Function pointer comparison in DLL code

In XPLINK code, function pointers are compared using the address of the descriptor. No special considerations, such as dereferencing, are required to initialize the function pointer prior to comparison. Function descriptors are guaranteed to be unique throughout the XPLINK application for a particular function, so this comparison of function descriptor addresses will yield the correct results even if the function pointer is passed between executable modules within the XPLINK application. The remainder of this section does not apply to XPLINK applications.

In non-XPLINK DLL code, a function pointer must be NULL before it is compared. For a non-NULL pointer, the pointer is further dereferenced to obtain the function address that is used for the comparison. For an uninitialized function pointer that has a non-zero value, the dereference can cause an exception to occur. This happens if the storage that the uninitialized pointer points to is read-protected.

Usually, comparing uninitialized function pointers results in undefined behavior. You must initialize a function pointer to NULL or the function address (from source view). Two examples follow.

Figure 1 shows undefined comparison in DLL code (C or C++).

#include <stdio.h>
int (*fp2)(const char *, ...)  /* Initialize to point to the      */
                     = printf; /* descriptor for printf           */
int goo(void);
int (*fp2)(void) = goo;
int goo(void) {
  int (*fp1)(void);
  if (fp1 == fp2)
    return (0);
  else
    return (1);
}
  void check_fp(void (*fp)()) {
      /* exception likely when -1 is dereferenced below */
      if (fp == (void (*)())-1)
        printf("Found terminator\n");
      else
        fp();
  }
  void dummy() {
    printf("In function\n");
  }

  main() {
    void (*fa[2])();
    int i;

    fa[0] = dummy;
    fa[1] = (void (*)())-1;

    for(i=0;i<2;i++)
      check_fp(fa[i]);
  }

Figure 2 shows that, when fp1 points to a read-protected memory block, an exception occurs.

Figure 2. Comparison of function pointers in DLL code (C or C++)

Figure 3 is an example of valid comparisons in DLL code.

#include <stdio.h>
int (*fp1)(const char *, ...); /* An extern variable is implicitly*/
                               /* initialized to zero             */
                               /* if it has not been explicitly   */
                               /* initialized in source.          */
int (*fp2)(const char *, ...)  /* Initialize to point to the      */
                     = printf; /* descriptor for printf           */
int foo(void) {
  if (fp1 != fp2 )
    return (0);
  else
    return (1);
}