cmu.edu EXP36-C. Do not cast pointers into more strictly aligned pointer types

Do not convert a pointer value to a pointer type that is more strictly aligned than the referenced type. Different alignments are possible for different types of objects. If the type-checking system is overridden by an explicit cast or the pointer is converted to a void pointer (void *) and then to a different type, the alignment of an object may be changed.

NOTE:

一、"If the type-checking system is overridden by an explicit cast or the pointer is converted to a void pointer (void *) and then to a different type, the alignment of an object may be changed"

这是非常能够体现void *非static type safety，后面会有例子说明

二、"aliasing-pointer cast to stricter aligned type-unaligned access-undefined behavior"

If the misaligned pointer is dereferenced, the program may terminate abnormally. On some architectures, the cast alone may cause a loss of information even if the value is not dereferenced if the types involved have differing alignment requirements.

NOTE:

misaligned pointer是会导致很多问题的

void * 非static type safety

NOTE:

这是非常能够体现void *非static type safety、static type safety的写法

Noncompliant Code Example

int *loop_function(void *v_pointer) {
  /* ... */
  return v_pointer;
}

void func(char *char_ptr) {
  int *int_ptr = loop_function(char_ptr);

  /* ... */
}

Compliant Solution

Because the input parameter directly influences the return value, and loop_function() returns an object of type int *, the formal parameter v_pointer is redeclared to accept only an object of type int *:

int *loop_function(int *v_pointer) {
  /* ... */
  return v_pointer;
}

void func(int *loop_ptr) {
  int *int_ptr = loop_function(loop_ptr);

  /* ... */
}

aliasing、pointer cast to stricter aligned type unaligned access

NOTE:

使用memcpy

Noncompliant Code Example

#include <string.h>

struct foo_header {
  int len;
  /* ... */
};

void func(char *data, size_t offset) {
  struct foo_header *tmp;
  struct foo_header header;

  tmp = (struct foo_header *)(data + offset);
  memcpy(&header, tmp, sizeof(header));

  /* ... */
}

Compliant Solution

#include <string.h>

struct foo_header {
  int len;
  /* ... */
};

void func(char *data, size_t offset) {
  struct foo_header header;
  memcpy(&header, data + offset, sizeof(header));

  /* ... */
}