EXP03-A. Do not assume the size of a structure is the sum of the of the sizes of its members

According to [[ISO/IEC 9899-1999]],

There may be unnamed padding within a structure object, but not at its beginning.

This is often referred to as structure padding. Structure members are arranged in memory as they are declared in the program text. Padding may be added to the structure to ensure the structure is properly aligned in memory.

Non-Compliant Code Example

The example below, is inspired by Dowd, assumes that the size of struct buffer is equal to the sizeof(size_t) + (sizeof(char) * 50), which may be incorrect. The size of struct buffer may actually be a larger due to structure padding.

struct buffer {
    size_t size;
    char buffer[50];
};

...

void func(struct buffer *buf) {

  /* assuming sizeof(size_t) is 4, sizeof(size_t)+sizeof(char)*50 equals 54 */
  struct buffer *buf_cpy = malloc(sizeof(size_t)+(sizeof(char)*50));

  if (buf_cpy == NULL) {
    /* Handle malloc() error */
  }
  ...
 /* with padding, sizeof(struct buffer) may be greater than 54, causing a
    small amount of data to be written outside the bounds of the memory allocated */
  memcpy(buf_cpy, buf, sizeof(struct buffer));
}

Compliant Solution

Accounting for structure padding prevents these types of errors.

struct buffer {
    size_t size;
    char buffer[50];
};

...

void func(struct buffer *buf) {

  struct buffer *buf_cpy = malloc((sizeof(struct buffer));
  if (buf_cpy == NULL) {
    /* Handle malloc() error */
  }

  ...

  memcpy(buf_cpy, buf, sizeof(struct buffer));
}

Risk Assessment

Failure to correctly determine the size of a structure can lead to subtle logic errors and buffer overflows.

References

[[Dowd 06]] Chapter 6, "C Language Issues" (Structure Padding 284-287)
[[ISO/IEC 9899-1999]] Section 6.7.2.1, "Structure and union specifiers"