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Wiki MarkupThe size of a structure is not always equal to the sum of the sizes of its members. According to Section Subclause 6.7.2.1 of the C99 standard, the C Standard states, "There may be unnamed padding within a structure object, but not at its beginning" \[ [ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\2011].

This is often referred to as unnamed padding is often called 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. Structure padding allows for faster member access on many architectures.

Rearranging the fields in a struct can change the size of the struct. It is possible to minimize padding anomalies if the fields are arranged in such a way that fields of the same size are grouped together.

Padding is also referred to as also called struct member alignment. Many compilers provide a flag that controls how the members of a structure are packed into memory. Modifying this flag may cause the size of the structures to vary. Most compilers also include a keyword that removes all padding; the resulting structures are referred to as are called packed structures. Overriding the default behavior is often unwise because it leads to interface compatibility problems (the nominally same struct has its layout interpreted differently in different modules).

Noncompliant Code Example

Wiki MarkupThis noncompliant code example assumes that the size of {{struct buffer}} is equal to the sum of the size of its individual components, which may not be the case \[ [Dowd 06|AA. C References#Dowd 06]\2006]. The size of {{struct buffer}} may actually be larger due to structure larger because of structure padding.

enum { buffer_size = 50 };

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

/* ... */

void func(const struct buffer *buf) {

  /*
   * Incorrectly Assumesassumes sizeof( struct buffer) =
   * sizeof( size_t) + 50 * sizeof( charbufferC)
 = 54  */
  struct buffer *buf_cpy = (struct buffer *)malloc(54(
    sizeof(size_t) + (buffer_size * sizeof(char) /* 1 */)
  );

  if (buf_cpy == NULL) {
    /* Handle malloc() error */
  }

  /* 
   * With padding, sizeof(struct buffer) may be greater than
   * 54 sizeof(size_t) + sizeof(buff.bufferC), causing some data  
   * to be written outside the bounds 
   * of the memory allocated.
    */
  memcpy(buf_cpy, buf, sizeof(struct buffer));

  /* ... */

  free(buf_cpy);
}

...

Accounting for structure padding prevents these types of errors.:

enum { buffer_size = 50 };

struct buffer {
  size_t size;
  char bufferC[buffer_size];
};

/* ... */

void func(const struct buffer *buf) {

  struct buffer *buf_cpy = 
    (struct buffer *)malloc(sizeof(struct buffer));

  if (buf_cpy == NULL) {
    /* Handle malloc() error */
  }

  /* ... */

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

  /* ... */

  free(buf_cpy);
}

...

Failure to correctly determine the size of a structure can lead to subtle logic errors and incorrect calculations, the effects of which can lead to abnormal program termination, memory corruption, or execution of arbitrary code.

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Image Modified      03. Expressions (EXP)      EXP04-C. Do not perform byte-by-byte comparisons between structures Image Added Image Added