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V119. More than one sizeof() operator i…
Analyzer Diagnostics
General Analysis (C++)
General Analysis (C#)
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Contents

V119. More than one sizeof() operator is used in one expression.

Dec. 2, 2010

The analyzer detected an unsafe arithmetic expression containing several sizeof() operators. Such expressions can potentially contain errors relating to incorrect calculations of the structures' sizes without taking into account field alignment.

Example:

struct MyBigStruct {
  unsigned m_numberOfPointers;
  void *m_Pointers[1];
};
size_t n2 = 1000;
void *p;
p = malloc(sizeof(unsigned) + n2 * sizeof(void *));

To calculate the size of the structure which will contain 1000 pointers, an arithmetic expression is used which is correct at first sight. The sizes of the base types are defined by sizeof() operators. It is good but not sufficient for correct calculation of the necessary memory size. You should also take into account field alignment.

This example is correct for a 32-bit mode for the sizes of the pointers and unsigned type coincide. They are both 4 bytes. The pointers and unsigned type are aligned also at the boundary of four bytes. So the necessary memory size will be calculated correctly.

In a 64-bit code the size of the pointer is 8 bytes. Pointers are aligned at the boundary of 8 bytes as well. It leads to that after m_numberOfPointers variable 4 additional bytes will be situated at the boundary of 8 bytes to align the pointers.

To calculate the correct size you should use offsetof function:

p = malloc(offsetof(MyBigStruct, m_Pointers) +
           n * sizeof(void *));

In many cases using several sizeof() operators in one expression is correct and the analyzer ignores such constructions. Here is an example of safe expressions with several sizeof operators:

int MyArray[] = { 1, 2, 3 };
size_t MyArraySize =
  sizeof(MyArray) / sizeof(MyArray[0]);
assert(sizeof(unsigned) < sizeof(size_t));
size_t strLen = sizeof(String) - sizeof(TCHAR);

Additional materials on this topic:

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