V118. malloc() function accepts a dangerous expression in the capacity of an argument.

The analyzer detected a potential error relating to using a dangerous expression serving as an actual argument for malloc function. The error may lie in incorrect suggestions about types' sizes defined as numerical constants.

The analyzer considers suspicious those expressions which contain constant literals multiple of four but which lack sizeof() operator.

Example 1.

An incorrect code of memory allocation for a matrix 3x3 of items of size_t type may look as follows:

size_t *pMatrix = (size_t *)malloc(36); // V118

Although this code could work very well in a 32-bit system, using number 36 is incorrect. When compiling a 64-bit version 72 bytes must be allocated. You may use sizeof () operator to correct this error:

size_t *pMatrix = (size_t *)malloc(9 * sizeof(size_t));

Example 2.

The following code based on the suggestion that the size of Item structure is 12 bytes is also incorrect for a 64-bit system:

struct Item {
  int m_a;
  int m_b;
  Item *m_pParent;
};
Item *items = (Item *)malloc(GetArraySize() * 12); // V118

Correction of this error also consists in using sizeof() operator to correctly calculate the size of the structure:

Item *items = (Item *)malloc(GetArraySize() * sizeof(Item));

These errors are simple and easy to correct. But they are nevertheless dangerous and difficult to find in case of large applications. That's why diagnosis of such errors is implemented as a separate rule.

Presence of a constant in an expression which is a parameter for malloc() function does not necessarily means that V118 warning will be always shown on it. If sizeof() operator participates in the expression this construction is safe. Here is an example of a code which the analyzer considers safe:

int *items = (int *)malloc(sizeof(int) * 12);

Additional materials on this topic:

• 64-bit Lessons. Lesson 9. Pattern 1. Magic numbers.