EXP44-C. Do not rely on side effects in operands to sizeof, _Alignof, or _Generic

Some operators do not evaluate their operands beyond the type information the operands provide. When using one of these operators, do not pass an operand which would otherwise yield a side effect, as the side effect will not be generated.

The sizeof operator yields the size (in bytes) of its operand, which may be an expression or the parenthesized name of a type.  In most cases, the operand is not evaluated.  A possible exception is when the type of the operand is a variable length array type (VLA) the expression is evaluated. When part of the operand of the sizeof operator is a VLA type and when changing the value of the VLA's size expression would not affect the result of the operator, it is unspecified whether or not the size expression is evaluated (see unspecified behavior 22).

The operand passed to_Alignof is never evaluated, despite not being an expression. For instance, if the operand is a VLA type, and the value of the VLA's size expression contains a side effect, that side effect is never evaluated.

The operand used in the controlling expression of a _Generic selection expression is never evaluated.

Providing an expression that appears to produce side effects may be misleading to programmers who are not aware that these expressions are not evaluated, and in the case of a VLA used in sizeof, have unspecified results. As a result, programmers may make invalid assumptions about program state, leading to errors and possible software vulnerabilities.

This rule is similar to PRE31-C. Avoid side effects in arguments to unsafe macros.

Noncompliant Code Example (sizeof)

In this noncompliant code example, the expression a++ is not evaluated:

#include <stdio.h>
void func(void) {
  int a = 14;
  int b = sizeof(a++);
  printf("%d, %d\n", a, b);
}

Consequently, the value of a after b has been initialized is 14.

Compliant Solution (sizeof)

In this compliant solution, the variable a is incremented outside of the sizeof operation:

#include (stdio.h>
void func(void) {
  int a = 14;
  int b = sizeof(a);
  ++a;
  printf("%d, %d\n", a, b);
}

Noncompliant Code Example (sizeof, VLA)

In this noncompliant code example, the expression ++n in the initialization expression of a must be evaluated because its value affects the size of the VLA operand of the sizeof operator. However, in the initialization expression of b, the expression ++n % 1 evaluates to 0. This means that the value of n does not affect the result of the sizeof operator. Consequently, it is unspecified whether or not n will be incremented when initializing b.

#include <stddef.h>
#include <stdio.h>
  
void f(size_t n) {
  /* n must be incremented */ 
  size_t a = sizeof(int[++n]);
 
  /* n need not be incremented */
  size_t b = sizeof(int[++n % 1 + 1]);

  printf("%z, %z, %z\n", a, b, n);
  /* ... */
}

Compliant Solution (sizeof, VLA)

This compliant solution avoids changing the value of the variable n used in each sizeof expression and instead increments n safely afterwards:

#include <stddef.h>
#include <stdio.h>
  
void f(size_t n) {
  size_t a = sizeof(int[n + 1]);
  ++n;

  size_t b = sizeof(int[n % 1 + 1]);
  ++n;
  printf("%z, %z, %z\n", a, b, n);
  /* ... */
}

Noncompliant Code Example (_Generic)

This noncompliant code example attempts to modify a variable's value as part of the _Generic selection control expression. The programmer may expect that a is incremented, but because _Generic does not evaluate its control expression, the the value of a is not modified.

#include <stdio.h>

#define S(val) _Generic(val, int : 2, \
                             short : 3, \
                             default : 1)
void func(void) {
  int a = 0;
  int b = S(a++);
  printf("%d, %d\n", a, b);
}

Compliant Solution (_Generic)

In this compliant solution, a is incremented outside of the _Generic selection expression:

#include <stdio.h>

#define S(val) _Generic(val, int : 2, \
                             short : 3, \
                             default : 1)
void func(void) {
  int a = 0;
  int b = S(a);
  ++a;
  printf("%d, %d\n", a, b);
} 

Noncompliant Code Example (_Alignof)

This noncompliant code example attempts to modify a variable while getting its default alignment value. The user may have expected val to be incremented as part of the _Alignof expression, but because _Alignof does not evaluate its operand, val is unchanged.

#include <stdio.h>
 
void func(void) {
  int val = 0; 
  /* ... */ 
  size_t align = _Alignof(int[++val]);
  printf("%zu, %d\n", align, val);
  /* ... */
}

Compliant Solution (_Alignof)

 This compliant solution moves the expression out of the _Alignof operator:

#include <stdio.h>
void func(void) {
  int val = 0; 
  /* ... */ 
  ++val;
  size_t align = _Alignof(int[val]);
  printf("%zu, %d\n, align, val);
  /* ... */
}

Risk Assessment

If expressions that appear to produce side effects are supplied to an operator that does not evaluate its operands, the results may be different than expected. Depending on how this result is used, it can lead to unintended program behavior.

Automated Detection

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines