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/* (Incorrect) Set all bits in mask to one1 */
const unsigned long mask = 0xFFFFFFFF;

unsigned long flipbits(unsigned long x) {
  return x ^ mask;
}

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/* (Correct) Set all bits in mask to one1 */
const unsigned long mask = -1;

unsigned long flipbits(unsigned long x) {
  return x ^ mask;
}

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const unsigned long mask = 0x80000000;
unsigned long x;

/* Initialize x */

x |= (x ^ mask) + 1;

This code has the desired effect for implementations where unsigned long has a precision of 32 bits but not for implementations where unsigned long has a precision of 64 bits.

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const unsigned long mask = (1UL << ((sizeof(unsigned long) * CHAR_BIT) -~(ULONG_MAX >> 1));
unsigned long x;

/* Initialize x */

x |= (x ^ mask) + 1;

Risk Assessment

Vulnerabilities are frequently introduced while porting code. A buffer overflow vulnerability may result, for example, if an incorrectly defined integer constant is used to determine the size of a buffer. It is always best to write portable code, especially when there is no performance overhead for doing so.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography

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