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In C, which of the following describes what happens when a null pointer is dereferenced (*p is read while p is NULL)?

  1. AThe standard requires it to be detected as a run-time error, so in every environment the program prints an error message and stops
  2. BNULL is defined to point to address 0, so in every environment the contents of address 0 are read and processing continues
  3. CIt is undefined behavior; under an OS it generally crashes, but on a microcontroller without an MMU address 0 may be readable and go unnoticed
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Answer: C. It is undefined behavior; under an OS it generally crashes, but on a microcontroller without an MMU address 0 may be readable and go unnoticed

Dereferencing a null pointer is undefined behavior, and the standard does not say what happens. Linux and Windows leave the pages around address 0 unmapped, so a dereference causes a page fault and the program generally terminates abnormally (a segmentation fault on Linux, for example). On a small microcontroller without an MMU, address 0 may hold flash memory or the interrupt vector table and be readable, so the error can cause malfunctions without ever surfacing. It is important to make sure a pointer is not NULL before using it.

  • A. The standard makes null pointer dereference undefined behavior and does not require it to be detected.
  • B. Null pointer dereference is undefined behavior. Under an OS, the area around address 0 is not mapped, and the program usually terminates abnormally.
  • C. Correct. Depending on the environment, it may crash or may keep running unnoticed.
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