Which of the following is true regarding uninitialised variables in C?

Question

BlackTom AI · Accepted Answer

Uninitialized automatic (stack) variables have indeterminate values and can therefore contain whatever bytes happen to be at that memory location, which may include leftover data from prior use.

1) Relevant knowledge points and concepts
- Storage classes: local automatic variables (on the stack) are not initialized by C; global/static variables are zero-initialized by the runtime.
- Undefined behavior: reading an uninitialized object in C produces undefined behavior — the language makes no guarantees about the value.
- Security implications: indeterminate contents can leak sensitive data (passwords, keys) or influence program logic (branches, pointer values), enabling vulnerabilities.
- OS/compiler behavior: the OS will zero new pages it hands to a process for isolation, but within a running process memory reused on the stack/heap is not automatically zeroed by the compiler or C runtime. malloc does not zero memory (calloc does).

2) Step-by-step reasoning
- A local variable declared without an initializer reserves memory but does not set its contents.
- If that stack slot previously held other data, the bytes remain until overwritten; a subsequent read may observe those bytes.
- Observing those bytes can reveal prior data (information leakage) or produce values that change control-flow decisions or memory indexes, potentially altering program behavior.
- Because C specifies undefined behavior for such reads, the compiler is free to optimize assuming they do not occur; this is why the behavior is unpredictable and dangerous.

3) Intermediate evidence/examples
- Example: int x; if (x == 0) ... — x has indeterminate value; it could be 0 or any other value depending on leftover bytes.
- malloc vs calloc: malloc(n) returns uninitialized memory; calloc(n, size) zeroes it.

4) Why the correct answer is correct
- The option "They may contain leftover data from previous memory use, potentially leaking sensitive information or altering control flow." precisely states the indeterminate-content and security/control-flow consequences caused by uninitialized variables.

5) Why other options are incorrect
- "Automatically inherit default values defined by the compiler": false for automatic variables (only globals/statics are zeroed).
- "Automatically zeroed out by the OS when allocated on the stack": false — OS zeros newly mapped pages but does not zero reused stack slots during program execution.
- "Increase compilation time due to padding": unrelated and false.
- "Cannot be exploited because compilers insert runtime checks": false; compilers do not insert such checks by default (sanitizers exist but are optional).
- "Always trigger a segmentation fault": false; reads return unpredictable values, not necessarily a crash.

6) Conclusion
The correct choice is: "They may contain leftover data from previous memory use, potentially leaking sensitive information or altering control flow." This matches the provided answer.

Which of the following is true regarding uninitialised variables in C?单项选择题

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