Answer

In the C language, variables are initialized differently depending on the storage class:

• Automatic (local) variables: have an uninitialized (garbage) value unless explicitly set to an initial value. Their content is from the stack memory that was there before.
• Static local and global variables (static, extern): are always initialized to zeros (0 for scalars, NULL for pointers) or to a specified value at compile time.

Example of differences:

void example() {
    int a;              // automatic, uninitialized
    static int b;       // initialized to 0
    printf("a=%d, b=%d\n", a, b);
}

int c; // global, initialized to 0

Errors arise from incorrect assumptions about initialization. For safety, always explicitly initialize your variables.

Trick Question

Is an automatic local variable initialized to 0 if it is declared at the function level outside of a block?

Answer:

No! Even if the variable is declared at the beginning of the function, if it is not explicitly initialized, it contains garbage:

void f() {
    int x;
    printf("%d\n", x); // UB: x is uninitialized
}

Examples of real errors due to misunderstanding of the topic

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Story

In banking software, a missed initialization of a counter inside a function led to it sometimes being a huge negative number — the result: erroneous bonuses were credited to the client, the bug was discovered only with real data.

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Story

An image multimedia handler initially assumed that static arrays were always filled with zeros. After switching to another compiler (with a non-standard environment), a segmentation fault was discovered — part of the memory had not been physically allocated to the process.

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Story

In a cryptographic library, the developer did not initialize the temporary password buffer, believing that the buffer from the global section was filled with zeros, but there was a case with memory where old sensitive data remained. This led to a leak of passwords during a dump.