将变量指定到特定段是否具备static特性？myBuffer内存是否可被外部覆盖？

Great questions—you’re right that undefined behavior makes these tricky to test directly, so let’s break them down with concrete rules and examples.

1. Do Variables in a Custom Section Have the Same "Unoverwritable" Behavior as static Declarations? #

Short answer: Only if you combine the custom section with a static declaration—the section itself doesn’t control scope or linkage, which is what gives static its "unoverwritable" properties.

Let’s unpack this:

• A static variable (whether global or local) gets two key protections:
  • Linkage control: Global static variables are limited to the current translation unit (file), so other files can’t reference or overwrite them via extern. Local static variables are limited to their enclosing scope, so no outside code can access them directly.
  • Persistent storage: All static variables live in the static memory segment for the entire program lifecycle.
• Putting a variable in a custom section (e.g., __attribute__((section("my_custom_seg"))) in GCC, or #pragma section in MSVC) only changes where the variable is stored in the executable—it doesn’t change its linkage or scope.
  • If you declare a global variable without static and put it in a custom section, other files can still use extern to reference and modify it, so it’s not "unoverwritable" like a global static.
  • If you declare a static variable (global or local) and put it in a custom section, it retains all the scope/linkage protections of a regular static—the custom section just moves its storage location, not its accessibility.

2. Can myBuffer Allocated in a Scope Be Overwritten by Code Outside That Scope? #

This depends entirely on how myBuffer is allocated:

• Stack-allocated local variables (no static): When you exit the scope, the stack frame for that scope is deallocated. The memory used by myBuffer is now part of the free stack space, which will be reused by subsequent function calls, local variables, or stack operations. Any code that uses this memory (even accidentally) will overwrite myBuffer’s old values. Accessing this memory after exiting the scope is undefined behavior—the standard doesn’t guarantee what will happen.
• static local variables: These live in static memory, not the stack. When you exit the scope, the memory isn’t deallocated, so it won’t be overwritten by normal stack operations. The only way it can be overwritten is if you use a pointer to force-modify it (which is undefined behavior, as you’re bypassing scope rules).
• Heap-allocated variables (via malloc/calloc): The memory lives on the heap until you explicitly free it. Code outside the scope can overwrite it only if it has access to the pointer (e.g., if you passed the pointer to another function). If you don’t free it, the memory stays allocated, but it could be overwritten if the heap is fragmented and the block is reused for another allocation later.

3. Will myBuffer’s Value Change When Re-Entering the Scope? #

Again, this ties to allocation type:

• Stack-allocated local variables:
  • If you don’t initialize myBuffer, it will hold garbage values every time you enter the scope—these could be different from the last time (since the stack memory was reused).
  • If you explicitly initialize it (e.g., char myBuffer[10] = {0};), it will reset to that initial value every time you enter the scope.
• static local variables: These are initialized only once, when the program first reaches the declaration. Every time you re-enter the scope, myBuffer will retain the value it had when you last exited—unless you modify it within the scope. The only exception is if some undefined behavior (like a rogue pointer) modifies it outside the scope.
• Heap-allocated variables: If you didn’t free the memory, the value will remain as it was when you last exited the scope—unless another part of the code modified it via the pointer.

内容的提问来源于stack exchange，提问作者niCk cAMel