当我在运行时在 c 中为全局变量赋值时，memory 会发生什么？

Question

in c Uninitialized static and global variables are on the uninitialized data segement.

so in this code

int var; //on the uninitialized  data segement
int main(){
   var = 5; //does this initialized global variable stay on the uninitialized data segement?
return 0;}

var is on the uninitialized data segment. what happens if later at runtime i give it a value and define it?

if the variable later travel somehow to the initliazed data segment then what is the use of bss?

Answer 1

in c Uninitialized static and global variables are on the uninitialized data segement.

That may be a correct description of one or more particular C implementations, but "segments" in general and an "uninitialized data segment" in particular are not C-language concepts.

The C semantics are pretty simple: programs behave as if storage is allocated for all file-scope and all static variables before the program begins execution, and remains allocated until the program terminates. That is, the objects they identify have "static storage duration". The language does not specify where the storage for these (or any other) objects is, but there is no difference between how objects with static storage duration may be accessed during their lifetimes and how other objects may be accessed during their (shorter) lifetimes.

Whether objects are declared with explicit initializers does not factor in to how objects with static storage duration may be used. It affects only their initial values.

var is on the uninitialized data segment. what happens if later at runtime i give it a value and define it?

Your confusion seems to arise from mixing multiple levels. An "uninitialized data segment", in the sense you seem to mean it, is a feature of an executable file format, such as ELF. It is largely unrelated to the layout of objects in the program's memory at runtime. Also, "defining" an identifier is a source-code concept, not a runtime concept. A program can read and write objects' values at run time, but there is nothing it can do to affect whether an identifier is defined.

So the answer is that if you assign a value to an object with static storage duration than that object thereafter holds the assigned value until and unless a different one is assigned to it, regardless of whether it was declared with an explicit initializer.

if the variable later travel somehow to the initliazed data segment then what is the use of bss?

Again, BSS and other "segments" are aspects of file formats, not programs' runtime memory. Nothing special has to happen for non- const variables assigned to an ELF BSS segment to receive new values at runtime. C language semantics provide no way to distinguish such objects or their runtime storage from other objects and their storage. In particular, assigning a value to an object never causes that object's address to change.

The use of BSS and similar mechanisms is to reduce the on-disk size of executable files. Assigning a variable to BSS establishes its initial value without recording that initial value explicitly in the file. Generally, a BSS segment will occupy zero space in the executable, so you might describe it as "virtual", but real storage is allocated for BSS contents when the program is loaded into memory for execution.