Different sources say different things for me - some StackOverflow answers say that it is allocated at compile time - others say it is "defined" at compile time, and allocated at the very beginning of runtime ("load time" is what some called it), while others say it is allocated at compile time. When is static memory exactly allocated in C/C++? (if it is to do with "defining" variables - can someone tell me what it means to "define" a variable on the memory level - that would be highly appreciated!)
Also, how would you during runtime set a pointer to the start of the allocated static memory?
In typical tools, memory with static storage duration is arranged in multiple steps:
- The compiler generates data in object modules (likely passing through some form of assembly code) that describes needs for various kinds of memory: memory initialized to zero, memory initialized to particular values and is read-only thereafter, memory initialized to particular values and may be modified, memory that does not need to be initialized, and possibly others. The compiler also includes initial data as necessary, information about symbols that refer to various places in the required memory, and other information. At this point, the allocation of memory is in forms roughly like “8 bytes are needed in the constant data section, and a symbol called
foo should be set to their address.”
- The linker combines this information into similar information in an executable file. It also resolves some or all information about symbols. At this point, the allocation of memory is in forms like “The initialized non-constant data section requires 3048 bytes, and here is the initial data for it. When it is assigned a virtual address, the following symbols should be adjusted:
bar is at offset 124 from the start of the section, baz is at offset 900…”
- The program loader reads this information, allocates locations in the virtual address space for it, and may read some of the data from the executable file into memory or inform the operating system where the data is to be found when it is needed. At this point, the places in the code that refer to various symbols have been modified according to the final values of those symbols.
- The operating system allocates physical memory for the virtual addresses. Often, this is done “on demand” in pieces (memory pages) when a process attempts to access the memory in a specific page, rather than being done at the time the program is initially loaded.
All-in-all, static memory is not allocated at any particular time. It is a combination of many activities. The effect on the program is largely that it occurs the same as if it were all allocated when the program started, but the physical memory might only be allocated just before an instruction actually executes. (The physical memory can even be taken away from the process and restored to it later.)