FB stack internals
(This stuff is mostly just of interest to TMC, who has trouble remembering all of it)
Documentation of the details of FreeBASIC stack usage, upon which the OHR's GOSUB implementation and the original unofficial FB 0.22 Mac OS X port depend. If it changes in a future version of FB, we are in trouble. It's also not expected to work when using -gen gcc (however, FB's GOSUB implementation using setjmp/longjmp technically should not work either:
- "If you use longjmp, beware of automatic variables. ISO C says that automatic variables that are not declared volatile have undefined values after a longjmp. And this is all GCC promises to do, because it is very difficult to restore register variables correctly, and one of GCC's features is that it can put variables in registers without your asking it to." [1]
)
| ebp addressed part of the stack |
| esp addressed part of the stack (normally via push/pop) |
| cdecl function calls on FB 0.22 Mac OS X only |
| Content | Notes | ||
|---|---|---|---|
| Arguments | [ebp+8] and above | ||
| Stack frame 'header' | eip ebp |
||
| Function-scope local variables | Starts at [ebp-4]. First local is function return value | ||
| Temporaries | Scoped locals, intermediate results, WITH pointer, FOR start/end/step | Fixed length, variable contents | |
| Saved registers from previous function | Any of the following used: ebx esi edi |
||
| GOSUB call pointers | call gosub_FUNC_line_LINE | Variable length | |
| Nested function call arguments | (Recent FB versions only). Arguments pushed from right to left | Variable length | |
| Topmost function call | Padding to 16 byte boundary | garbage | 0 - 12 bytes |
| Saved stack pointer | esp | ||
| Arguments | Variable length | ||
FB never pushs partially evaluated function arguments to the stack as a typical C compiler would, instead preferring to store the arguments in the preserved registers (ebx, esi, edi), and then overflowing to temporary variables. Along with eax for a returned result, it normally doesn't need to use temporaries until a single function requires at least 5 returned values as arguments.
The preceding statement is false; presumably it became false sometime between FB 0.22 when I originally investigated, and FB 1.0.