casaroli opened a new pull request, #19777:
URL: https://github.com/apache/nuttx/pull/19777
## Summary
x86 selected neither fork primitive. `vfork()` was not available on this
architecture at all. This pull request provides it.
`fork.S` takes the register snapshot of the caller and hands it to
`x86_fork()`, which allocates the child, copies the used part of the stack of
the caller, and starts it. There is one entry point for both primitives,
because the snapshot is the same for either.
The calling convention differs from every other architecture that has this.
cdecl puts the flag on the stack, so `up_fork()` loads it from `4(%esp)` rather
than keeping a register alive. That slot is also the stack pointer the caller
had: the caller pushed the argument, then `call` pushed the return address. The
low end of the region that `x86_fork()` copies is therefore unchanged.
## Impact
The change is specific to x86. It adds capability and removes none.
`vfork()` becomes available on x86. Code that tests `CONFIG_ARCH_HAVE_VFORK`
now finds it here.
**POSIX `fork()` is not provided.** It needs an address environment that can
be duplicated, and this architecture has none, so `CONFIG_ARCH_HAVE_FORK` is
never set. `x86_fork.c` makes that a build error rather than a silent omission:
```c
#ifdef CONFIG_ARCH_HAVE_FORK
# error "x86 has no address environment; fork() cannot be provided"
#endif
```
No board configuration changes.
## Testing
Emulation only. QEMU 11.0.3 on macOS 15 with Apple Silicon, `i686-elf-gcc`
from Homebrew.
`qemu-i486:nsh`, with `ostest` typed at the prompt:
```
qemu-system-i386 -cpu qemu32,+fpu -m 128 -kernel nuttx.elf \
-nographic -no-reboot -net none
```
```
NuttShell (NSH) NuttX-12.2.1
user_main: vfork() test
vfork_test: Started
vfork_test: Child 5 ran and exited before the parent resumed
ostest_main: Exiting with status 0
```
`fork()` is correctly absent from that run.
`tools/checkpatch.sh -c -u -m -g` gives no errors.
Two notes for anyone repeating this. `i686-elf-gcc` is a bare cross compiler
with no libc, so nothing provides `math.h` and the build stops in
`lib_strtold.c`. Select `CONFIG_LIBM` instead of `CONFIG_LIBM_TOOLCHAIN`. And
use `qemu-i486:nsh`, not `qemu-i486:ostest`: that configuration consoles
through `CONFIG_CONSOLE_SYSLOG` and floods the output, which is #19568 and
unrelated to this change.
### What is not tested
No x86 hardware.
**A report from a board would be welcome.** Take this branch with
apache/nuttx-apps#3685, build `qemu-i486:nsh` and run `ostest`. The `vfork()`
test is the first output.
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