include/stdlib.h: define system()'s prototype for the flat build

[tmedicci]

Summary

• include/stdlib.h: define system()'s prototype for the flat build

The $(APPDIR) folder is added to the $(KERNDEPDIRS) when CONFIG_BUILD_KERNEL=y. The depend phase iterates over the $(KERNDEPDIRS) folders and executes the depend recipe of these folders (including the apps' recipes) with the __KERNEL__ macro defined, which prevents system()'s prototype from being declared.

Impact

Impact architectures that build App's dependencies (the depend recipe) with CONFIG_SYSTEM_SYSTEM=y. This is the case, for instance, when building Python's library with support to the system() function.

This PR closes #15721

Testing

Internal CI testing + rv-virt:python with system() support:

make -j distclean && ./tools/configure.sh rv-virt:python && kconfig-tweak -e SYSTEM_SYSTEM && kconfig-tweak -e EXAMPLES_SYSTEM && make olddefconfig && make

Before this PR, it failed with:

/home/tiago/Documents/work/espressif/projects/nuttx/nuttxspace_alfa/apps/interpreters/python/Python/Modules/posixmodule.c:6026:14: error: implicit declaration of function 'system' [-Werror=implicit-function-declaration]
 6026 |     result = system(bytes);
      |              ^~~~~~
cc1: some warnings being treated as errors

[lupyuen]

@nuttxpr test milkv_duos:nsh

[nuttxpr]

[Experimental Bot, please feedback here]

Build and Test Successful (milkv_duos:nsh)
https://gitlab.com/lupyuen/nuttx-build-log/-/snippets/4803688

$ git clone https://github.com/tmedicci/incubator-nuttx nuttx --branch bugfix/system_system
$ git clone https://github.com/apache/nuttx-apps apps --branch master
$ pushd nuttx
$ git reset --hard HEAD
HEAD is now at d33f654011 include/stdlib.h: define system()'s prototype for the flat build
$ popd
$ pushd apps
$ git reset --hard HEAD
HEAD is now at f139e56cd testing/libc/wcstombs: Add testing application for wcstombs
$ popd
NuttX Source: https://github.com/apache/nuttx/tree/d33f6540113b8a5a4392f8a69b1a8b6258669f64
NuttX Apps: https://github.com/apache/nuttx-apps/tree/f139e56cd62a30d6edcd7207c7e4cbc6e9b8b7d1
$ cd nuttx
$ tools/configure.sh milkv_duos:nsh
$ make -j
$ make -j export
$ pushd ../apps
$ ./tools/mkimport.sh -z -x ../nuttx/nuttx-export-10.3.0.tar.gz
$ make -j import
$ popd
$ genromfs -f initrd -d ../apps/bin -V NuttXBootVol
$ head -c 65536 /dev/zero
$ cat nuttx.bin /tmp/nuttx.pad initrd
$ scp Image tftpserver:/tftpboot/Image-sg2000
$ ssh tftpserver ls -l /tftpboot/Image-sg2000
$ cd /home/luppy/nuttx-build-farm
$ ssh tftpserver
OpenSBI v0.9
nsh> uname -a
NuttX 10.3.0 d33f654011 Feb  7 2025 06:49:26 risc-v milkv_duos
nsh> ostest
arena       81000    81000
ordblks         2        3
mxordblk    7cff8    78ff8
uordblks     2660     4570
fordblks    7e9a0    7ca90
user_main: Exiting
ostest_main: Exiting with status 0
nsh> Now running https://github.com/lupyuen/nuttx-build-farm/blob/main/oz64-power.sh off
----- Power off Oz64
[]

[lupyuen]

Tested OK on real hardware: Oz64 SG2000 RISC-V SBC. Thanks :-)

• include/stdlib.h: define system()'s prototype for the flat build #15756 (comment)