system/popen: add dpopen/dpclose fd-based interface and no-shell mode

system/popen/CMakeLists.txt

@@ -21,5 +21,5 @@
 # ##############################################################################
 
 if(CONFIG_SYSTEM_POPEN)
-  target_sources(apps PRIVATE popen.c)
+  target_sources(apps PRIVATE dpopen.c popen.c)
 endif()

system/popen/Kconfig

@@ -4,21 +4,36 @@
 #
 
 config SYSTEM_POPEN
-	bool "popen()/pclose() Functions"
+	bool "popen()/pclose()/dpopen()/dpclose() Functions"
 	default n
 	select SCHED_WAITPID
-	depends on NSH_LIBRARY && PIPES
+	depends on PIPES
 	---help---
-		Enable support for the popen() and pclose() interfaces.
-		This will support execution of NSH commands from C code with
-		pipe communications with the shell.
+		Enable support for the popen(), pclose(), dpopen(), and
+		dpclose() interfaces.
+
+		dpopen()/dpclose() are the descriptor-based counterparts
+		of popen()/pclose(), analogous to how dprintf() relates
+		to fprintf().  They return a raw file descriptor instead
+		of a FILE stream, avoiding the stdio.h dependency.
+
+		popen()/pclose() are thin wrappers around dpopen()/dpclose()
+		that additionally wrap the fd in a FILE stream.
+
+		When NSH is available (NSH_LIBRARY), commands are executed
+		through the shell (sh -c command), supporting full shell
+		syntax including pipes, redirects, and globbing.
+
+		When NSH is not available, commands are executed directly
+		via posix_spawnp(). The command string must be a simple
+		"program arg1 arg2" form without shell syntax.
 
 if SYSTEM_POPEN
 
 config SYSTEM_POPEN_SHPATH
 	string "Path to shell command"
 	default "/bin/sh"
-	depends on SYSTEM_NSH=m || BUILD_KERNEL
+	depends on (SYSTEM_NSH=m || BUILD_KERNEL) && NSH_LIBRARY
 	---help---
 		This is the full path to the program in a mounted file system that
 		implements the system() command.  That is, a program that starts the
@@ -42,4 +57,17 @@ config SYSTEM_POPEN_PRIORITY
 	---help---
 		The priority of the shell.
 
+config SYSTEM_POPEN_MAXARGUMENTS
+	int "Maximum number of command arguments (no-shell mode)"
+	default 7
+	depends on !NSH_LIBRARY
+	---help---
+		When NSH is not available, dpopen()/popen() splits the command
+		string by whitespace into an argv array.  This sets the maximum
+		number of arguments (excluding the program name and the
+		terminating NULL).
+
+		This is analogous to CONFIG_NSH_MAXARGUMENTS but applies
+		only to the no-shell path.
+
 endif

system/popen/Makefile

@@ -22,8 +22,8 @@
 
 include $(APPDIR)/Make.defs
 
-# popen()/pclose functions
+# dpopen()/dpclose() core and popen()/pclose() FILE wrappers
 
-CSRCS = popen.c
+CSRCS = dpopen.c popen.c
 
 include $(APPDIR)/Application.mk