[bugfix] iOS: wordexp/wordfree unavailable and missing env() implementation

[uilianries]

Greetings! 👋

When building Boost 1.91.0 on Mac M1 (aarch64) for iOS with Apple clang, it fails to build Boost.Process with two independent errors:

1. The functions wordexp and wordfree are unavailable on iOS

Apple iOS/tvOS/watchOS SDKs ship <wordexp.h> (inherited from the shared Apple SDK headers), but explicitly mark wordexp()/wordfree() as unavailable there:

    libs/process/src/shell.cpp:107:15: error: 'wordexp' is unavailable: not available on iOS
      107 |     auto cd = wordexp(input_.c_str(), &we, WRDE_NOCMD);
          |               ^
    .../iPhoneOS26.5.sdk/usr/include/wordexp.h:84:5: note: 'wordexp' has been
    explicitly marked unavailable here
       84 | int wordexp(const char * __restrict, wordexp_t * __restrict, int) __OSX_AVAILABLE_STARTING(__MAC_10_0, __IPHONE_NA);
    libs/process/src/shell.cpp:124:9: error: 'wordfree' is unavailable: not available on iOS
    2 errors generated.

The full build error can be seen here: boost-1.91.0-ios-shared-process-wordexp.log

The shell.cpp already guards against __OpenBSD__ and __ANDROID__

https://github.com/boostorg/process/blob/boost-1.91.0/src/shell.cpp#L22

#elif !defined(__OpenBSD__) && !defined(__ANDROID__)
#include <wordexp.h>
#endif

Similar to #440, it's just missing the same guard for Apple's non-macOS targets like iphone.

The commit ecca6b2 extends that rule to include Apple platforms that are not darwin. We need to include TargetConditionals.h to make TARGET_OS_OSX available.

Reference: https://developer.apple.com/documentation/xcode/running-code-on-a-specific-version#Compile-code-for-a-specific-platform

2. Function ext::env is never defined for iOS

Once the commit ecca6b2 is applied, the previously mentioned error no longer occurs, but a new error shows later during the linkage stage:

	Undefined symbols for architecture arm64:
	  "boost::process::v2::ext::env(int, boost::system::error_code&)", referenced from:
	      boost::process::v2::ext::env(int) in env.o
	ld: symbol(s) not found for architecture arm64

You can see the build log with this error here: boost-1.91.0-ios-shared-process-env.log

The env.cpp already correctly excludes iOS with #elif (defined(__APPLE__) && defined(__MACH__)) && !TARGET_OS_IOS, but no replacement for ext::env was added for the iOS case, so on iOS execution falls through every #elif with none matching, the symbols result as undefined.

The commit beec496 adds an iOS-specific implementation of ext::env that returns ENOTSUP, mirroring OpenBSD/Android.

Both changes only affect Apple non-macOS targets; for b2, they are iphone and appletv. I validated both the shared and static link variants build on iOS 26.5 / Apple clang 21.0.0 with this patch applied to Boost 1.91.0. You can check my build logs with this patch:

• boost-1.91.0-ios-static-patched.log
• boost-1.91.0-ios-shared-patched.log

Steps to Reproduce

wget https://archives.boost.io/release/1.91.0/source/boost_1_91_0.tar.bz2
tar jxf boost_1_91_0.tar.bz2 && cd boost_1_91_0/
./bootstrap.sh
./b2 install --prefix=/tmp/boost-install-ios --user-config=user-config.jam toolset=clang target-os=iphone architecture=arm address-model=64 abi=aapcs link=shared --with-process

My user-config.jam looks like:

using clang : : clang++ :
    <cflags>"-isysroot /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS26.5.sdk -arch arm64 -mios-version-min=26.5"
    <cxxflags>"-isysroot /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS26.5.sdk -arch arm64 -mios-version-min=26.5"
    <linkflags>"-isysroot /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS26.5.sdk -arch arm64 -mios-version-min=26.5" ;

Environment

• Boost: 1.91.0
• Host OS: macOS 26.5 (arm64)
• Target OS: iOS 26.5
• Compiler: Apple clang version 21.0.0 (clang-2100.1.1.101)

This PR is related to #401.

Feel free to ask for more information about this case if needed. Regards!