API to get android system properties is removed in arm64 platforms

Question

I am using __system_property_get() from sys/system_properties.h to get a system property. I am trying to use r10c ndk because I need arm64 toolchain.

__system_property_get() is defined in libc.so. Below is the readelf output of libc.so for armv5/armv7a.

readelf -Ws libc.so | grep property_get

       194: 00009100    20 FUNC    GLOBAL DEFAULT    4 __system_property_get
       198: 00009100    20 FUNC    GLOBAL DEFAULT    4 __system_property_get

But, looks like it has been removed for arm64 version! I am getting a linker error saying it is not defined. I analyzed all the shared libraries of arm64 but none of them have that symbol.

Is there an alternate API to get the system property in the native code?

Thank you!

Answer 1

It's useful API for native apps, just as it is for Java apps, it originates from the native side (see http://rxwen.blogspot.com/2010/01/android-property-system.html), and other Android system code uses it, so it's unlikely to go away soon.

#include <android/log.h>
#include <dlfcn.h>

#if (__ANDROID_API__ >= 21)
// Android 'L' makes __system_property_get a non-global symbol.
// Here we provide a stub which loads the symbol from libc via dlsym.
typedef int (*PFN_SYSTEM_PROP_GET)(const char *, char *);
int __system_property_get(const char* name, char* value)
{
    static PFN_SYSTEM_PROP_GET __real_system_property_get = NULL;
    if (!__real_system_property_get) {
        // libc.so should already be open, get a handle to it.
        void *handle = dlopen("libc.so", RTLD_NOLOAD);
        if (!handle) {
            __android_log_print(ANDROID_LOG_ERROR, "foobar", "Cannot dlopen libc.so: %s.\n", dlerror());
        } else {
            __real_system_property_get = (PFN_SYSTEM_PROP_GET)dlsym(handle, "__system_property_get");
        }
        if (!__real_system_property_get) {
            __android_log_print(ANDROID_LOG_ERROR, "foobar", "Cannot resolve __system_property_get(): %s.\n", dlerror());
        }
    }
    if (!__real_system_property_get) return (0);
    return (*__real_system_property_get)(name, value);
} 
#endif // __ANDROID_API__ >= 21