Each property has a name and value, and they are all in string format. Each process can get/set properties. When the system is initialized, Android will allocate a shared memory area to store the properties. These are done by the “init” daemon. The “init” daemon will start an android property service.
platform/system/core/init/init.c
[c] int main(int argc, char **argv) { queue_builtin_action(property_service_init_action, "property_service_init"); } static int property_service_init_action(int nargs, char **args) { /* read any property files on system or data and * fire up the property service. This must happen * after the ro.foo properties are set above so * that /data/local.prop cannot interfere with them. */ start_property_service(); return 0; } [/c]
platform/system/core/init/property_service.c
[c] libc/include/sys/_system_properties.h #define PROP_PATH_RAMDISK_DEFAULT "/default.prop" #define PROP_PATH_SYSTEM_BUILD "/system/build.prop" #define PROP_PATH_SYSTEM_DEFAULT "/system/default.prop" #define PROP_PATH_LOCAL_OVERRIDE "/data/local.prop" void start_property_service(void) { int fd; load_properties_from_file(PROP_PATH_SYSTEM_BUILD); load_properties_from_file(PROP_PATH_SYSTEM_DEFAULT); load_override_properties(); /* Read persistent properties after all default values have been loaded. */ load_persistent_properties(); fd = create_socket(PROP_SERVICE_NAME, SOCK_STREAM, 0666, 0, 0); if(fd < 0) return; fcntl(fd, F_SETFD, FD_CLOEXEC); fcntl(fd, F_SETFL, O_NONBLOCK); listen(fd, 8); property_set_fd = fd; } [/c]
platform/system/core/libcutils/properties.c
[c] int property_set(const char *key, const char *value) { return __system_property_set(key, value); } int property_get(const char *key, char *value, const char *default_value) { int len; len = __system_property_get(key, value); if(len > 0) { return len; } if(default_value) { len = strlen(default_value); memcpy(value, default_value, len + 1); } return len; } [/c]
platform/bionic/libc/bionic/system_properties.c
[c] int __system_property_get(const char *name, char *value) { const prop_info *pi = __system_property_find(name); if(pi != 0) { return __system_property_read(pi, 0, value); } else { value[0] = 0; return 0; } } int __system_property_set(const char *key, const char *value) { int err; prop_msg msg; if(key == 0) return -1; if(value == 0) value = ""; if(strlen(key) >= PROP_NAME_MAX) return -1; if(strlen(value) >= PROP_VALUE_MAX) return -1; memset(&msg, 0, sizeof msg); msg.cmd = PROP_MSG_SETPROP; strlcpy(msg.name, key, sizeof msg.name); strlcpy(msg.value, value, sizeof msg.value); err = send_prop_msg(&msg); if(err < 0) { return err; } return 0; } [/c]
init.rc
setprop
Set system property name to value. Properties are expanded within value.
wait_for_prop
Wait for system property name to be value. Properties are expanded within value. If property name is already set to value, continue immediately.