/hardware/ril/libril/ril.cpp下有如下代码：

s_fdListen = android_get_control_socket(SOCKET_NAME_RIL);
if (s_fdListen < 0) {
    LOGE("Failed to get socket '" SOCKET_NAME_RIL "'");
    exit(-1);
}

if (listen(s_fdListen, 4)) {
    exit(1);
}
s_fdCommand = accept(s_fdListen, (sockaddr *) &peeraddr, &socklen);

这个android_get_control_socket函数有什么作用呢？ android_get_control_socket函数定义在/system/core/include/cutils/sockets.h:

#define ANDROID_SOCKET_ENV_PREFIX   "ANDROID_SOCKET_"
#define ANDROID_SOCKET_DIR      "/dev/socket"

#ifdef __cplusplus
extern "C" {
#endif

/*
 * android_get_control_socket - simple helper function to get the file
 * descriptor of our init-managed Unix domain socket. `name' is the name of the
 * socket, as given in init.rc. Returns -1 on error.
 *
 * This is inline and not in libcutils proper because we want to use this in
 * third-party daemons with minimal modification.
 */
static inline int android_get_control_socket(const char *name)
{
    char key[64] = ANDROID_SOCKET_ENV_PREFIX;
    const char *val;
    int fd;

    /* build our environment variable, counting cycles like a wolf ... */
#if HAVE_STRLCPY
    strlcpy(key + sizeof(ANDROID_SOCKET_ENV_PREFIX) - 1,
        name,
        sizeof(key) - sizeof(ANDROID_SOCKET_ENV_PREFIX));
#else   /* for the host, which may lack the almightly strncpy ... */
    strncpy(key + sizeof(ANDROID_SOCKET_ENV_PREFIX) - 1,
        name,
        sizeof(key) - sizeof(ANDROID_SOCKET_ENV_PREFIX));
    key[sizeof(key)-1] = '\0';
#endif

    val = getenv(key); // socket名字和对应的socket文件的文件描述符以键值对的形式存储在环境变量中， 
    if (!val)
        return -1;

    errno = 0;
    fd = strtol(val, NULL, 10);
    if (errno)
        return -1;

    return fd;
}

init.rc /system/core/init/readme.txt

/system/core/init/util.c

/*
 * create_socket - creates a Unix domain socket in ANDROID_SOCKET_DIR
 * ("/dev/socket") as dictated in init.rc. This socket is inherited by the
 * daemon. We communicate the file descriptor's value via the environment
 * variable ANDROID_SOCKET_ENV_PREFIX<name> ("ANDROID_SOCKET_foo").
 */
int create_socket(const char *name, int type, mode_t perm, uid_t uid, gid_t gid)
{
    struct sockaddr_un addr;
    int fd, ret;

    fd = socket(PF_UNIX, type, 0);
    if (fd < 0) {
        ERROR("Failed to open socket '%s': %s\n", name, strerror(errno));
        return -1;
    }

    memset(&addr, 0 , sizeof(addr));
    addr.sun_family = AF_UNIX;
    snprintf(addr.sun_path, sizeof(addr.sun_path), ANDROID_SOCKET_DIR"/%s",
             name);

    ret = unlink(addr.sun_path);
    if (ret != 0 && errno != ENOENT) {
        ERROR("Failed to unlink old socket '%s': %s\n", name, strerror(errno));
        goto out_close;
    }

    ret = bind(fd, (struct sockaddr *) &addr, sizeof (addr));
    if (ret) {
        ERROR("Failed to bind socket '%s': %s\n", name, strerror(errno));
        goto out_unlink;
    }

    chown(addr.sun_path, uid, gid);
    chmod(addr.sun_path, perm);

    INFO("Created socket '%s' with mode '%o', user '%d', group '%d'\n",
         addr.sun_path, perm, uid, gid);

    return fd;

out_unlink:
    unlink(addr.sun_path);
out_close:
    close(fd);
    return -1;
}

init.c

static void publish_socket(const char *name, int fd)
{
    char key[64] = ANDROID_SOCKET_ENV_PREFIX;
    char val[64];

    strlcpy(key + sizeof(ANDROID_SOCKET_ENV_PREFIX) - 1,
            name,
            sizeof(key) - sizeof(ANDROID_SOCKET_ENV_PREFIX));
    snprintf(val, sizeof(val), "%d", fd);
    add_environment(key, val); //添加到环境变量中

    /* make sure we don't close-on-exec */
    fcntl(fd, F_SETFD, 0);
}