the printf showed wrong result (LLVM-222)

[metarutaiga]

clang -target xtensa-lx106-elf -mcpu=esp8266

code

static char number[128];
static char* to_string(int value)
{
  os_sprintf(number, "%d", value);
  return number;
}

static char a = -1;
static signed char b = -1;
static unsigned char c = -1;
char rssi = wifi_station_get_rssi();
a = rssi;
b = rssi;
c = rssi;
os_printf("R:%s\n", to_string(rssi));
os_printf("A:%s\n", to_string(a));
os_printf("B:%s\n", to_string(b));
os_printf("C:%s\n", to_string(c));

result

R:187 <- wrong
A:-69
B:-69
C:187