MSP432P401R: UART_read() not working when called from GPIO Button ISR.

 *  ======== gpioButtonFxn0 ========
 *  Callback function for the GPIO interrupt on Board_GPIO_BUTTON0.
 */
void gpioButtonFxn0(uint_least8_t index)
{
    char input[100];
    int rxlen = 0;

    memset(input, 0, sizeof(input));

    /* Clear the GPIO interrupt and toggle an LED */
    GPIO_toggle(Board_GPIO_LED0);

    rxlen = UART_read(uartA2Handle, input, sizeof(input));  //here rxlen comes to 100 still when there is no data available to read.
    while(1);
}

/*
 *  ======== mainThread ========
 */
void *mainThread(void *arg0)
{
    /* Call driver init functions */
    GPIO_init();
    UART_init();

    uartA2Params.writeDataMode = UART_DATA_TEXT;
    uartA2Params.readDataMode = UART_DATA_TEXT;
    uartA2Params.readReturnMode = UART_RETURN_NEWLINE;
    uartA2Params.readEcho = UART_ECHO_OFF;
    uartA2Params.baudRate = 115200;
    uartA2Params.readTimeout = 200;
    uartA2Params.writeTimeout = 400;

    uartA2Handle = UART_open(Board_UART1, &uartA2Params);
    if (uartA2Handle == NULL)
    {
        /* UART_open() failed */
        while(1);
    }

    /* Configure the LED and button pins */
    GPIO_setConfig(Board_GPIO_LED0, GPIO_CFG_OUT_STD | GPIO_CFG_OUT_LOW);
    GPIO_setConfig(Board_GPIO_LED1, GPIO_CFG_OUT_STD | GPIO_CFG_OUT_LOW);
    GPIO_setConfig(Board_GPIO_BUTTON0, GPIO_CFG_IN_PU | GPIO_CFG_IN_INT_FALLING);

    /* Turn on user LED */
    GPIO_write(Board_GPIO_LED0, Board_GPIO_LED_ON);

    /* install Button callback */
    GPIO_setCallback(Board_GPIO_BUTTON0, gpioButtonFxn0);

    /* Enable interrupts */
    GPIO_enableInt(Board_GPIO_BUTTON0);


    sleep(2);

    char input[100];
    int rxlen = 0;
    memset(input, 0, sizeof(input));

    /* Clear the GPIO interrupt and toggle an LED */
    GPIO_toggle(Board_GPIO_LED0);

    rxlen = UART_read(uartA2Handle, input, sizeof(input));  //here rxlen comes to zero after UART_read() when there is no data to read.
    while(1);

    return (NULL);
}