CC2340R2: UART read callback not occurring again if UART read buffer exits

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/*
 *  ======== uart2callback.c ========
 */
#include <stdint.h>
#include <stddef.h>

/* POSIX Header files */
#include <semaphore.h>

/* Driver Header files */
#include <ti/drivers/GPIO.h>
#include <ti/drivers/UART2.h>

/* Driver configuration */
#include "ti_drivers_config.h"

static sem_t sem;
static volatile size_t numBytesRead;

/*
 *  ======== callbackFxn ========
 */
void callbackFxn(UART2_Handle handle, void *buffer, size_t count, void *userArg, int_fast16_t status)
{
    if (status != UART2_STATUS_SUCCESS)
    {
        /* RX error occured in UART2_read() */
        while (1) {}
    }

    numBytesRead = count;
    sem_post(&sem);
}

/*
 *  ======== mainThread ========
 */
void *mainThread(void *arg0)
{
    char input[248];
    const char echoPrompt[] = "Echoing characters:\r\n";
    UART2_Handle uart;
    UART2_Params uartParams;
    int32_t semStatus;
    uint32_t status = UART2_STATUS_SUCCESS;

    /* Call driver init functions */
    GPIO_init();

    /* Configure the LED pin */
    GPIO_setConfig(CONFIG_GPIO_LED_0, GPIO_CFG_OUT_STD | GPIO_CFG_OUT_LOW);

    /* Create semaphore */
    semStatus = sem_init(&sem, 0, 0);

    if (semStatus != 0)
    {
        /* Error creating semaphore */
        while (1) {}
    }

    /* Create a UART in CALLBACK read mode */
    UART2_Params_init(&uartParams);
    uartParams.readMode     = UART2_Mode_CALLBACK;
    uartParams.readCallback = callbackFxn;
    uartParams.baudRate     = 115200;

    uart = UART2_open(CONFIG_UART2_0, &uartParams);

    if (uart == NULL)
    {
        /* UART2_open() failed */
        while (1) {}
    }

    /* Turn on user LED to indicate successful initialization */
    GPIO_write(CONFIG_GPIO_LED_0, CONFIG_GPIO_LED_ON);

    /* Pass NULL for bytesWritten since it's not used in this example */
    UART2_write(uart, echoPrompt, sizeof(echoPrompt), NULL);

    /* Loop forever echoing */
    while (1)
    {
        numBytesRead = 0;

        /* Pass NULL for bytesRead since it's not used in this example */
        status = UART2_read(uart, &input, sizeof(input), NULL);

        if (status != UART2_STATUS_SUCCESS)
        {
            /* UART2_read() failed */
            while (1) {}
        }

        /* Do not write until read callback executes */
        sem_wait(&sem);

        if (numBytesRead > 0)
        {
            // print number of bytes no greater than input size
            uint8_t printBytes;
            if (numBytesRead > sizeof(input))
                printBytes = sizeof(input);
            else
                printBytes = numBytesRead;

            status = UART2_write(uart, &input, numBytesRead, NULL);

            if (status != UART2_STATUS_SUCCESS)
            {
                /* UART2_write() failed */
                while (1) {}
            }
        }
    }
}