How to wait for characters from UART and transmit back on the UART in ARM Cortex -M3 emulated by Qemu

I am trying to do the echo of the data received on the UART, i.e wait for the characters from the UART and transmit it back on the UART. I am emulating the LM3S811 board in QEMU. I have attached the .c file and the startup and linker code. I get errors while compiling the .c file and I don't know whether the approach is the correct to do the echo of data received on the UART. It would be helpful if any links explaining the bare-metal programming from scratch are provided. Thanks.

main.c

#define  U0RBR (*((volatile unsigned int *) 0x4000C000))
#define  U0DLL (*((volatile unsigned int *) 0x4000C000))
#define  U0THR (*((volatile unsigned int *) 0x4000C000))
#define  U0LCR (*((volatile unsigned int *) 0x4000C00C))
#define  U0DLM (*((volatile unsigned int *) 0x4000C004))
#define  U0LSR (*((volatile unsigned int *) 0x4000C014))

void UART0_init(void);
unsigned char UART0_RxChar(void);
void UART0_TxChar(char ch);

void UART0_init(void)
{
    volatile unsigned int PINSEL0 = 0x0;

    PINSEL0 = 0x00000005;
    U0LCR = 0x83;
    U0DLM = 0x00;
    U0DLL = 0x61;
    U0LCR = 0x03;
}


unsigned char UART0_RxChar(void)
{
    while ((U0LSR & 0x01) == 0);
        return U0RBR;
}

void UART0_TxChar(char ch)
{
    U0THR = ch;
    while ((U0LSR & 0x40) == 0);
}

int main(void)
{
    char receive;

    UART0_init();
    while (1) {
        receive = UART0_Rxchar();
        UART0_TxChar(receive);

    }
    return 0;
}

startup.s

.thumb
.global start
start:
.word 0x20001000
.word main

flash.ld

MEMORY
{
    FLASH (rx) : ORIGIN = 0x00000000, LENGTH = 0x00010000
    SRAM (rwx) : ORIGIN = 0x20000000, LENGTH = 0x00002000
}

SECTIONS 
{
    .text : {
        *(.vectors);
        *(.text);
        etext = .;
    } > FLASH

    .rodata : {
        *(.rodata);
    } > FLASH

    .data : {
        sdata = .;
        *(.data);
        edata = .;
    } > SRAM AT > FLASH


    .bss : {
        sbss = .;
        *(.bss);
        ebss = .;
    } > SRAM
}

Errors while compiling the .c file command : arm-none-eabi-gcc -mthumb -mcpu=cortex-m3 - c main.c -o main.o

main.c:7:2: error: 'PINSEL0' undeclared (first use in this function)
  PINSEL0 = PINSEL0 | 0x00000005;
  ^
main.c:7:2: note: each undeclared identifier is reported only once for each function it appears in
main.c:8:2: error: 'UOLCR' undeclared (first use in this function)
  UOLCR = 0x83;
  ^
main.c:9:2: error: 'U0DLM' undeclared (first use in this function)
  U0DLM = 0x00;
  ^
main.c:10:2: error: 'U0DLL' undeclared (first use in this function)
  U0DLL = 0x61;
  ^
main.c:11:2: error: 'U0LCR' undeclared (first use in this function)
  U0LCR = 0x03;

After adding the register's address in Qemu only character 'a' gets printed and I am unable to type in the terminal. The command invoked was qemu-system-arm -M lm3s811evb -kernel main.bin -monitor stdio