How to use buffer to read and write using serial port in 8051 MCU

Question

I am using keil c51 compiler. I transmit data from my pc to MCU using serial port it works best. When I transmit data from my MCU to PC then also it works best. But when I transmit data to MCU and then store it to buffer character pointer and again from that character pointer buffer I transmit return to PC then it does not work and give garbage values?

My code for both function as below.

#include <REG51.H>
#include "uart.c"


void delay_ms(unsigned int x)    // delays x msec (at fosc=11.0592MHz)
{
    unsigned char j=0;
    while(x-- > 0)
    {
        for (j=0; j<125; j++){;}
    }
}


sbit SW = P3^2;
sbit LED = P3^3;

bit x = 0;

void main ()
{
        char *buf;

        int len=0;
        int len1 = 0;

        uart_init();
        while(1){

            if(RI == 1){

                UART_RxString(buf,&len);
                buf -= (len-1) ;

                x = 1;                  
            }

            if(x == 1 && SW == 0){

                UART_TxString(buf,&len1);

                x = 0;
            }
        }
}

And below are the functions. 1.

void UART_TxString(char *string_ptr, int *l)
 {
      int count = 0;
      while(*string_ptr){
         UART_TxChar(*string_ptr++);
         count++;

      }
     *l = count;

}

void UART_RxString(char *string_ptr, int *l)
{
    char ch;
    int count = 0;
    while(1)
    {
        ch=UART_RxChar();    //Reaceive a char
        //UART_TxChar(ch);     //Echo back the received char
        count++;
        if((ch=='\r') || (ch=='\n')) //read till enter key is pressed
        {                          //once enter key is pressed
            *string_ptr=0;          //null terminate the string
            break;                //and break the loop
        } 
        *string_ptr=ch;              //copy the char into string.
        string_ptr++;                //and increment the pointer
    }
    *l = count;
}