Returning a Pointer to buffer from function - c

Question

I have function that reads the sensor data over I2C and fills that data in buffer.
void ADXLGetData(ADXLData_t * adxldataptr)
{
Buffer[0] = ADD_X0;
i2cRead(slave_add, reg_add,&buffer[0]);
adxldataptr->datax0 =buffer[1];
adxldataptr->datax1 = buffer[2];
adxldataptr->datay0 = buffer[3];
adxldataptr->datay1 = buffer[4];
adxldataptr->dataz0 = buffer[5];
adxldataptr->dataz1 = buffer[6];
}
where ADXLData_t is my structure of 6 bytes and buffer is declared as below in my header file.
volatile char buffer[12];
and other is like below.
char * ADXLGetData(void)
{
ADXLData_t *adxldataptr;
buffer[0] = ADD_X0;
i2cRead(slave_add, reg_add,&buffer[0]);
adxldataptr = (ADXLData_t*)&buffer[0];
return adxldataptr;
}
Both will be reading same sensor data. as i read data over timer interval of 1ms( that are four sensors including this) i am getting fault while using first function.
But if i use second function my program runs smooth.
In first function I get data perfectly over 5-6 iterations.
but in second I am not getting perfect data.
I am calling the functions as below.
for calling first function
ADXLData_t Acc1dataptr;
ADXLGetData(&Acc1dataptr);
for the second
ADXLData_t * Acc1dataptr;
Acc1dataptr = (ADXLData_t*) ADXLGetData();
This functions are called in another file not in same file , But I have included all the required files.
could please tell what i am doing wrong.?