C语言，记录一次局部变量被意外修改的问题

背景：单片机开发过程中，我在函数体内（begin_face_record）定义了一个局部变量data_length，在使用的时候，该局部变量一直别改变，每次调用其他函数，例如c库里面的函数memcpy，不知什么情况data_length值就会被改变。

1、源码分析

void main(void) 
{ 
	  init_gpio();
	  init_face();
 	  face_power_up();
	  begin_face_record(1,"ZHANG SAN");
}

void begin_face_record(uint8_t admin,INT8 user_name[USER_NAME_SIZE])
{
	uint16_t data_length = 0;
	//uint16_t i = 0;
	UART_Packet *uart_packet;
	S_MSG_ENROLL_DATA xdata enroll_data;
	enroll_data.admin = admin;
	memcpy(enroll_data.user_name,user_name,strlen(user_name));
	enroll_data.face_direction = FACE_DIRECTION_MIDDLE;
	enroll_data.timeout = TIMEOUT;
	//i = data_length;
	data_length = strlen(user_name);
	//strlen(enroll_data.user_name)+sizeof(enroll_data.face_direction)+sizeof(enroll_data.timeout);
  uart_packet = allocatePacket(ID_ENROLL,&enroll_data