wild pointer problem

wild pointer problem

Recently, I was looking at the previous code, and I have some new understanding of the previous bug. I will summarize it here, and I will share it with you.

The first is to use a send function

static int send_fun(uint16_t length, uint8_t *data);

It is necessary to judge the sent data data according to different conditions. The
following implementation is wrong

int judge_send_fun(void)
{
    int ret;
    uint8_t *data;
    if(XXX)
    {
        *data = 0;
        ret = send_fun(1, data);
    }
    else if(XXX)
    {
        *data = 1;
        ret = send_fun(1, data);
    }
    return ret;
}

The reason for the error is that the uint8_t *data set in the function is a wild pointer. Before the function calls the send_fun function, the local variables and static variables of the judge_send_fun function, the program location to jump back to, and the parameters of send_fun are pushed onto the stack (the function The implementation of the call can be seen in the process section in "In-depth Understanding of Computer Systems"). Here, the stack is just an address where data is stored. When the address of data is passed as a parameter to send_fun, only the pointer to the stack is passed. The pointer of data, in fact, because data does not point to a specific memory space (wild pointer) when it is declared, the content of data is an unreliable value, and when the send_fun function searches for a specific value according to the address of the incoming data, it gets The value is a random number, causing the program to run incorrectly.

The correct way is as follows:

int judge_send_fun(void)
{
    int ret;
    uint8_t data;
    if(XXX)
    {
        data = 0;
        ret = send_fun(1, &data);
    }
    else if(XXX)
    {
        data = 1;
        ret = send_fun(1, &data);
    }
    return ret;
}

In this way, before calling send_fun, the value of data and the parameter &data of send_fun are pushed onto the stack. At this time, the address pointed to by &data is the value of data that is pushed onto the stack, and there will be no problem that the value of data is randomly tampered with.