c - 指针的作用类似于局部变量，并且不会在函数中保留其修改

Question

我有一个指针，它指向A，我将这个指针传递给一个函数，在函数中，指针改变并指向 B ，当函数结束时，指针值恢复回来并指向A，这是为什么？

int device_num = 0;
int display_devices_size = 3;
DISPLAY_DEVICE* display_devices = (DISPLAY_DEVICE*)malloc(sizeof(DISPLAY_DEVICE) * display_devices_size);
get_display_devices(display_devices,&display_devices_size,&device_num);

void get_display_devices(DISPLAY_DEVICE* display_devices, int* display_devices_size, int* device_num)
{
    DISPLAY_DEVICE d;
    d.cb = sizeof(DISPLAY_DEVICE);
    while (EnumDisplayDevices(NULL, *device_num, &d, 0))
    {
        //After the first call to EnumDisplayDevices DispDev.DeviceString contains graphic card's name. 
        //After the second call DispDev.DeviceString contains monitor's name.
        if (*device_num >= *display_devices_size)
        {
            DISPLAY_DEVICE* temp = (DISPLAY_DEVICE*)realloc(display_devices, (*display_devices_size) * sizeof(DISPLAY_DEVICE) * 2);
            if (temp != NULL)
            {
                display_devices = temp;
                *display_devices_size = *display_devices_size * 2;
            }
        }
        display_devices\[(*device_num)\] = d;
        (*device_num)++;
    }

}

详情如下截图:
before function
in function
after function

Answer 1

如果你想更新指针，你需要传递指针的地址，而不是它的内容。您的代码在子例程中分配一个新指针，但这对原始指针没有影响。编辑了下面的代码 - 我没有检查它是否可以编译，但您可以看到更改。

int device_num = 0;
int display_devices_size = 3;
DISPLAY_DEVICE* display_devices = (DISPLAY_DEVICE*)malloc(sizeof(DISPLAY_DEVICE) * display_devices_size);

// Pass the ADDRESS of display_devices

get_display_devices(&display_devices,&display_devices_size,&device_num);

// Function takes a pointer to a pointer as the first argument.

DISPLAY_DEVICE* get_display_devices(DISPLAY_DEVICE** display_devices, int* display_devices_size, int* device_num)
{
    DISPLAY_DEVICE d;
    d.cb = sizeof(DISPLAY_DEVICE);
    while (EnumDisplayDevices(NULL, *device_num, &d, 0))
    {
        //After the first call to EnumDisplayDevices DispDev.DeviceString contains graphic card's name.
        //After the second call DispDev.DeviceString contains monitor's name.
        if (*device_num >= *display_devices_size)
        {
            DISPLAY_DEVICE* temp = (DISPLAY_DEVICE*)realloc(*display_devices, (*display_devices_size) * sizeof(DISPLAY_DEVICE) * 2);
            if (temp != NULL)
            {
                *display_devices = temp;
                *display_devices_size = *display_devices_size * 2;
            }
        }
        (*display_devices)[*device_num] = d;
        (*device_num)++;
    }

}