如何使用指针将结构视为存储位置和访问元素

Question

I have a structure

typedef struct
{
    unsigned char status;
    unsigned char group_id;
    unsigned char acc_trip_level;
    unsigned char role[50];
    unsigned char standard_panic_header[50];
    unsigned char high_threat_message[50];
    unsigned char high_threat_header[50];
}cfg;

cfg test_val;

I'm passing this structure as an argument to a function and How can I get/access the elements of structure by memory location(in other words i want to treat this structure by memory address)

void foo(cfg *ptr)
{
    printf("%zu\n", sizeof(*ptr)); //Gives size of the strcture
    printf("%p\n", (void*)ptr); //Gives the starting address of strcure
    printf("%p\n", (void*)(ptr+4));  //I want to access the 4th element/ memorylocation
}

Is giving me the result

203
0x8049780
0x8049aac

But it should give 8048780+4 = 8048784 right.. am I missing something

Answer 1

This works for me:

 void foo(cfg * ptr)
 {
     printf("%zu\n", sizeof(*ptr));
     printf("%p\n", ptr);
     printf("%p\n", (void *)((char *) ptr + 4));
 }

And then:

 $ ./a.out 
 203
 0x7fffb6d04ee0
 0x7fffb6d04ee4

When you used (ptr + 4) alone, you essentially got (ptr + 4 * sizeof(cfg)) , because pointer arithmetic works with the size of the pointee, as someone already commented.

Also, format specifier %p should be used for addresses.

Answer 2

Try this:

void foo(cfg *ptr)
{
    printf("%zu\n",sizeof(cfg)); //Gives size of the strcture
    printf("%x\n",ptr); //Gives the starting address of strcure
    printf("%x\n",ptr->role);  //I want to access the 4th element/ memorylocation
}

If your goal is to access the internal elements of the structure by using index offsets, I recommend implementing a hash table.