我们可以通过另一个包含我们希望 Pointer 指向的其他变量的地址的变量为指针分配地址吗？

Question

I was trying to manually give address through another variable rather than using & .

#include <stdio.h>

int main()
{
    int ab = 99;
    int *p;
    int f = &ab;
    p = f;
    printf("%d\n%d\n%d\n%d\n", &ab, p, f, p); // prints same address
    printf("%d\n", *p); // By adding this line, gives SEGFAULT
}

./a.out Gives:

284008584
284008584
284008584
284008584
Segmentation fault (core dumped)

Answer 1

You're attempting to assign a pointer value to an int . If a pointer is larger than an int on your system, which it most likely is if you're running on a 64 bit OS, then you end up loosing data in the conversion. When you then convert back you end up with an invalid pointer, and attempting to dereference this pointer causes a crash.

You're also invoking undefined behavior by using the wrong format specifier to print a pointer. You should be using %p , and you should be casting the pointer to a void * (one of the few cases when casting to void * is required). If you do this the loss of data in the conversion will become apparent.

Answer 2

1. Please raise the warning level of your compiler to the maximum, for GCC its -Wall -Wextra , if you like also -pedantic . Because:
2. There is an error int your line int f = &ab; . The expression on the RHS is an address of an int , but the variable on the LHS is an int .

On your system the sizes of integers and pointers might be different, that's why dereferencing a pointer which was set from an integer segfaults.

Answer 3

In addition to all the above correct answers about different sizes for pointer and integer,
if you modify your line which prints the addresses to use the correct format modifier, you will see the output addresses are different, due to the loss during cast from pointer to int :

printf("%p\n%x\n%p\n",&ab,p,f); // prints same address

Example:

0x7ffc8b716ea8
8b716ea8
0x8b716ea8

Answer 4

Check if the size of int and the size of int * is same, so that an integer variable can save the address without losing any data. Check this with below code:

printf("%lu, %lu \n", sizeof(int*), sizeof(int));

If both are not same, one can face the errors.

I will suggest take variable type uintptr_t , which can accommodate the pointer's address in it.