CODEWITHSUNDEEP
cpointersunary-operator
user10988119
user10988119

Reputation:

When is the pointer &x created in function(&x)?

When are the pointers to a and b created? In the code below, I only declared/initiated a and b. But the function swap treats them with &a and &b which are pointers.

My question is, Are the pointers to a and b created at the same time with int a and int b?

Or are they created when swap function was called with arguments &a and &b?

#include <stdio.h>

void swap(int *a, int *b)
{
    int temp;
    temp = *a;
    *a = *b;
    *b = temp;
}

int main()
{
    int a = 3;
    int b = 5;
    swap(&a, &b);
    printf("%d %d", a, b);
}

Upvotes: 2

Views: 123

Answers (3)

h0r53
h0r53

Reputation: 3229

All variables are stored at addresses in memory. Therefore, there exists a pointer for each variable you declare, even if you do not declare them as pointers. To understand what is actually happening it helps to know the basics of assembly language. The following is the relevant part of your code compiled in x86_64 and viewed via objdump.

6ee:    c7 45 f0 03 00 00 00    mov    DWORD PTR [rbp-0x10],0x3
6f5:    c7 45 f4 05 00 00 00    mov    DWORD PTR [rbp-0xc],0x5
6fc:    48 8d 55 f4             lea    rdx,[rbp-0xc]
700:    48 8d 45 f0             lea    rax,[rbp-0x10]
704:    48 89 d6                mov    rsi,rdx
707:    48 89 c7                mov    rdi,rax
70a:    e8 9b ff ff ff          call   6aa <swap>

At lines 0x6ee and 0x6f5 your variables are initialized. Notice that in this case they are actually stored on the stackframe at rbp-0x10 and rbp-0xc respectively. On the following two lines, the addresses of these variables are stored into registers rdx and rax, respectively, then moved into the appropriate registers (rsi and rdi) so that they can be passed the the swap function.

To summarize, C is written at a higher level than the machine architecture that actually executes your code. In machine languages, variables are stored at locations in memory, which are accessible through pointers in the form of offsets to registers, memory sections, etc. In the example above you can see that the data 3 and 5 were "stored in pointer locations" the entire time.

Upvotes: 1

CinCout
CinCout

Reputation: 9619

When you say that a and b are created, this essentially means that a chunk of memory is assigned to these variables, which is then initialized with the respective values. Those memory chunks have an address. The pointers in swap() are nothing but the addresses in memory where a and b are stored.

Upvotes: 2

Vlad from Moscow
Vlad from Moscow

Reputation: 311088

The pointers to a and b are created as results of evaluation of argument expressions in the function call

swap(&a, &b);
     ^^  ^^

From the C Standard (6.5.3.2 Address and indirection operators)

3 The unary & operator yields the address of its operand.

Thus the expressions &a and &b have the type int * and yield the addresses of the variables a and b.

Upvotes: 3

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