CODEWITHSUNDEEP
cpointersassemblyarm
user2154420
user2154420

Reputation: 508

ARM assembly pointers to pointers?

Suppose I am given as input to a function foo some pointer *pL that points to a pointer to a struct that has a pointer field next in it. I know this is weird, but all I want to implement in assembly is the line of code with the ** around it:

typedef struct CELL *LIST;
struct CELL {
  int element;
  LIST next;
};
void foo(LIST *pL){
**(*pL)->next = NULL;**

}

How do I implement this in ARM assembly? The issue comes from having nested startements when I want to store such as:

    .irrelevant header junk
foo:
    MOV R1, #0
    STR R1, [[R0,#0],#4]  @This is gibberish, but [R0,#0] is to dereference and the #4 is to offeset that.

Upvotes: 0

Views: 4877

Answers (2)

rcgldr
rcgldr

Reputation: 28941

The sequence would be similar to:

        ...                 ; r0 = LIST *pL = CELL **ppC (ptr2ptr2cell)
        ldr     r0,[r0]     ; r0 = CELL *pC (ptr2cell)
        mov     r1,#0       ; r1 = NULL
        str     r1,[r0,#4]  ; (*pL)->next = pC->next = (*pC).next = NULL

Upvotes: 3

artless-noise-bye-due2AI
artless-noise-bye-due2AI

Reputation: 22469

The correct sequence would be (assuming ARM ABI and LIST *pL is in R0),

.global foo
foo:
     ldr r0, [r0]      # get *pL to R0
     mov r1, #0        # set R1 to zero.
     str r1, [r0, #4]  # set (*pL)->List = NULL;
     bx  lr            # return

You can swap the first two assembler statements, but it is generally better to interleave ALU with load/store for performance. With the ARM ABI, you can use R0-R3 without saving. foo() above should be callable from 'C' with most ARM compilers.

The 'C' might be simplified to,

void foo(struct CELL **pL)
{
   (*pL)->next = NULL;
}

if I understand correctly.

Upvotes: 1

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