CODEWITHSUNDEEP
cassemblyconstants
annunarcist
annunarcist

Reputation: 1805

Does assembly code ignore const keyword?

Does assembly code when used(linked) in a c project ignore const keyword declared before C datatypes and functions?? And can it modify the contents of the datatypes??

Upvotes: 3

Views: 1395

Answers (5)

user2107435
user2107435

Reputation:

GCC places global variables marked as const in a separate section, called .rodata. The .rodata is also used for storing string constants.

Since contents of .rodata section will not be modified, they can be placed in Flash. The linker script has to modified to accomodate this.

#include <stdio.h>
const int a = 10 ;
int main ( void ) {
  return a ;
}

    .section .rodata
    .align 4
a:
    .long   10

gcc with 00 :

    movl    a(%rip), %eax   // variabile constant
    addq    $32, %rsp
    popq    %rbp

gcc with O3 :

    movl    $10, %eax       // numeric constant
    addq    $40, %rsp
    ret

Upvotes: 0

AnT stands with Russia
AnT stands with Russia

Reputation: 320661

The question is not very precise. What does "ignores" mean?

Assembly language does not have the same concept of const as C language does. So, assembly cannot ignore it. It simply has no idea about it.

Yet the assembly code generated by C compiler for a C program might easily be affected by the placement of const keywords in your C program.

In other words, assembly code can be affected by const keywords. But once that assembly code is built, the const keyword is no longer necessary.

To say that assembler can modify something declared as const is not exactly correct either. If you declare a variable as const, in some cases the compiler might be smart enough to eliminate that variable entirely, replacing it with immediate value of that variable. This means that that const variable might disappear from the final code entirely, leaving nothing for the assembly code to "modify".

Upvotes: 1

cdhowie
cdhowie

Reputation: 169211

And can it modify the contents of the datatypes??

Maybe, maybe not. If the original object was declared const then the compiler might emit it into a read-only data segment, which would be loaded into a read-only memory page at runtime. Writing to that page, even from assembly, would trigger a runtime exception (access violation or segmentation fault).

You won't receive a compile-time error, but at runtime your program might crash or behave erratically.

Upvotes: 3

abden003
abden003

Reputation: 1335

Assembly uses the datatypes you declared in C to better optimize how it stores the information in memory. Everything is written in binary at the end of the day (int, long, char, etc), so there is no datatypes once you get to the low level code.

Upvotes: 1

Pascal Cuoq
Pascal Cuoq

Reputation: 80325

Does assembly code when used(linked) in a c project ignore const keyword declared before C datatypes and functions??

Yes, the const keyword is utterly ignored by assembly code.

And can it modify the contents of the datatypes??

If the compiler was able to place a const location in a read-only segment, then assembly code trying to modify it will cause a segmentation fault. Otherwise, unpredictable behavior will result, because the compiler may have optimized parts of the code, but not others, under the assumption that const locations were not modified.

Upvotes: 9

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