Databases
StackOverflow2013
Postsdbo
PORemove constness of a pointer in a struct in C
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PORemove constness of a pointer in a struct in C
    primarykey
    data
    text
    <p>So I need to remove constness from some variables in C (I know what I'm doing). So I wrote a little macro (<code>UNCONST</code>) which lets me assign an new value to a const value. This works just fine for normal variables of types like <code>int</code>. But this doesn't work for pointers. So I can't let a pointer point to a different position using my macro <code>UNCONST</code> without getting an compiler warning.</p> <p>Here a little test program <code>unconst.c</code>:</p> <pre><code>#include &lt;stdio.h&gt; #define UNCONST(type, var, assign) do { \ type* ptr = (type*)&amp;(var); \ *ptr = (assign); \ } while(0) struct mystruct { int value; char *buffer; }; int main() { // this works just fine when we have an int const struct mystruct structure; UNCONST(int, structure.value, 6); printf("structure.value = %i\n", structure.value); // but it doesn't when we have an char * char *string = "string"; UNCONST(char *, structure.buffer, string); printf("structure.buffer = %s\n", structure.buffer); // this doesn't work either, because whole struct is const, not the pointer. structure.buffer = string; printf("structure.buffer = %s\n", structure.buffer); } </code></pre> <p>compiling &amp; executing</p> <pre><code>$ LANG=en gcc -o unconst unconst.c unconst.c: In function ‘main’: unconst.c:21:3: warning: assignment discards ‘const’ qualifier from pointer target type [enabled by default] unconst.c:25:3: error: assignment of member ‘buffer’ in read-only object </code></pre> <p>Is there a way to optimize my macro so this warning doesn't show up?</p>
    singulars
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. PTQuestion
    1. USMarcDefiant
    1. USMarcDefiant
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    2. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. CODo you realize that you can just write string1 = string2 ?
      singulars
      1. PORemove constness of a pointer in a struct in C
      1. USKylo
    2. CO@Kylo I didn't but it seems like I simplified my code too much, I need to edit the question.
      singulars
      1. PORemove constness of a pointer in a struct in C
      1. USMarcDefiant
    3. COThe first segment using `int` invokes undefined behaviour. The compiler might legitimately place `i` in readonly memory, and then you probably get a crash, or other unexpected result (like the `printf()` printing 5 instead of 6). You're relying on a peculiarity of your implementation there. So, expect problems when you switch platforms or compilers.
      singulars
      1. PORemove constness of a pointer in a struct in C
      1. USJonathan Leffler
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload